Ecology of the seagrass meadows of the west coast of Florida: a community profile

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September 1989
THE ECOLOGY OF THE
SEAGRASS MEADOWS
OF THE WEST COAST
OF FLORIDA:
A
Community
Profile
a
Minerals Management Service
and
Fish and Wildlife Service
U.S. Department of the Interior
Biological Report 85(7.25)
September 1989
THE ECOLOGY OF THE SEAGRASS MEADOWS OF THE WEST
COAST OF FLORIDA: A COMMUNITY PROFILE
Joseph C. Zieman
Rita T. Zieman
Department of Environmental Sciences
University of Virginia
Charlottesville, VA 22903
Project Officer
Edward Pendleton
U.S. Fish and Wildlife Service
National Wetlands Research Center
1010 Gause Boulevard
Slidell, LA 70458
Conducted in Cooperation With
Minerals Management Service
Gulf of Mexico
U.S. Department of the Interior
Fish and Wildlife Service
Research and Development
Washington, D C 20240
DISCLAIMER
The opinions and recommendations expressed in this report are those of the authors
a n d d o n o t n e c e s s a r i l y r e f l e c t t h e v i e w s o f t h e U . S . F i s h a n d W i l d l i f e S e r v i c e , n o r
does the mention of trade names constitute endorsement or recommendations for use by
the Federal Government.
Library of Congress Cataloging-in-Publication Data
Zieman, Joseph C.
The ecology of the seagrass meadows of the west coast of Florida.
(Biological report ; 85 (7.25)
“National Wetlands Research Center.”
“Conducted in cooperation with Minerals Management Service, Gulf of Mexico.”
“September 1989.”
Bibliography: p.
1. Marine ecology--Florida. 2. Marine ecology--Mexico, Gulf of. 3.
Seagrasses--Florida--Ecology. 4. Seagrasses--Mexico, Gulf of--Ecology. I.
Zieman, Rita T. II. Pendleton, Edward C. III. National Wetlands Research Center
(U.S.) IV. United States. Minerals Management Service. V. Title. VI. Series:
Biological report (Washington, D.C.) ; 857.25.
QH105.F6Z49 1989 574.5’2636’0916364 89-600197
Suggested citation:
Z i e m a n , J . C . , and R.T. Zieman. 1989. The ecology of the seagrass meadows of the west
coast of Florida: a community profile. U.S. Fish Wildl. Serv. Biol. Rep.
85(7.25). 155 pp.
PREFACE
Seagrass beds have come to be known as
extremely productive and valuable coastal
wetland resources. They are critical
nursery areas for a number of fish,
shrimp, and crab species, and support the
adults of these and other species that
forage around seagrass beds, preying on
the rich and varied fauna that occur in
these habitats. Seagrass beds support
several endangered and threatened
species, including sea turtles and
manatees along the west coast of Florida,
the geographic area covered in this
profile.
For these reasons and others, seagrass
beds or meadows have been the topic of
several of the reports in this community
profile series. This report, covering
the seagrass community of the Florida
Gulf of Mexico coastline from south of
Tampa Bay to Pensacola, is the fifth
community profile to deal with submerged
aquatic vegetation beds; others in the
series have synthesized ecologic data on
seagrasses of south Florida, eelgrass
beds in the Pacific Northwest and along
the Atlantic coast, and kelp forests of
the central California coastline.
These reports in total represent a
major effort toward summarizing and
synthesizing what is known of the
ecologic structure, functioning, and
values of these marine and estuarine
communities. This profile in particular
builds on the author's earlier profile on
the seagrass meadows of south Florida.
As will become apparent to the reader,
while enough is known to describe the
gulf coast seagrass community, there has
been little study of the finer points of
the structure and function of seagrass
beds in this region. To shed light on
the ecology of Thalassia, Syringodium,
and Halodule meadows on Florida's a.,ulf
coast, one is forced to extrapolate a
good deal from information from studies
conducted on the south and southeast
Florida coasts and elsewhere. However,
in so doing the author has been able to
update his own earlier communitv profile.
Thus, The Ecology -of the- Seagra;s'Meadows
-of th-e West Coast of Florida is not only
a synthexof &IC, but also serves
as- a state-of-the-art review of
subtropical seagrass ecology and a
this series, the profile finally high-lights
how much is still left to learn
about these valuable natural habitats.
iii
CONVERSION FACTORS
Multiply
millimeters (mm)
centimeters (cm)
meters (m)
meters (m)
kilometers (km)
kilometers (km)
BY To Obtain
0.03937 inches
0.3937 inches
3.281 feet
0.5468 fathoms
0.6214 statute miles
0.5396 nautical miles
square meters (m’) 10.76
square kilometers (km’) 0.3861
hectares (ha) 2.471
liters (I)
cubic meters (m3)
cubic meters (m3)
0.2642 gallons
35.31 cubic feet
0.0008110 acre-feet
milligrams (mg)
grams (9)
kilograms (kg)
metric tons (1)
metric tons (1)
0.00003527 ounces
0.03527 ounces
2.205 pounds
2205.0 pounds
1.102 short tons
kilocalories (kcal)
Celsius degrees (‘C)
3.968 British thermal units
1.8(%) + 32 Fahrenheit degrees
inches 25.40
inches 2.54
feet (ft) 0.3048
fathoms 1.829
statute miles (mi) 1.609
nautical miles (nmi) i ,852
square feet (ft2)
square miles (mi2)
acres
gallons (gal)
cubic feet (ft3)
acre-feet
ounces (02)
ounces (02)
pounds (lb)
pounds (lb)
short tons (ton)
British thermal units (Etu) 0.2520
Metric to U.S. Customary
U.S. Customary to Metric
0.0929 square meters
2.590 square kilometers
0.4047 hectares
3.785 liters
0.02831 cubic meters
1233.0 cubic meters
28350.0 milligrams
28.35 grams
0.4536 kilograms
0.00045 metric tons
0.9072 metric tons
kilocalories
Fahrenheit degrees (“F) 0.5556 (“F - 32) Celsius degrees
square feet
square mrles
acres
millimeters
centimeters
meters
meters
kilometers
kilometers
iv
CONTENTS
PREFACE ...........................................................................
CONVERSION FACTORS ................................................................
FIGURES ...........................................................................
TABLES ............................................................................
ACKNOWLEDGMENTS ...................................................................
CHAPTER 1. INTRODUCTION ..........................................................
:*:
Seagrass Ecosystems .....................................................
1:3
The Seagrasses of the West Coast of Florida .............................
Physical Environment ....................................................
:::
Geologic Environment ....................................................
Succession and Ecosystem Development ....................................
CHAPTER 2 AUTECOLOGY OF FLORIDA GULF COAST SEAGRASSES ...........................
2.1 Plant Morphology and Growth .............................................
z
Reproduction ............................................................
214
Plant Constituents ......................................................
Physiological Ecology ...................................................
2.4.1 Environmental Tolerances and Responses ...........................
2.4.2 Photosynthetic Carbon Fixation ...................................
2.4.3 Isotopic Fractionation ...........................................
2.5 Nutrient Uptake and Supply ..............................................
CHAPTER 3 DISTRIBUTION, BIOMASS, AND PRODUCTIVITY ...............................
3.1 Distribution ... ........................................................
3.1.1 Seagrass Distribution in Tampa Bay ...............................
3.1.2 Seagrass Distribution in the Big Bend Area .......................
3.2 Biomass .................................................................
3.2.1 Seagrass Biomass in Tampa Bay ....................................
3.2.2 Seagrass Biomass in the Big Bend Area ............................
3.3 Productivity ............................................................
3.3.1 Seagrass Productivity in Tampa Bay ...............................
3.3.2 Seagrass Productivity in the Big Bend Area .......................
CHAPTER 4. COMPONENTS OF THE SEAGRASS COMMUNITY ..................................
4.1 Algal Associates ........................................................
4.1.1 Phytoplankton ....................................................
4.1.2 Benthic Algae ....................................................
4.1.3 Epiphytic Algae ..................................................
4.2 Invertebrates ...........................................................
4.3 Fishes ..................................................................
V
Page
iii
iv
vii
viii
ix
1
11
:2'
14
;:
23
23
24
27
27
27
29
34
::
37
:;
40
t;
t:
46
48
Page
4.4 Reptiles ................................................................
4.5 Birds ...................................................................
4.6 Mammals .................................................................
CHAPTER 5. STRUCTURAL AND FUNCTIONAL RELATIONSHIPS IN SEAGRASS SYSTEMS...........
5.1
5.2
5.3
CHAPTER 6
6.1
6.2
6.3
6.4
6.5
The Relationship of Structure, Shelter, and Predation ...................
5.1.1 Fauna1 Abundance and Structure ...................................
5.1.2 Structure and Predation ..........................................
5.1.3 Fauna1 Sampling: The Problems of Gear and Technique ..............
General Trophic Structure ...............................................
5.2.1 Seagrass Grazers .................................................
5.2.2 Epiphyte Seagrass Complex ........................................
5.2.3 Detrital Feeding .................................................
5.2.4 Carnivory........................................................
5.2.5 Trophies and Ontogenetic Development
of Grassbed Fishes ...............................................
Decomposition and Detrital Processing ...................................
INTERFACES WITH OTHER SYSTEMS .........................................
Salt Marsh and Mangrove .................................................
Gulf Reefs ..............................................................
Continental Shelf .......................................................
Export of Seagrass ......................................................
Nursery Grounds .........................................................
6.5.1 Blue Crabs .......................................................
6.5.2 Shrimp ...........................................................
6.5.3 Fish...............................~ .............................
6.5.4 Detached Macrophytes as Nursery Habitat ..........................
CHAPTER 7. HUMAN IMPACTS AND APPLIED ECOLOGY .....................................
7.1 Dredging, Filling, and Other Physical Damage ............................
7.1.1 Acute Stress .....................................................
7.1.2 Other Physical Damage ............................................
7.1.3 Chronic Effects ..................................................
7.2 Eutrophication and Sewage ...............................................
7.3 Oil .....................................................................
7.4 Temperature and Salinity ................................................
7.5 Paper Mill Effluents ....................................................
7.6 Disturbance, Recolonization, and Restoration ............................
7.6.1
7.6.2
7.6.3
7.7 Final
REFERENCES.....
APPENDIX: FISH
Disturbance ......................................................
Recolonization ...................................................
Restoration ......................................................
Thoughts ..........................................................
SpECIEj.;;kVEVs.IN.SbUjH'ANb'WESiikN'iib~~~~::::::::::::::::::::::
53
53
54
58
58
58
60
66
67
68
70
70
71
72
72
77
77
79
81
83
84
84
85
86
86
88
88
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90
91
91
91
93
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95
95
96
99
101
103
131
vi
Number
FIGURES
Page
17
18
19
20
21
22
23
24
Z
27
28
Location Map of Florida ...................................................... 1
The seagrasses of the west Florida coast ..................................... 4
Temperatures at four locations in coastal Florida ........................... 5
Coastal geology of the Florida west coast .................................... 8
General morphology of a Thalassia plant ...................................... 12
Flowers of Thalassia and Syringodium (Durako) ................................ 13
Temperature responses of Thalassia and Syringodium
on the west Florida coast .................................................... 19
Carbon isotopic variation at two locations in Florida ........................ 24
Seagrass coverage in Tampa Bay, 1879 and 1982 ................................ 28
Seagrass meadow types in Tampa Bay ........................................... 29
Seagrass distribution and density in Big Bend area ........................... 30
Seagrass species distribution in the Big Bend area ........................... 31
Depth distribution of seagrass biomass in Apalachee Bay ...................... 32
Seasonal cycle of Thalassia at two stations in Apalachee Bay ................. 37
Location of recently fixed carbon photosynthate in Thalassia ................. 38
Seasonal changes in productivity of seagrasses and red
algae in Apalachee Bay ....................................................... 39
Distribution and diversity of benthic marine plants in the
Gulf of Mexico ............................................................... 42
Representative temperate (Carolinean) invertebrate communities ............... 47
Representative tropical (West Indian) invertebrate communities ............... 48
Comparison of fauna1 abundance between seagrass beds and
adjacent habitats ............................................................ 58
Salt-marsh zonation in Florida ............................................... 79
Life histories of spotted sea trout and snook on the west
Florida coast ................................................................ 80
Fauna1 zonation of west Florida shelf ........................................ 82
Eastern Gulf of Mexico current patterns during summer ........................ 83
Blue crab spawning migrations and larval transport ........................... 85
Channel through grassbed with open-water dredge disposal area
in Tampa Bay ................................................................. 89
Dredged and filled areas in Boca Ciega Bay ................................... 90
Crystal River power plant .................................................... 93
vii
TABLES
Number
1
234
z789
1':
12
13
14
15
16
17
18
19
Page
Precipitation statistics for gulf coast stations .............................
Sediment characteristics of the west coast of Florida ........................
Constituents of seagrasses
!
...................................................
Seasonal changes in protein and carbohydrates in seagrasses in Tampa Bay
15
..... 18
Representative values of seagrass biomass
Biomass partitioning in seagrasses
.................................... 35
...........................................
Seagrass biomass in Tampa Bay area
36
........................................... 36
Representative values of seagrass productivity ...............................
Macroalgae of seagrass communities of west Florida
37
Algal epiphytes of the seagrasses of Florida
........................... 43
..............................
Fishes of Apalachee Bay...................................................:::
Seagrass-associated fish families
:;
..............................
Seagrass and wetlands associated birds of the Florida west coast:::::::::::::
Comparative abundance of organisms in seagrass and sand habitats
z:
.............
Influence of seagrass community structure on fauna1 abundance
59
................
Wetlands acreage on the west Florida coast
61
.......................... 77
Historic human impact on Florida seagrass meadows...................:::::::: 1 97
Seagrass damage and restoration assessment
Success of seagrass restoration techniques
................................... 99
................................... 100
. . .
VIII
ACKNOWLEDGMENTS
The production of this manuscript turned
into a much larger project than was
envisioned when we began. During that time,
many people contributed greatly towards the
production of the final product. The
authors are especially grateful to Richard
L. Iverson who contributed both written
material and numerous published and
unpublished figures for Chapters 2 and 3.
We wish to give special thanks to our
project officer, Edward C. Pendleton, who
has been unusually conscientious throughout
all aspects of the project, starting with
the outline and continuing to the final
production. We especially thank him for his
patience in the face of numerous
unanticipated delays.
Critical comments and reviews were made
by Ron Phillips, Robin Lewis, Mike Durako,
Laura Gabanski, Cheryl Vaughn, Charles Hill,
Robert Rogers, David Moran, Ed Pendleton,
and Lorna Sicarello. Robin Lewis and Mike
Durako also thoughtfully provided several
illustrative photographs.
Sue Lauritzen did the layout and Daisy
Singleton and Joyce Rodberg keyboarded the
final draft.
ix
CHAPTER 1. INTRODUCTION
1 .l SEAGRASS ECOSYSTEMS
Seagrass meadows are recognized today as
one of the most important communities in
shallow coastal waters. Rapidly growing
seagrass leaves serve as the basis of a
productive grazing and detrital food web,
while the canopy structure formed by these
leaves offers shelter and protection from
predation for innumerable small organisms,
many of which are the juveniles of
important commercial species. The coastal
waters of Florida are especially rich in
seagrass resources. The two largest
seagrass meadows in Florida have received
little human disturbance thus far. The
largest, in Florida Bay, is approximately
5,500 km2, and is protected from
large-scale human impact because it is
mostly within the boundaries of Everglades
National Park. The second largest bed is
just off the northwest coast of Florida,
between Tarpon Springs and St. Marks, and
is approximately 3,000 km2 (Iverson and
Bittaker 1986). Other seagrass meadows,
especially those within urbanized
estuaries, have not fared as well. Lewis
et al. (1985a) found that in 1982, Tampa
Bay contained 5,750 ha of seagrass cover.
From old maps and aerial photographs they
estimated the historical coverage to be
nearly 31,000 ha, thus showing a reduction
to less than 20% of the historical
coverage.
The coastline of western Florida is a
major ecocline for the tropical seagrass
species. Although the distance is not
great, about 650 km from Florida Bay to
Apalachicola Bay, it represents a shift
from a region in the south where tropical
seagrasses reach their highest
development, to areas that are the
northern limits of distribution for
several of the species, notably Thalassia
and Syringodium. While this report
addresses the west and northwest coast of
Florida, the area of central interest for
this community profile is the region from
Tampa Bay to Apalachicola Bay (Figure 1).
This region contains the large offshore
beds of the Big Bend area, as well as
several representative estuarine systems.
It is largely defined by the available
data base for the Florida west coast.
Compared with seagrass meadows in
southern Florida, communities of western
Florida and the northeastern Gulf of
Mexico have received little attention from
the research community; therefore, this
community profile will refer to data from
south Florida and the Caribbean when
Apalachicolo
BQY
G U L F O F
MEXICO
Figure 1. Location map of Florida.
1
comparable studies from western Florida do
not exist. Interestingly, the west coast
area was the location of the seminal
seagrass studies of Florida, in
particular, and the Southeast, in general.
This work culminated in the monograph on
the seagrasses of Florida by Ronald C.
Phillips published in 1960. Within the
past 10 years, research on these systems
has accelerated in the bays and estuaries
of north Florida and in central Florida;
however, less work has been done on the
large offshore bed between these two
regions. This extensive seagrass meadow
is unique among Florida's seagrass
resources since it is truly offshore, and
does not lie behind any form of protective
barrier.
Seagrass ecosystems are among the
richest, most productive, and most
important of all coastal systems. They
are also paradoxical in nature--
simultaneously simple and complex. They
are simple in that there are few species
of seagrasses, unique marine angiosperms
that live and carry out their life cycle
in seawater. Vast and extensive undersea
meadows stretching for hundreds of
kilometers may be composed of only one to
perhaps four species. The ecosystems,
however, are complex ,because there are
hundreds to thousands of species of
associated flora and fauna that inhabit
the seagrass meadows and utilize the food,
substrate, and shelter provided by the
plants.
The pioneering work of Petersen (1918)
in the Baltic region provided the first
documentation of the value of seagrass
beds to shallow coastal ecosystems. These
studies demonstrated how the primary
production from these plants was channeled
through the detrital food web and
supported the rich commercial fisheries of
the region. Despite the thoroughness and
quality of Petersen's work, only in the
past two decades have the richness and
value of seagrass ecosystems begun to be
realized (Wood et al. 1969; McRoy and
McMillan 1977; Zieman and Wetzel 1980).
The first conceptualization of the
functions of seagrasses was provided by
Wood et al. (1969). The generalizations
have now been shown to be applicable to a
wide variety of systems and situations.
The following is an updated version
(Zieman 1982) of the earlier conceptual
framework.
1.
2.
3.
4.
5.
High Production and Growth
The ability of seagrasses to exert a
major influence on the marine seascape
is due in large part to their
extremely rapid growth and high net
productivity. The leaves grow at
rates of typically 5 mm per day, but
growth rates of over 10 mm per day are
not uncommon under favorable
circumstances.
Food and Feeding Pathways
The photosynthetically fixed energy
from the seagrasses may follow two
general pathways: direct grazing of
organisms on the living plant material
or utilization of detritus from
decaying seagrass material, primarily
leaves. The export of seagrass
material, both living and detrital, to
a location some distance from the
seagrass bed allows for further
distribution of energy away from its
original source.
Shelter
Seagrass beds serve as a nursery
ground, that is, a place of both food
and shelter, for the juveniles of a
variety of finfish and shellfish of
commercial and sportfishing
importance.
Habitat Stabilization
Seagrasses stabilize the sediments
in two ways: the leaves slow and
retard current flow to reduce water
velocity near the sediment-water
interface, which promotes
sedimentation of particles as well as
inhibiting resuspension of both
organic and inorganic material.
Secondly, roots and rhizomes form a
complex, interlocking matrix with
which to bind the sediment and retard
erosion.
Nutrient Effects
The production of detritus
promotion of sedimentation
leaves of seagrasses provide
and the
by the
organic
matter for the sediments and maintain
an active environment for nutrient
recycling. Epiphytic algae on the
leaves of seagrasses have been shown
2
to fix nitrogen, thus adding to the
nutrient pool of the region. In
addition, seagrasses have been shown
to take up nutrients from the
sediments, transporting them through
the plant and releasing the nutrients
into the water column through the
leaves, thus acting as a nutrient
pump.
In addition to providing
shelter, the seagrass leaves
food resource in coastal
habitat and
are a major
ecosystems,
functioning through three major pathways:
direct herbivory, detrital food webs, and
export to adjacent ecosystems. Direct
herbivory on green seagrass leaves is
confined to a small number of species and
is most prevalent in tropical and
subtropical regions, especially in the
vicinity of coral reefs. Since the time
of Petersen (1918), the detrital food web
has been considered the main trophic
pathway in seagrass meadows, and current
studies continue to support this concept,
although direct herbivory can be locally
important in some areas (Zieman et al.
1984a; Thayer et al. 1984). In addition
to the internal utilization of seagrasses
as a food source, many beds, especially
those dominated by Syringodium, export
large quantities of organic material to
other distant ecosystems.
In the subtropical waters of south
Florida, seagrass meadows often bridge
large areas between the mangrove and coral
reef communities, while also serving as a
primary nursery and feeding ground
themselves (Zieman 1982). On the west
coast of Florida, they function in a
similar manner, as nurseries and feeding
grounds, but also serve as an interface
between the coastal salt marsh communities
and offshore habitats of the eastern Gulf
of Mexico.
1.2 THESEAGRASSESOFTHEWESTCOASTOF
FLORIDA
Seagrasses compose the relatively small
group of monocots which have evolved the
ability to carry out their life cycle
completely submerged in the marine
environment. Worldwide, they include 2
families divided into 12 genera and
approximately 45 species. The
Potamogetonaceae include 9 genera and 34
species and are represented on the west
coast of Florida by Syringodium filiforme
Kutz, whose common name is manatee grass,
and Halodule wrightii Ascherson, shoal
grass; the Hydrocharitaceae contains 3
genera with 11 species (Phillips 1978), of
which Thalassia testudinum Konig, (turtle
grass), and two species of Halophila, H.
engelmanni Acherson and H. decipiens
Ostenfeld, are found in the waters of the
west coast of Florida. Ruppia maritima
Linneaus (widgeon or ditch grass)
euryhaline angiosperm found abundantly in
fresh waters and in the marine environment
grows primarily in lower salinity areas.
The small number of species occurring in
these waters, and their distinctive gross
morphologies (Figure 2) preclude the need
for a dichotomous key, although systematic
works such as den Hartog (1970) and
Tomlinson (1980) are available for
comparison of seagrasses in other areas.
Phillips (1960a) still provides the best
treatment of local species.
The three dominant species of the open
coastal waters are Thalassia testudinum,
Syringodium filiforme, and Halodule
wrightii.
Thalassia is the largest and most robust
of the west Florida seaarasses. and the
densest growth in the vast" grassded of the
Big Bend area is dominated by a mixture of
this species and Syringodium (Iverson and
Bittaker 1986). While this species is not
abundant in the lower salinity waters of
Tampa Bay (Lewis et al. 1985a), it is the
dominant seagrass in the adjacent waters
of Boca Ciega Bay (Taylor and Saloman
1968; Bauersfeld et al. 1969), and in the
Tarpon Springs area (Phillips 1960a).
Among the local seagrasses, Syringodium
is distinctive in having cylindrical
leaves which are quite brittle and
buoyant, and thus are readily broken off
and exported from the immediate area by
winds and currents. This species is more
widely distributed in Tampa Bay than is
Thalassia (Phillips 1960a; Lewis et al.
-and while it is codominant in the
Big Bend grassbed, its biomass is
generally lower than that of Thalassia in
the mixed stands of that area, although
3
Halodule wrightii
Syringodium filiforme
Thalassla testudlnum
Halophila engelmanni Halophila decipiens Ruppia maritima
Figure 2. The seagrasses of the west Florida coast.
there are localized areas where it is
abundant.
Halodule, which has narrow leaves and a
shmot system,
pioneer species in
is recognized as the
the successional
development of grassbeds in the gulf and
Caribbean. It is more tolerant of low
salinity than both Thalassia and
Fy
and thus occurs in areas of
ampa Bay where those seagrasses cannot
survive
1985a).
(Phillips 1960a; Lewis et al.
As its common name, shoal grass,
indicates, it is often found in shallow
waters where it is subjected to repeated
4
exposure to the atmosphere. In the Big
Bend grassbed, this plant often forms both
the shallowest shoreward fringe of and the
deepest, outermost stands of seagrass, and
exhibits different morphologies in the two
zones (Phillips 1960b; McMillan 1978;
R.L. Iverson, unpubl. data).
1.3 PHYSICAL ENVIRONMENT
The west coast of Florida has a mild
maritime climate varying from temperate in
the north to semitropical * the
southernmost regions. For much':f the
year the southern portion of Florida is
dominated by the southeasterly trade
winds, while the airflow in the northern
and central portion is from the west,
under the influence of the westerlies and
accompanying cyclones (counterclockwise
circulation about a center of low
pressure) in the winter, and the western
margin of the Bermuda-Azores anticyclone
(clockwise circulation about a center of
high pressure) in summer.
The resulting differences in temperature
patterns are evident in Figure 3, which
shows the average monthly water
temperatures at several locations from
Pensacola to Key West (McNulty et al.
1972). The Cedar Key station is in the
center of the region under consideration
here. Both the average and maximum summer
temperatures vary little among the
stations, with highs around 33 OC. Most
obvious are the lower winter temperatures
and greater seasonal range at the northern
stations. Key West has a monthly low
average of 22 OC and a range of 14-26 OC
during January, while Cedar Key has a
January average temperature of 13.5 'C
with a range of 4-22 OC.
Earle (1969) found a similar pattern
with inshore gulf temperatures of 13-15 OC
in the north and 22.6-22.9 OC in the
Florida Keys. However, north of Cedar
Key, extreme winter lows of O-5 OC have
been recorded. The average winter
temperatures in the northern gulf in
winter are similar to the summer high
temperatures in New England, and Earle
(1969) noted that many winter species in
the northern gulf are the same as the
summer species in New England waters.
Precipitation generally increases
northward and westward along the Florida
coast from a low of 100 cm annually at Key
West to 163 cm at Pensacola (Table 1).
However, in the region from Tampa to
Apalachicola the precipitation is
relatively uniform with a minimum annually
of 118 cm at Cedar Key to a maximum of 140
cm at Apalachicola with about half of the
annual amount falling between June and
September. The average annual and monthly
Key West St. Petersburg Cedar Key Pensacola
J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D
Figure 3. Temperatures atfourlocations in coastal Florida (from McNulty eta11972).
5
Table 1. Precipitation statistics for coastal stations on the eastern Gulf of Mexico
(from Jordan 1973).
Precipitation, Precipitation, Precipitation,
Mean Annual June-Sept. Dec.-March
(inches) G) G)
Mobile 65.5 41.4 34.9
Pensacola 63.4 43.3 30.0
Apalachicola 56.2 52.5 25.8
Tallahassee 56.9 47.5 28.5
Cedar Key 46.6 55.9 23.8
Tampa 51.6 60.2 20.6
Fort Meyers 53.3 63.6 14.3
Everglades 54.7 62.5 12.4
Key West 40.0 48.0 17.4
rates show the general patterns, but the
extreme months and years are highly
variable and can have severe effects on
the local biota. For Cedar Key, annual
rainfall has varied between 68-208 cm,
while monthly values at Apalachicola have
varied from a low of 0.03 cm to a high of
57 cm.
In the shallow waters of the estuaries
and the inshore gulf, water temperature
and salinity are locally affected by both
seasonal and isolated storms. The most
severe storms are tropical hurricanes with
their high winds, heavy rainfall, and
often devastating storm surges.
Hurricanes occur most frequently in the
late summer months when the oceanic
surface temperatures are at their highest,
but can occur in any month. The
probability of encountering hurricane
force winds in any one year varies greatly
along the Florida coast, being 1 in 8 at
Key West and Pensacola, 1 in 17 at
Apalachicola and St. Marks, and 1 in 25 at
Tampa-St. Petersburg (Bradley 1972). In
addition to the immediate local effects of
these storms, water quality is affected
following their passage by greatly
increased runoff from rivers and streams,
accompanied by increased turbidity and
biochemical oxygen demand.
In most locations, seagrass beds are
relatively protected from the surges of
large storms. However, in the Big Bend of
Florida these beds are subject to the full
force of storm waves. In 1985, two
hurricanes, "Kate" and "Elena" passed
directly through the area causing
localized disruption and bottom scouring.
Qualitative observations of stations
sampled before and after the hurricanes
suqqested complete recovery of the denser
inshore beds' of Thalassia, Syringodium,
and Halodule and the sparse offshore
Halophila beds in the vicinity of Tarpon
Sorinas Shelf Associates
1986):
(Continental
In the vicinity of Cedar Key,
where Hurricane "Elena" stalled for about
48 hours, seagrasses appeared to be
recovering, but at a slower rate than the
other site.
Tidal ranges are low to moderate along
most of the Florida west coast. From
Florida Bay northward to St. Joseph Bay
the tides are predominately semi-diurnal
(McNulty et al. 1972), shifting to diurnal
west of this point. Throughout the entire
area, the mean diurnal range is 0.5-1.1 m.
Daily ranges at Tampa Bay are 0.6-0.8 m.
Just north of Tampa Bay, the range
increases to 1.1 m until Apalachee Bay
where it begins to decrease slightly and
reaches 0.4-0.7 m at Apalachicola Bay.
Offshore circulation is dominated by two
large counter-rotating gyres. The
northern one is influenced by coastal
estuarine waters, while the southern one
is influenced by waters from Florida Bay.
In addition, there are periodic incursions
of the loop current with waters from the
6
tropical Caribbean and the Yucutan Channel
(Chew 1955; Austin 1970).
1.4 GEOLOGIC ENVIRONMENT
The present Florida peninsula is the
emergent portion of the Floridan Plateau,
consisting of layers of limestone and
unconsolidated sediments over a base of
sandstone and volcanic rocks (Puri and
Vernon 1959; NcNulty et al. 1972). The
limestone and ancient sediments are at
least 1,000 m in thickness over the entire
region. The rivers that. enter the gulf
east of Apalachicola Bay drain the coastal
plain, carrying small amounts of sediments
that are primarily carbonates and
anhydrites (McNulty et al. 1972). From
Apalachicola Bay westward, the rivers
drain areas of the Piedmont plateau and
the Appalachian highlands, and carry
primarily elastic sediments. Table 2
gives the characteristics of sediments for
several locations on the west Florida
coast.
The coastline of west Florida has been
divided and classified (Figure 4)
according to several different criteria
and schemes, including coastal beach and
interface characteristics (Price 1954;
Tanner 1960; McNulty et al. 1972), fauna1
community affinities (Lyons and Collard
1974), and underlying substrates and
outcrops (Brooks 1973). The coastal
divisions resulting from these differing
schemes are very highly correlated, and
the divisions used in this paper are a
combination of the above schemes.
The coastline west of Lighthouse Point,
near Apalachicola Bay, is the northern
gulf barrier coastline, with attached sand
Table 2. Sediment characteristics of the west Florida coast (from Folger 1972).
Carbonate
Location Organic content content Texture
Florida Bay
Whitewater
Bay
Gullivan Bay
(very open)
Port Charlotte
Harbor
Tampa Bay
Apalachicola
Bay
St. Joseph
Bay
Pensacola Bay
Average = 2.1% west
1% on shelf
l%-4% in lagoon
O.l%-1.0%
maximum = 3.1%
0.5%-2.0%
0.5%-4.5%
__
up to 90%
(Quartz 3.5% east,
up to 30%)
Up to 65%
(quartz 5%-10%)
lo%-40% typically
Locally to 60%-80%
Quartz 4%-8% near
islands
maximum = 24%
-_
0.5%-40%
lO%-40%
lo%-80%
1.3%-5%
Median size east = 0.025mm
west = 0.028mm
W = 70% silt, 30% sand
Non CaCOs = fine to very
fine sand
Very fine to fine sand
Variable, typically
sand sized
Variable, very coarse
sand to clay
Variable, very coarse
sand and gravel to clay
Coarse sand to silt
7
rend beaches and
-low d u n s , o” G u l f ;
,;do, marshes and
c
Figure4. Coastalgeologyof the Florida west coast (from McNultyet al. 1972).
elastic
barrier
found to
beaches alternating with barrier islands. that they carry little suspended
A similar attached beach-barrier island material to form beaches or
interface exists from Anclote Key islands (Ross 1973) such as those
southward along the western edge of the the south or to the west.
central and lower Florida peninsula.
Along both the northern gulf and the Of equal or greater importance
central and lower peninsula, the barrier t_ he. region between St. Marks ant
is that
II Tarpon
beaches and spits enclose the major
estuaries and lagoons. However, the Big
Bend, the upper coastline of the
peninsula, is unique for the region in
that it is an extensive area with no
offshore barrier, where rivers, creeks,
and marshes grade directly into the
eastern Gulf of Mexico. A number of
physical, geological, and hydrological
features interact to produce this effect.
The rivers of the Big Bend are notable in
Springs is one of the few examples
world-wide of a zero-energy coastline
(Murali 1982). This is defined as a coast
where "the average breaker heights are
3-4 cm or less, and there is no signi-ficant
littoral transport of sand" (Murali
1982). The major factors that contribute
to this phenomenon include the wide,
gently sloping shelf; the divergence of
approaching wave trains into the large,
expanding coastal concavity; the location
8
of the coast in a generally upwind direc-tion;
a small supply of new sediment; and
the wave dampening effects of old sub-merged
beaches and the submerged seagrass
meadows (Murali 1982). Although the
presence of submerged seagrass meadows
interfacing directly with salt marshes
has been considered to be a contributing
factor to the zero-energy coast, it is
more likely that their presence in this
area is in fact the result of existing low
energy conditions, as seagrass beds are
rare on open oceanic, unprotected coasts.
Once established, the seagrass beds could
enhance the effects of those primary
factors responsible for reduced energy
conditions.
1.5 SUCCESSIONANDECOSYSTEM DEVELOPMENT
Throughout their range, few plants
participate in the successional sequence
leading to seagrasses because there are so
few marine plants that can colonize soft
sediments. In general, this sequence
consists only of the seagrasses and the
rhizophytic green algae. Seagrasses are
vital to the coastal ecosystem because
they are the only plants capable of
providing the basis for a mature,
productive ecosystem in these regions.
Few other systems are so dominated and
controlled by a single species as a climax
Thalassia or Zostera meadow.
Odum (1974) classified Thalassia beds as
"natural tropical ecosystems with high
diversity." Compared to other natural
systems, tropical seagrass beds are
regions of very high diversity, but this
can be misleading. These comparisons were
made at a time when high diversity was
equated with high biological stability.
The prevailing concept was that the
multitude of different organisms, with
their widely differing requirements and
interactions, functioned as a highly
intricate web structure that lessened the
importance of each link to the maintenance
of the total system. There was much
natural redundance built into such
systems. The problem is that at climax
there is one species for which there is no
redundancy - the seagrass. If the
seagrass disappears, the entire associated
community disappears along with it; there
is no other organism that can sustain and
support the system.
The initial colonizers are typically
rhizophytic macroalgae, of which various
species of Halimeda and Penicillus are the
most common, although species of Caulerpa,
Udotea, Rhipocephalus, and Avrainvilla
occur also. These algae have some
sediment binding capability, but their
ability to stabilize the sediments is
minimal and their major function in the
early successional stage seems to be the
contribution of sedimentary particles as
they die and decompose.
Halodule wrightii, the local pioneer
species of seagrasses, colonizes readily
either from seed or rapid vegetative
branching. The carpet laid by Halodule
further stabilizes the sediment surface;
the numerous leaves forming a better
buffer to protect the integrity of the
sediment surface than the algal
communities. In some sequences
Syringodium will appear next, intermixed
with Halodule at one edge of its
distribution and Thalassia at the other.
However, it is the least constant member
of this sequence and is frequently absent.
In areas with consistent disturbance and
sediments low . content
Syringodium may ber:me %Fniist abundant
species. It is commonly found lining
natural channels with hiqh velocity waters
and higher turbidity than Thalaisia can
tolerate.
As successional development proceeds,
Thalassia will begin to colonize the
region. Its strong straplike leaves and
massive rhizome and root system
efficiently trap and retain particles,
increasing the organic matter of the
sediment. The sediment height rises until
the rate of deposition and erosion of
sediment particles is in balance. This is
a function of the intensity of wave
action, current velocity, and leaf
density.
In shallow-water successional sequences
leading to Thalassia, the early stages are
often characterized by low sediment
organic matter and open nutrient supply;
that is, the community relies on nutrients
brought in from adjacent areas by water
movement as opposed to i-n situ-
9
regeneration. With the progression from
rhizophytic algae to Thalassia, there is a
progressive increase in the below ground
biomass of the community as well as the
portion exposed in the water column. With
the progressive increase in leaf area of
the plants, the sediment trapping and
particle retention increases. This
material adds organic matter to further
fuel the sedimentary microbial cycles.
In summary, as species succession occurs
in these shallow marine systems, important
structural changes occur. The most
obvious change with community development
is the increase in leaf area, which
provides an increase in surface area for
the colonization of epiphytic algae and
fauna, with the surface area of the climax
community being many times that of either
the pioneer seagrass, Halodule, or the
initial algal colonizers. In addition to
providing a substrate, the increasing leaf
area also increases the leaf baffling and
sediment trapping effects. Thus, as the
canopy component increases, so does the
material in the sediment. Thalassia, the
climax species, has the highest leaf area,
the highest total biomass, and by far the
greatest amount of material in the
sediments of any of the successional
stages.
10
CHAPTER 2. AUTECOLOGY OF FLORIDA GULF COAST SEAGRASSES
2.1 PLANT MORPHOLOGY AND GROWTH
Seagrasses worldwide show a remakable
similarity in their structure and growth
(den Hartog 1970; Zieman and Wetzel 1980).
For the seagrasses of the northwest coast
of Florida, we shall focus primarily on
the growth and morphology of Thalassia,
considering this as representative of the
local species.
Detailed descriptions of the anatomy and
morphology of Thalassia were presented by
Tomlinson and -966) and Tomlinson
(1969a, 1969b, 1972). Flat, straplike
leaves with rounded tips emerge from erect
short shoots which branch laterally from
horizontal rhizomes at regular intervals.
In this species rhizomes occur from 1 to
25 cm below the sediment surface, but are
typically found in the depth range of
3-10 cm. (The rhizomes of Halodule and
Halo hila are near the surfamften
& While the rhizomes of
Syringodium are generally found at an
intermediate depth, in strong currents,
they may be exposed, even extending up
into the water column.) Roots of
Thalassia emerge from the rhizomes and the
short shoots. Much smaller in cross
section than rhizomes, the roots vary in
length according to sediment composition
and depth.
On a Thalassia short shoot, new leaves
grow on alternating sides of a central
meristem that is enclosed by old leaf
sheaths. New growth on leaves is produced
by the basal meristem, thus the base of a
leaf is the freshest, youngest portion.
Short shoots of this species typically
have two to five leaves at a time.
Studies of
morphology have
temporal and spat
seagrass growth and
revealed patterns of
ial variation. Grassbeds
11
in areas of relatively low productivity in
Biscayne Bay, Florida, averaged 3.3 leaves
per short shoot, while in the more
productive meadows of the Florida Keys,
plants averaged 3.7 leaves per short shoot
(Zieman 1975a). The width of leaves
increased with age of the short shoot,
reaching maximum width five to seven
shoots back from the growing rhizome tip
(Figure 5). Leaf width can also reflect
morphogeographic variation: in Florida,
Durako and Moffler (1981) identified the
effects of a latitudinal stress gradient
in leaves of Thalassia seedlings, with the
greatest widths occurring in the Keys and
the narrowest leaves found in northern
Florida. In another study, leaf widths
did not reflect a latitudinal or stress
gradient, but showed sexual differences:
female short shoots tended to have
narrower leaves than male shoots (Durako
and Moffler 1985a). Transplant
exoeriments found that narrow-leaved
plants of Thalassia, Syringodium, and
Halodule from the north coast of the Gulf
of Mexico continued to produce narrow
leaves, and broader-leaved plants from the
southern gulf and Caribbean likewise
continued to produce wider leaves, even
when moved to different habitats (McMillan
1978).
Thalassia leaves in Biscayne Bay grew an
average of 2.5 mm/day in length, but
growth rates as high as 1 cm/day were
measured over periods of 15-20 days
(Zieman 1975a). Leaf growth rate in
Thalassia usually decreases exponentially
with leaf age (Patriquin 1973; Zieman
1975a). In contrast, leaf elongation in
Syringodium proceeded at a relatively
steady rate throughout the growth phase
(Fry 1983). The first few leaves produced
on a new Thalassia short shoot are reduced
in size and are tapered; the regular
straplike leaves are produced at a rate of
AVERAGE LEAF WIDTH (MM)
8 5 8 7 7.5 7 4
L E A F
BRANCH OR
S H O R T S H O O T - .
DISTANCE BETWEEN BRANCHES (CM)
Figure 5. General morphology of a Thalassia plant.
one-new-leaf-per-short-shoot every 14-16
days. The rate of leaf production in
Biscayne Bay was dependent on temperature,
with low growth occurring in the cooler
winter months (Zieman 1975a). Less
seasonal variation was found in the
tropical Caribbean waters of Barbados and
Jamaica by Patriquin (1973) and Greenway
(1974) respectively. Durako and Moffler
(1981) found a gradient of root and leaf
growth in Thalassia seedlings, from high
rates in the Florida Keys to low growth
rates in north Florida waters.
In Tampa Bay, Durako and Moffler (1985c)
found pronounced seasonal patterns in
maximum leaf lengths of Thalassia. There
was a slight decrease in the middle of
summer, coincident with high temperatures
and floral production, but maximum lengths
were much less in the cold winter months,
reflecting both leaf die-off and depressed
growth rates due to exposure to low
temperatures. A pattern of spatial
variation was evident, with shorter leaves
occurring in the middle of the grassbed
where the water was shallower.
2.2 REPRODUCTION
Vegetative reproduction in seagrasses
accounts for their capacity to produce
high biomass and area1 cover; however,
sexual reproduction is important in
providing the genetic plasticity for
successful adaptation and competition in
the species. Studies of flower production
in the seagrasses considered here have
focused primarily on Thalassia. This
plant is sexually dimorphic, producing
separate male and female flowers. Grey
and Moffler (1978) found that short shoots
occurring on a common rhizome segment
produced flowers of the same sex,
suggesting that Thalassia is also
dioecious, that is, has separate male and
female plants.
12
Flower production in Florida populations
of Thalassia occurs from April to August
or September, peaking in June (Orpurt and
Boral 1964; Grey and Moffler 1978) (Figure
6). While Phillips (1960a) found no
flowering north of Tarpon Springs, more
recent studies have revealed flowering in
the grassbeds of the Florida panhandle
(Marmelstein et al. 1968; Phillips et al.
1981). The percent of short shoots in a
grassbed bearing reproductive structures
varies greatly: less than I_% of plants
from north Florida beds reproduced
sexually, while reproductive densities in
plants from south and central Florida
ranged from l% to 15% (Phillips 1960a;
Orpurt and Boral 1964; Zieman 1975). More
recently Moffler et al. (1981) found
reproductive densities of 44% in Tampa
Bay. A later study in Tampa Bay recorded
reproductive densities of 11.4%, 20.7%,
and 10.0% for 1981, 1982, and 1983,
respectively, and found that increased
numbers of male flowers accounted for the
higher reproductive density of 1982
(Durako and Moffler 1985b). Spatial
density distributions showed higher
numbers of female plants occurred on the
fringes of the bed where short shoots are
generally younger, while more male plants
were found in the center on presumably
older short shoots. This pattern could
reflect an age-related sexual expression
in the plants, although environmental
factors and clonal differences also can
influence leaf width (Durako and Moffler
1985b). (Thalassia seed production in
Tampa Bay was apparently low compared with
south Florida and probably could not
provide an adequate supply for restoration
projects (Lewis and Phillips 1981).
Phillips (1960a) found flowering Ruppia
abundant in Tampa Bay; however, he did not
observe seedling germination. Flower and
fruit production in Ruppia of this area
peak in May and disappear in June (Lewis
et al. 1985a). Phillips (1960a) did not
find reproductive Halodule, Syringodium,
or Halophila in Tampa Bay; however,
several reproductive specimens of Halodule
were later found in nearby waters (Lewis
et al. 1985a). Although reproductive
plants are rare in Syringodium, female
plants have been collected in the bay
(Lewis et al. 1985a). Zimmerman and
Figure 6. Flowers of Thalassia (left) and Syringodium (right) (photo by M. J. Durako.)
13
Livingston (1976b) found a number of
flowering Syringodium plants in their
Apalachee Bay samples in May, 1972. These
authors also found numerous flowering
plants of Ruppia in May and June.
In 1 aboratory studies, cultures of
Thalassia, Syringodium, Halodule, and
Halophila engelmannii flowered under
continuous light, suggesting that
flowering was independent of day length.
The temperature range for flowering in
these plants was 22-26 OC (McMillan 1982).
Lewis et al. (1985a) also found that
flower production in Thalassia was
probably controlled by factors other than
photoperiod.
2.3 PLANT CONSTITUENTS
Because of their high productivity and
wide distribution, seagrasses are
recognized as a potentially important food
source in shallow coastal marine systems.
The fact that this abundant food source is
subjected to relatively low levels of
direct grazing on the living plant
material has prompted studies of the
chemical constituents and relative food
value of seagrasses. Various authors have
performed such constituent analyses for
the seagrasses considered here
(Burkholder et al. 1959; Bauersfeld et al.
1969; Walsh and Grow 1972; Lowe and
Lawrence 1976; Bjorndal 1980; Dawes and
Lawrence 1980; Vicente et al. 1980; Dawes
and Lawrence 1983). A summary of these
results is given in Table 3. Dawes and
Lawrence (1980) noted that the differences
in sample preparation and chemical
analyses employed make direct comparison
of the data difficult, and subsequently
proposed a procedure to standardize
analyses so that future data will be
comparable, making it possible to
determine the effect of seasonal and other
environmental changes on the chemical
content of the plants.
The relative amount of protein in the
plant tissues has been used as a measure
of the potential food value of tropical
seagrasses. Comparative studies have
shown that turtle grass leaves are roughly
equal in percent protein to phytoplankton
and Bermuda grass (Burkholder et al. 1959)
and 2 to 3 times higher than 10 species of
tropical forage grass (Vicente et al.
1980).
Walsh and Grow (1972) found that
Thalassia protein content compared
favorably with reported values for grain
crops: corn contained from 9.8% to 16%
protein, sorghum 8.6% to 16.5%, and wheat
8.3% to 12%. Various studies of the
protein content of Thalassia leaves have
yielded results ranging from a low of 3%
of dry weight for unwashed epiphytized
leaves (Dawes et al. 1979) to a maximum of
29.7% for leaves rinsed with .distilled
water (Walsh and Grow 1972). The low
value for unwashed leaves reflects the
inclusion of sea water salts, and possibly
sediment particles which settle on leaves,
into the total dry weight. Values more
typically range between 10% and 15% of dry
weight.
Dawes and Lawrence (1983) and Durako and
Moffler (1985c) have reported spatial and
temporal variations of protein content.
In Tampa Bay values for Thalassia and
S rin odium varied from 8% to 22% and from
*, respectively, with maximum
values occurrinq in the summer (Table 4).
Thalassia leaves collected in July 1979
from Tamna Bav. Kev West. and Glovers
Reef, Belize,-' showed a significant
increase in protein content from Tampa to
Belize, even though the sites were similar
in depth, salinity, and temperature (Dawes
and Lawrence 1983). If such a latitudinal
trend holds, Thalassia from the Big Bend
area, for which constituent analyses have
not been performed, could have even lower
protein content, and thus lower food
value. Such a decrease in nutritional
value might be reflected in the results of
Kitting et al. (1984), who found that
several seagrass "detritivores" in the
northern gulf actually derived most of
their nutrition from epiphytes.
The new growth of the basal portions of
leaves of Thalassia are higher in protein
and lower in Inorganic content (Cawes and
Lawrence 1980). The green turtle has been
shown to exploit this fact in its pattern
of grazing: a patch of seagrass is
initially cropped, with the upper older
portions of the leaves left to float away,
and such patches are subsequently
maintained for a period of time by
repeated grazing (Bjorndal 1980). Thus,
14
Table 3. Constituent analysis of seagrasses (from Zieman 1982).
Season/ % as Carbo-
Species Component date Referenced Ash
Energy
Nitrogen Protein Fat hydrates (kcal/g) Reference
Thalassia Leaves February %DW 24.8 2.1 (13.1) 0.5 35.6 1.99 Burkholder
et al. 1959
Annual %AFDW 1.6-4.8 25.7
mean %DW 24.5 (10.3-29.7)
January
April
July
October
Mean
%DW 29
37
33
44
x
8 0.9
9 4.0
22 1.0
23.6 4.66 Walsh and
Grow 1972
?
45
50
44
2.4 Dawes and
3.0 Lawrence
3.1 1980
41 2.6
E 2.8
13
13
22 0
2.0
%DW
(unwashed)
%DW
(washed)
47.3 11.0 0.7
13.0 0.5
38
24.8 35.6
%DW 24.7 9.1 2.3 63.9
Bauersfeld
et al. 1969
July-
August
Lowe and
Lawrence
1976
January
August
%DW 16.7 Bjorndal
1980
17 Vicente
et al.
1978
(Continued)
Table 3. (Continued).
Species Component
Season/ % as Carbo- Energy
date Referenced Ash Protein Fat hydrates (kcal/g) Reference
Thalassia Rhizomes
Roots
Photosynthesis
inactive part
of short shoot
Rhizomes
Syringodium Leaves
Leaves
Short shoots
photosynthesis
inactive
parts
Annual
mean
January
April
July
October
Mean
January
April
July
October
Mean
July-
August
January
April
July
October
Mean
January
April
July
October
Mean
%DW
%AFDW
%DW
23.8
50.5
24.1
5.8-12.2
11.0
19.6
15.0
72.1
%DW 39
51
48
56
7E
9 1.0
7 0.5
16 0.7
8 0.8
lo 0.8
51 2.7
42 2.2
35 2.5
35 2.0
a 2.4
%DW 26
24
33
36
m
9 0.5 65 3.2
8 1.6 66 3.4
16 0.2 51 3.0
7 1 1
i-0
A
0.9
56 2.8
m 3.1
%DW 27.0 3.10 3.4 66.3
%DW 30
28
33
32
x
9 1.7 59
8 6.2 58
13 4.0 50
13
ii
1.8
3.4
53
!Z
%DW 28 10 1.3 61
27 11 3.6 58
31 14 0.9 54
41 11 1.1 47
Z? Tz 1.7 !Z
4.88 Walsh and Grow 1972
Bauersfeld et al.
1969
Dawes and Lawrence
1980
Lowe and Lawrence
1976
3.1 Dawes and Lawrence
2.4 1980
3.2
3.1
3.0
3.2
3.3
3.1
2.6
3.1
(Continued)
Table 3. (Concluded).
Species Component
Season/ % as Carbo-date
Referenced Ash Protein
Energy
Fat hydrates (kcal/g) Reference
January %DW 16 9
April 18 5
July 17 12
October 19 6
Mean 18 8
1.0 74 3.6
4.7 72 3.7
0.1 71 3.6
0.5
1.6
75 3.5
73 3.6
Syringodium Rhizomes
January %DW 32
April 25
July 25
October 26
Mean T
January
April
July
October
Mean
%DW 25
29
36
34
Ti
January
April
July
October
Mean
%DW 14
17 8
i-G 8
19 1.0 48
18 3.2 54
19 1.2 55
Halodule Leaves 3.1 Dawes and Lawrence
3.5 1980
3.3
3.3
3.3
14
T;s
1.4
1.7
59
55
Short shoots
photosynthesis
inactive part
59
898
1.1
3.5
0.8
A1 2
1.7
69
59
55
56
60
3.2
3.0
2.9
2.9
3.0
Rhizomes 0.7
1.6
0.1
;1 1
0.9
76
74
70
74
T-4
3.7
3.7
3.4
3.6
3.6
Table 4. Seasonal content of protein and soluble carbohydrates (% dry weight) in Tampa Bay (after
Dawes 1987).
Species Component January April July October
Thalassia testudinum
Leaves Protein 8
Carbohydrate 6
99
22
9
13
7
Protein 9 8 16 7
Carbohydrate 12 21 24 36
Rhizomes
Syringodium filiforme
Leaves 13
20
Protein 9 8 13
Carbohydrate 22 16 18
Protein 9 5 12 16
Carbohydrate 36 38 50 46
Rhizomes
Halodule wrightii
Leaves Protein 19 18
Carbohydrate 14 19
19
15
14
13
Protein 9 7 8 8
Carbohydrate 43 40 43 54
Rhizomes
higher levels of soluble carbohydrates in
Thalassia rhizomes compared to the leaves.
In Tampa Bay, seasonal variation in
soluble carbohydrate content occurred in
the rhizomes of both Thalassia and
Syringodium, reflecting production and
storage of starch during summer and fall
(Dawes and Lawrence 1980). Mean values
for the lipid content of Thalassia leaves
varied from 1.2% to 4.2%, and were
comparable to the "fat" content of
tropical terrestrial grasses.
the turtles create a more energetically
and nutritionally rich food source, and,
indeed, Zieman et al. (1984) found that
the nitrogen content of leaves within
turtle patches was similar to the content
of the basal portions of ungrazed leaves
and higher than the upper portions of
those leaves.
The values reported for ash content of
Thalassia leaves range from 45% dry weight
for unwashed samples to a low of about 25%
for samples rinsed in fresh water.
Samples washed in ambient seawater
contained 29%-44% ash (Dawes and Lawrence
1980). Thalassia rhizomes from the west
coast of Florida had ash content
significantly lower than did the leaves,
with mean values ranging from 2l% to 26%
dry weight (Dawes and Lawrence 1983).
Cell wall carbohydrates (cellulose,
hemicellulose, and lignin) accounted for
45%-60% of the dry weight of turtle grass
leaves (Bjorndal 1980; Vicente et al.
1980). Dawes and Lawrence (1983) found
2.4 PHYSIOLOGICAL ECOLOGY
2.4.1 Environmental Tolerances
and Responses
a. Temperature. The range of thermal
tolerance in tropical organisms is
often about half that of their
temperate counterparts. Although
the upper temperature limits are
similar, the tropical organisms have
18
reduced cold tolerance. McMillan
(1979) found a gradient of chill
tolerance in Florida seagrasses,
with those from northern Florida
most tolerant of low temperatures
and those of the Florida Keys least
tolerant. After growing in culture
for 22 months, Thalassia seedlings
maintained their original pattern of
chill tolerance: those from
Apalachee Bay, Florida, showed less
damage than seedlings from the
Florida Keys and St. Croix.
In the thermally impacted waters
of Anclote Estuary north of Tampa
Bay, Barber and Behrens (1985) found
that maximal growth in Syringodium
occurred between 23 and 29 OC, and
between 23 and 31 OC in Thalassia
(Figure 7). In the cooler months,
thermally impacted stations had
higher productivities than did
non-impacted areas, but in the
summer months, Syringodium
productivity was depressed at the
warmer stations when the upper
thermal tolerance limit of this
seagrass was exceeded. In Apalachee
Bay, Zimmerman and Livingston
(1976b) found that Syringodium
tolerated lower temperatures than
Thalassia, which suffered leaf kill
when temperatures fell below 15 OC.
Some defoliation of Thalassia also
occurred when summer temperatures
rose to 30 OC.
Thalassia
IO 15 20 25 30
TEMPERATURE (“C)
Figure 7. Temperature responses of Thalassia
Behrens 1985).
In Texas waters, vigorous growth
of Ruppia in the spring correlated
with cool temperatures rather than
lowered salinities (Pulich 1985).
Phillips (1960a) reported a
temperature range of 7-35 OC for
Ruppia in Tampa Bay; growth and
reproduction was highest with spring
temperatures, and decreased when
high summer temperatures were
reached. In Texas, a similar pattern
of temperature response was evident
for Ruppia in a mixed stand with
Halodule; in contrast, Halodule
biomass at that site peaked in the
warmer summer months and declined in
the fall. Phillips (1960a) reported
that Halodule in Tampa Bay suffered
winter leaf kill, even at sites
which were always submerged.
However, Halodule suffered little
leaf kill in Apalachee Bay, where
the minimum winter temperature was 9
OC. This temperature was a new
minimum reDorted for HaloDhila
en elmanni (Zimmerman and Livingston i!kijT
b. Salinity. Although the seagrasses
considered here are able to tolerate
fluctuations in salinity, the
optimum concentration for growth
varies among the species.
Experiments with transplanted
seagrasses showed that, of the
species considered here, Halodule
had the broadest salinity tolerance,
4.0
3.0
2.0
1.0
Syrinqodium
0
1 1 1 I
10 15 20 25 30 35
TEMPERATURE (“C)
and Syringodium on the west Florida coast (after Barber and
19
/ -
Thalassia and Syringodium
(==E;c$;;ea)t;eere intermediate, and
most stenohaline.
Ruppia sho;;im a wide tolerancz:
ranging freshwater
hypersaline conditions (McMillan
1974). Evaluating the upper limits
of salinity tolerance, McMillan and
Mosely (1967) found that Halodule
(=Diplanthera) tolerated the highest
salinities, surviving up to 80 ppt,
followed, in order, by Thalassia,
Ruppia, and Syringodium, with
Halophila's relative tolerance
aoparentlv somewhere between
Halodule -and Syringodium McMillan
salinities was Ruppia, followed by
Halodule, Thalassla, Syringodium
(=Cymodocea), and Halophila. While
both Ruppia and Halodule exhibit
broad ranges of salinity tolerance,
the former is the only seagrass
species, of this region capable of
surviving in freshwater. According
to McMahan (1968), Halodule does not
survive in salinities less than 3.5
ppt and has an optimum salinity of
44 ppt.
Thalassia and Syringodium do not
grow in areas of low salinity in the
eastern Gulf of Mexico (Phillips
1960a), and were not reported in
areas with salinities less than
about 17 ppt in the northern
seagrass bed (Zimmerman and
Livingston 1976). Turtle grass can
survive short periods of exposure to
extremes ranging from a low of 3.5
ppt (Sculthorpe 1967) to a maximum
of 60 ppt (McMillan and Mosely
1967); however, significant leaf
loss frequently follows exposure to
salinity extremes. The optimum
salinity reported for this seagrass
ranges 24-35 ppt (Phillips 1960;
McMillan and Mosely 1967; Zieman
1975a). In turtle grass, maximum
photosynthetic activity occurred in
fullstrength seawater, and decreased
linearly with decreasing salinity
(Hammer 1968b). The effect of
freshwater runoff following a
hurricane was considered more
damaging to seagrasses than the
effects of high winds and tidal
surge (Thomas et al. 1961).
According to Humm (I973),
Halophila does not tolerate reduced
salinities; however, Zimmerman and
Livingston (1976b) found a bed of y.
engelmanni off the mouth of the
Econfina River, an area of
relatively low salinity.
addition, Earle (1972) report::
Halophila occurring at depths
ranging from intertidal to 13 m, and
Strawn (1961) found this species
mixed with Halodule and Ruppia on an
old oyster bar. Thus, it appears
that tialophila may in fact be quite
euryhaline.
Ruppia traditionally has been
considered a brackish-water species
(Verhoeven 1975) and, indeed, among
the seagrasses it alone can be
maintained in tap water (McMillan
1974). Phillips (1960a) reported
that it occurred most frequently in
salinities below 25 ppt, which
correspond with the findings of
Zimmerman and Livingston (1976b).
In the Big Bend seagrass bed, Ruppia
was observed growing near river
mouths (R.L. Iverson, Forida State
University, Tallahassee; pers.
comm.) However, Dawes (1974) found
it growing in areas of relatively
high but stable salinity in the
lower portions of Tampa Bay.
transplants
Ruppia
survived in salinities
up to 74 ppt (McMillan and Mosely
1967); this species also grew in
Texas waters at a site where
salinities averaged 25-32 ppt and at
hypersaline
for several
another site where
conditions persisted
months (Pulich 1985).
has been observed
hypersaline waters in
(J. Fourqurean, Un
aRupplia s o
growing in
Florida Bay
iversity of
Virginia, Charlottesville; pers.
comm.). Thus Ruppia also appears to
be quite euryhaline.
The oxygen contained in the
column of seagrass beds
generally provides a supply adequate
to meet the respiratory demands of
the plants themselves and associated
organisms. In Thalassia beds,
20
photosynthetic oxygen production can
be so high that bubbles escape from
the leaf margins in the late
afternoon. The seagrasses are less
susceptible to low oxygen
concentrations than the animals of
the grassbeds; nevertheless, leaf
mortality and increased microbial
activity coincided with lowered
oxygen levels in Japanese Zostera
beds (Kikuchi 1980). Low 0s levels
do slow their rate of respiration,
and when internal O2 concentrations
are lowered, the plant's rate of
resgiration is controlled by
diffusion of oxygen from the water
column. During the night, the
respiratory demand of the seagrasses
and associated plant and animal
communities can lower concentrations
of the surrounding waters (Durako et
al. 1982). In Puerto Rico (Odum et
al. 1960) and in Florida and Texas
(Odum and Wilson 1962) nighttime
oxygen concentrations were typically
4-7 mg 0s L-l, and a low of 2-3 mg
02 L-l recorded on a calm night in
August during an extreme low tide.
d. TLhigeh t.fact that well-developed
seaarass beds do not occur at depths
greater than 10 m has been
considered indirect evidence that
photosynthesis in seagrasses
requires high light intensity, and
that light penetration limits the
depth to which seagrasses can grow
(Humm 1956; Buesa 1975; Wiginton and
McMillan 1979). Gessner and Hammer
(1961) suggested that increased
hydrostatic pressure, as well as
decreased light, may limit
photosynthesis suggesting that light
is probably not the sole factor
restricting photosynthesis at depth;
however, there were no significant
pressure effects on photosynthetic
rates of Thalassia and Syringodium
for plants collected from various
depths near Buck Island, St. Croix,
and subjected to 1 and 3 atmospheres
of pressure (R.L. Iverson,
Department of Oceanography, Florida
State University; unpubl. data). It
therefore unlikely that the
i:essures that exist over the depth
gradients where these seagrasses are
found can explain the significant
decrease in Thalassia biomass at
the limit of its depth distribution.
However, the maximum depth at which
seagrasses occur does indeed
correlate with the available light
regime. Buesa (1975) reported the
following depth maxima for the
seaarasses off the northwest coast
of _ Cuba:
Syringodium,
decipiens 24
englemann;, 14.
Thalassia
16.5 m;
.3 m; and
4 m.
14 m;
Halophila
Halophila
Of the visible light spectrum, the
longer red wavelengths are absorbed
in the first few meters in both
clear and turbid waters. The clear
tropical waters of the Caribbean Sea
are enriched in blue light, while in
turbid shallow waters, such as parts
of Florida Bay and coastal waters of
Texas, enrichment of green
wavelength occurs. In a study of
the effects of specific wavelengths
of light on seagrass photosynthesis,
Buesa (1975) found that Thalassia
responded best to red light-)
and S rin odium grew best with blue
wave-0 nm). Wiginton and
McMillan (1979) reported increasing
chlorophyll a to chlorophyll b
ratios for seagrasses obtained from
increasing depths near Buck Island,
St. Croix, but concluded that light
quantity rather than light quality
was the primary environmental
determinant of seagrass depth
distribution along the Buck Island
gradient. Thalassia growing in
outer Florida Bay has considerably
more non-photosynthetic tissue than
compensation light-energy level
(below which annual net increase of
total plant biomass cannot occur)
for Thalassia growing in tropical
habitats is less than the
compensation light energy level for
Syringodium and for Halodule as a
conseauence of the
demands
respiratory
created by the 'greatei
orooortion of nonohotosvnthetic
tissue mass of Thalassia <n those
habitats.
Humm (1973) observed that Ruppia
occurred in areas of low light and
21
high turbidity. Phillips (1960a)
also noted the growth of this
species in areas of poor light
penetration.
in addition to!%$!$%ng~~
light of deeper waters also grows in
areas of high turbidity (Zimmerman
and Livingston 1976b).
e. Current. Seagrass biomass and
production are greatly influenced by
current velocity (Conover 1968).
The maximum standing crops for both
Thalassia and Zostera were found
where current velocities averaged
0.5 m set-l. Rapid currents are
thought to disrupt diffusion
gradients and increase the
availability of CO2 and nutrients to
the plants (Conover 1968). In south
Florida, the densest stands of
Thalassia and Syringodium are found
in the tidal channels separating
mangrove islands. Off the coast of
Nicaragua, samples from mangrove
tidal channels had a leaf standing
crop of 262 g dry weight m-2 and
total biomass of 4,570 g m-2. By
comparison, values for samples from
a quiescent lagoon were 185 and
1,033 g m-2 respectively (McRoy,
Zieman, and Ogden, unpubl. data).
Strong currents can affect the
structure of seagrass beds. In some
areas of high current, lunate
features called blowouts occur
(Patriquin 1975). These
crescent-shaped erosional features
migrate through the bed in the
direction of the current.
Recolonization takes place at the
trailing edge of the blowout, and
here the successional sequence of
seagrass colonization can be
observed.
f. Sediment. Seagrasses are found in a
variety of substrates, ranging in
texture from fine muds to coarse
sands. Because they are rooted
plants, they do have minimum
sediment-depth requirements, which
differ among the species.
Halodule's shallow surficial root
system allows it to colonize thin
sediments in areas of minimal
hydraulic stability (Fonseca et al.
1981). Thalassia is more robust,
reauirina up to 50 cm of sediment
for lush-growth, although it occurs
shallower
G72).
sediments (Zieman
In the Bahamas, Thalassia
did not occur in sediments less than
7 cm deep (Scoffin 1970). Phillips
(1962) reported that seagrasses in
Tampa Bay grew only in muddy sand
substrates, and patches of pure sand
were unvegetated.
Reduced sediments seem always to
be associated with well-developed
Thalassia beds and most likely
reflect the greater importance of
sediment-nutrient content and
microbial nutrient recycling in
meadows of this species, rather than
a specific requirement for reducing
conditions. Halodule is generally
thought to grow in more aerobic
substrates; however, Pulfch (1985),
working in Texas waters, postulated
that Ruppia normally occurs in
low-nutrient sediments while
Halodule prefers organic-rich
sediments where sulfate reduction is
substantial. Phillips (1960a)
reported that Syringodium
distribution was apparently
independent of sediment type and
this species is found in both
oxidized and reduced sediments
(Patriquin and Knowles 1972).
Ruppia is generally found in finer
substrates than the above species
(Phillips 1960a), while Halophila
grows in a wide range of substrates
from muddy sands to limestone, and
even on mangrove roots (Earle 1972).
In the Big Bend area of the west
coast of Florida, Iverson and
Bittaker (1986) also recorded
Thalassia growing in coarser
sediments than the other species of
that northern grassbed. Syringodium
and Halodule biomass were greater in
fine sediments (Iverson and Bittaker
1986). Similarly, Buesa (1975)
found that Thalassia off the
northwest coast of Cuba grew in
coarser sediments than Syringodium
or Halophila.
g. Exposure. Thalassia and Syringodium
are subtidal plants and do not
22
tolerate exposure to the air. While
Thalassia does grow on flats that
are infrequently exposed, unless
such exposure is brief, desiccation
will cause leaf kill. Halodule can
withstand repeated exposure at low
tide, and is most abundant between
neap-low and spring-low tide marks
higher
iG6Oa).
salinities (Phillips
In low-salinity intertidal
areas, Ruppia and Halodule often
occur in mixed stands (Phillips
1962; Earle 1972). Dawes (1987)
noted that Ruppia forms extensive
meadows on flats where it can be
exposed to intense sun and appears
to tolerate a degree of desiccation.
2.4.2 Photosynthetic Carbon Fixation
Three separate biochemical pathways by
which plants can fix inorganic carbon
photosynthetically have been identified.
The majority of terrestrial plants
utilize the Cs pathway, in which CO2 is
initially incorporated into a three-carbon
product. In the C, pathway, found
primarily in plants from tropical and
arid areas, a four-carbon product results
from the first step of CO;! incorpora-tion.
The third pathway, CAM (crassula-cean
acid metabolism), by which plants
take up CO:! at night and store it as
malic acid until daytime when it is then
used in photosynthesis, occurs in water-stressed
plants such as desert succu-lents.
A major factor in the differences
of photosynthetic physiology between Cs
and C4 is the greater efficiency of
refixation of photorespired CO2 found in
the C, plants (Hough 1974; Moffler et al.
1981). High rates of refixation have
been detected, however, in some Cs plants
with specialized leaf anatomy and gas
lacunae (Sondergaard and Wetzel 1980) and
may be implicated in seagrass carbon
metabolism (Beer and Wetzel 1982). Sea-grass
leaves possess large internal
lacunar spaces which facilitate gas
transport (Zieman and Wetzel 1980). The
presence of these lacunae and the absence
of stomata provide the plants with a
relatively closed pool of carbon dioxide,
thus promoting recycling of CO*.
Seagrasses share with C4 plants such
physiological adaptations as high thermal
and light optima for photosynthesis and
high productivity rates. Although
Thalassia was originally thought to be a
C, plant, Beer and Wetzel (1982), using
radiolabel led HCOm3, concluded that both
this seagrass and Halodule were
intermediaries between Cs and C4 in their
carbon metabolism. Syringodium and
Zostera exhibited the most typically Cs
pattern of the seagrasses studied.
2.4.3 Isotopic Fractionation
A significant result of the differences
in carbon metabolic pathways is that
imprints are left in the form of
characteristic ratios of the stable
isotopes of carbon in the plant tissues
produced. In biochemical reactions,
plants do not utilize 12C and 13C in the
exact ratios found in the environment,
but discriminate between the two,
favoring the lighter isotope. Plants
using the C3 pathway are relatively
depleted in 13C, while C4 plants have
higher ratios of l3C to l2C. The
relative content of 1% to 12C is
compared to the isotopic ratio of a
standard and expressed as a "del" value
(6) as follows:
13C/l2C
(513c, sample lx103
13c/12c standard
The range of 613C values for Cs plants
is -24 to -34 ppt, while C4 plants vary
from -6 to -9 ppt (Smith and Epstein
1971). Seagrass values, particularly
those of Thalassia and Syringodium, are
similar to those of the C4 plants.
McMillan et al. (1980) reported that 45
of 47 species examined fell within the
range of -3 to -19 ppt, with only two
species of Halophila having lower values.
Samples of Thalassia from the Gulf of
Mexico and the Caribbean range from -8.3
to -12.5 ppt, with a mean of -10.4 ppt.
Halophila had similar isotopic
composition, with means of -10.2 ppt for
gulf and -12.6 ppt for Caribbean samples.
Syringodium, by comparison, had
relatively fewer negative numbers, with a
mean of -5 ppt and a range of -3 to -9.5
PPt. This species has a greater
proportion of lacunar spaces, and the
lacunae are more completely partitioned
than those of the other seagrasses
considered. This greater lacunar
23
isolation presumably enhances the
recycling of C02, which occurs in C,
plants! and thus the similarity in
isotopic composition is not unexpected.
Tropical seagrasses in general have
values less neaative than those of
temperate specie;. A study of Zostera
showed little seasonal variation in
isotopic composition (Thayer et al.
1978). The isotopic composition can
vary, however, with habitat (Smith et al.
1976; Zieman et al. 1984c). McMillan and
Smith (1982) found that seagrasses grown
in laboratory cultures had more negative
values, that is, were more depleted in
the heavier 13C than samples from the
natural environment. They concluded that
such results could reflect differences in
carbon sources and in recycling of
internal carbon.
Since plants have characteristic
isotopic compositions, and the animals
that consume them retain to within t2 ppt
the same ratio (DeNiro and Epstein 1978;
Fry et al 1978), these isotope
"signatures" provide a useful tracer for
food chain studies (Figure 8). In the
marine environment, seagrasses have
isotopic ratios distinct from other
marine plants. Thus carbon derived from
seagrasses (-3 to -15 ppt) is
distingished from that of marine
macroalgae (-12 to -20 ppt), particulate
organic carbon and phytoplankton (-18 to
-25 ppt) and mangroves (-24 to -27 ppt)
(Fry and Parker 1979). In Texas,
sediment organic matter within a seagrass
bed was more depleted in 13C compared to
sediment organic matter from adjacent
bays without seagrasses (Fry et al.
1977), and the same pattern was reflected
in the animals (Fry 1981). The 6 13C of
the polychaete worm Diapatra cuprea
varied from an average of -13 ppt in
seagrass-dominated areas to -18 ppt where
phytoplankton were the dominant primary
producers (Fry and Parker 1979). Similar
trends were observed for fish and shrimp.
The utility of
method of food
restricted at the
cost of analytical
limitations of data
this carbon isotope
chain analysis is
present by the high
equipment and by the
interpretatio_n^. When
a consumer organism has a 6 l"C value
which falls within a range specific for a
particular plant source, the relationship
is readily apparent; however, if the
animal has a de1 value falling between
two identifiable pl,ant groups, it is
unclear whether this represents a food
source which itself has a value
intermediate between the two known groups
or whether the organism is consuming a
mixed diet.
2.5 NUTRIENTUPTAKEANDSUPPLY
Seagrasses are highly productive plants
that can grow in low-nutrient
environments; thus, the manner in which
plant nutrient demands are met is of
particular interest. Since seagrasses
occupy both the water column and the
CARBON ISOTOPE ANALYSIS OF PRODUCERS AND CONSUMERS
6 13c
ROOKERY BAY ---- T H PA PP ELMM
PINE CHANNEL ---- S P P T H L A MM
! 11 ll! lrrl!llrl!ll rl!llll!
-5 -10 -15 -20 -25 -30
Legend: T = Thalassia P = Pink Shrimp E = Epiphytes M = Mangroves L = Litter A = Macroalgae
H = Halodule S = Syringodium
Figure 8. Carbon isotopic variation at two locations in Florida (after Zieman et al. 1984c).
24
sediments, controversy existed in the
past over whether nutrients were taken in
throuah the leaves or the roots. The
tempeiate seagrass Zostera is capable of
takina in nutrients both from the water
columi and through the roots (McRoy and
Barsdate 1970); however, uptake through
the root system was shown to be faster
and more efficient (Penhale and Thayer
1980). McRoy and Barsdate (1970) found
that Zostera could translocate ammonium
and phosphate from the sediments to the
leaves and excrete these nutrients into
surrounding waters. Such nutrient
pumping may be important only in
sediments with high nutrient
concentrations (Penhale and Thayer 1980).
Studies of nutrient supply to
seagrasses have concentrated on nitrogen
and phosphorus because these, along with
carbon, are the primary constituents of
plant material. In Zostera beds in
Chesapeake Bay, the addition of
commercial fertilizer containing both
nitrogen and phosphorus stimulated leaf
growth (Orth 1977a). Harlin and
Thorne-Miller (1981) observed similar
growth enhancement when inorganic
ammonium and phosphate were added to
waters overlying Zostera beds in a Rhode
Island bay. The relative importance of
these major nutrients in limiting plant
growth has not been determined and
probably depends on local nutrient
supplies and processes.
Three sources of nitrogen are available
to the plants: microbially recycled
nitrogen from organic matter in the
sediment, dissolved ammonium and nitrate
in the water column, and ammonium from
the microbial fixation of dissolved NZ.
Sources of organic matter for
decomposition in the sediment include
animal excretions and dead roots and
rhizomes, Sediment bacteria convert the
organic nitrogen to ammonia in the anoxic
zone, which begins only a few millimeters
below the surface. Ammonia that is not
quickly bound either by biological uptake
or chemical adsorption by sediment
particles can diffuse upward to the
aerobic zone, where it can then diffuse
into overlying waters or be converted to
nitrate by nitrifying bacteria. Nitrate
concentrations are low in the sediments;
nitrate is either rapidly assimilated or
converted to N2 by denitrifying bacteria.
Patriquin (1972) and Capone and Taylor
(1980) identified the recycled organic
material as the primary source of
nitrogen for leaf growth; however,
nitrogen fixed by sediment microbes
could supply 20% to 50% of the plants'
requirements (Capone and Taylor 1980).
In contrast, Capone et al. (1979) found
that fixation by phyllosphere microbes
contributed primarily to epiphyte growth.
The relative importance of the different
nitrogen pools to the plants is indicated
by such factors as sediment
characteristics and water column
concentrations.
Inorganic phosphorus, unlike nitrogen,
has no gaseous phase and does not change
valence state in normal environmental
reactions. Thus the source of phosphorus
to the seagrasses is dissolved inorganic
orthophosphate (PO,), derived either from
the breakdown of organic matter or from
the weathering of minerals, some of which
are biologically precipitated. While
water-column concentrations in tropical
waters are normally low, phosphate may be
quite abundant in the sediments. High
levels of HCl-extractable phosphate were
found in the carbonate sediments of
seagrass beds of Barbados, but pore-water
concentrations and concentrations in
overlying waters were low (Patriquin
1972). Because the high sediment
concentrations probably reflected
undissolved phosphate not available for
uptake by the plants, Patriquin concluded
that the nutrient-poor overlying waters
were the primary source of phosphate to
the seagrasses. Sediment type influences
the dissolution of phosphate, and,
therefore, its availability to the
plants. Silicious sediments readily
exchange phosphate with overlying waters
(Nixon et al. 1980), but carbonate
sediments tend to absorb phosphate,
removing it from solution. Rosenfeld
(1979) reported that pore-water phosphate
concentrations of Florida Bay sediments
were two orders of magnitude lower than
concentrations in Long Island Sound pore
waters and attributed the difference to
calcium carbonate adsorption of
phosphate.
Terrestrial runoff also can be an
important factor affecting the
25
concentration of dissolved nutrients. In limiting plant growth can be expected to
Apalachicola Bay, nutrient concentration vary accordingly. At this time, the
peaks coincided with periods of maximum degree to which phosphorous and nitrogen
river discharge (Myers and Iverson 1981). are limiting the growth of Florida's
The bays and estuaries of the northwest seagrasses is still unknown, and is a
coast of Florida vary widely in sediment timely and important topic for further
composition and terrestrial input; thus, research.
the supply of phosphate and its role in
26
CHAPTER 3. DISTRIBUTION, BIOMASS, AND PRODUCTIVITY
3.1 DISTRIBUTION
Distribution of seagrasses along the
west coast of Florida is unique in that
the plants not only occur in protected
estuarine grassbeds typically found along
the Gulf of Mexico coast (represented in
this area by the grassbeds in embayments
such as Rookery Bay, Charlotte Harbor,
Tampa Bay, and St. Joseph Bay), but also
form an extensive offshore bed located
along the coastal reach between the St.
Marks River and Tampa Bay, known as the
Big Bend area.
have been studied most extensively (Thorne
1954; Phillips 1962; Taylor and Saloman
1968; Lewis and Phillips 1980; Lewis et
al. 1985a). While this estuary has
received intense human impact and cannot
be considered necessarily typical or
representative of west Florida bays, the
abundance of information on Tampa Bay
seagrasses provides a useful base for
comparison with other areas.
Seagrass distribution in the eastern
Gulf of Mexico has been investigated at
several different levels of spatial
resolution. Humm (1956) reported
seagrasses observed at specific sites
along the northern coast of the Gulf of
Mexico. Phillips (1960a) described the
general location of seagrasses around the
Gulf of Mexico based on literature reports
and on field surveys. Bauersfeld et al.
(1969) and Earle (1972) estimated area1
seagrass distribution in the eastern Gulf
of Mexico using indirect methods. McNulty
et al. (1972) reported seagrass
distribution within embayments and
estuaries in the eastern Gulf of Mexico
based on field observations and on
analysis of aerial photography. While the
seagrass distribution within embayments
adjacent to the northeastern Gulf of
Mexico has been reasonably well described,
the spatial extent and biomass of
seagrasses of the Big Bend area have been
only recently investigated (Continental
Shelf Associates 1985; Iverson and
Bittaker 1986).
Thorne (1954) identified five seagrasses
occurring in the bay: Thalassia, Syringo-dium,
Halodule, Ruppia maritima and Halo-pehnilgae
l m a n n i i . In his survey of sea-grasses
of Tampa Bay, Phillips (1962)
reported the presence of all species but
Halophila; however, this seagrass was
later observed in the bay by Lewis and
Phillips (1981) and Moffler and Durako
(reported in Lewis et al. 1985a).
Phillips (1962) noted that the southern
part of the bay was dominated by Diplan-athera
(Halodule) and in the northern
part Ruppia was more abundant, presumably
due to a salinity gradient. Thalassia is
relatively sparse in Tampa Bay, possibly
because of low salinities (Phillips 1962),
but is the dominant species in the adja-cent
waters of Boca Ciega Bay (Pomeroy
1960; Phillips 1962; Taylor and Saloman
1968). Lewis et al. (1985a) estimated
that the current distribution of sea-grasses
in the bay, covering 5,750 ha
(14,203 acres) represents a reduction of
81% of historical coverage prior to human
impact (Figure 9).
3.1.1 Seagrass Distribution in Tampa Bay
a. Seagrass associations. There are
five types of seaqrass meadows found
in Tampa Bay (Figure 10). Mid-bay
shoal perennial beds contain Tha-lassia,
Syringodium, and Halodule,
but rarelv RuoDia. due to either the
Among the estuarine grassbeds of the
west coast of Florida, those of Tampa Bay
fast currents or increased salini-ties
found on the shoals where these
27
10 mi
15 km
Figure 9. Seagrass coverage in Tampa Bay in 1879 and 1982 (after Lewis et al. 1985).
beds grow. Healthy fringe perennial
beds contain all five species found
in the bay. In these beds, Ruppia
is found in the shallowest water
close to shore, followed by almost
pure stands of Halodule, Thalassia,
and Syringodium, respectively, as
depth increases. These meadows
generally have an unvegetated sand
bar separating the seagrasses from
the main body of the bay. Stressed-fringe
perennial beds are similar to
their healthy counterparts except
coverage is reduced, and migration
of a destabilized sand bar
eventually leads to the dis-appearance
of the bed. These beds
occur in areas of the bay where
phytoplankton are abundant, possibly
competing with the benthic
macrophytes. Finally, colonizing
perennial grassbeds are found in
bands in the euphotic zone of man-made
fill areas. The dominant soe-ties
here are Halodule and Syringo-dium,
presumably because their rhi-zomes
are more readily fragmented
and dispersed to unvegetated areas.
b. Sediment effects. According to
Thorne (1954) seagrass distribution
in the Gulf of Mexico was limited to
soft marl, mud, or sand substrates.
Phillips (1962) found that all sea-grasses
in Tampa Bay grew in muddy
sand: while sandy substrates
remained unvegetated. The sediments
of the bay contain varying amounts
of carbonates, which may be impor-tant
in determining the availability
of essential nutrients.
C. Depth distribution. Phillips (1962)
reported that seagrass growth was
limited to depths of less than one
fathom (2 m) in the turbid waters of
Tampa Bay. Syringodium dominates
below the spring low-tide mark, and in
deeper water frequently occurs in
mixed stands with Thalassia (Humm
1956; Phillips 1960a; Phillips 1962).
Shallow areas are dominated by Ruppia
28
MBS(P)
HF(P)
S F (PI
Figure 10. Seagrass meadow types in Tampa Bay.
MBS(P) = mid-bay shoal perennial; HF(P) = healthy
fringe perennial; SF(P) q stressed fringe perennial; E
q ephemeral; C(P) = colonizing perennial (after Lewis
et al. 1985).
and Halodule (Phillips 1962). Three
mOrDhOl oaicallv distinct forms of
Halbdule-in the bay were identified
according to depth distribution
Dwarfed plants occurred in areas
exposed at neap and spring low tides,
while subtidal plants were more robust
(Phillips 1960d). Salinity rather
than tidal exposure was thought to
control the distribution of Ruppia in
the bay (Phillips 1962).
3.1.2 Seagrass Distribution in
the Big Bend Area
The Big Bend seagrass bed overlies
drowned karst topography which extends
from the town of St. Marks south to Tarpon
Springs. The sediments of this low energy
region are composed of clay and silicious
sand over limestone. Results of recent
investigations suggest that seagrasses are
the dominant benthic feature of the
nearshore environment from St. Marks to
Tampa (Iverson and Bittaker 1986;
Continental Shelf Associates 1985)
Analysis of a photographic composite
obtained from aerial photography
(Continental Shelf Associates 1985)
revealed some broad-scale patterns in
seagrass distribution with beds of
greatest density in shallow water well
removed from river mouths. Beds of lesser
density extended as far as 112 km offshore
(Figure 11).
Samples for characterization of seagrass
distribution in eastern Gulf of Mexico
coastal waters were taken by Iverson and
Bittaker (1986) from St. Marks to Tampa
during the month of October for several
years. Visual observations of the
presence or absence of different seagrass
species were made at each of about 300
stations in the Big Bend area of north
Florida (Figure 12). Samples for
estimation of seasonal seagrass biomass
changes were collected within a 10 m
radius of a metal marker stake located in
1 m water depth at stations near the
Florida State University Marine Laboratory
at Turkey Point, and in St. Joseph Bay.
The line marking the outer limit of the
seagrass beds in Figure 12 indicates the
maximum depth to which seagrass coverage
of about 80% or more of the bottom
extended within each major area.
Vegetation covered about 3,000 km*, with
seagrasses occurring as a band varying
from 11 to 35 km wide between St. Marks
and Tarpon Springs, Florida.
All six species of seagrasses presented
in Chapter 1 were found in the Big Bend
grassbeds. Halodule occasionally formed
both the innermost and the outermost
monospecific stands in this area.
Shallow-water Halodule growing on shoals
often exposed at low tide, generally had
short, narrow leaves, and deep-water
Halodule was tall with wider leaves
(Iverson and Bittaker 1986).
Shallow-water and deep-water forms of
Halodule appear to be morphologically
different clones (Phillips 1960b; McMillan
1978). Shallow areas not exposed on low
tides contained mixtures of Thalassia,
Syringodium, and Halodule. Densest
portions of the seagrass bed were
dominated by Thalassia and Syringodium in
29
‘7.
-CRY5
Bl
:TAL
Y
Figure 11. Seagrass distribution and density in the Big Bend area (adapted from Continental Shelf
Associates 1985).
30
OUTER LIMIT OF
SEAGRASS BEDS
29” N
EM0 w+
GULF OF
50 kilometers
SrSteinhatchee
;PRINGS
casassa
PHOTO TRANSEC’I-.%?=Y
I * 1
SEAGRASS PRESENT
0 Thalassia testudinum
Q Syrlngodium f i l i f o r m e
(3 Halodule wriahtii
Q Halophila enqelmanni
Q Halophtla decipiens
Q Ruppia maritima
9m. MLW -c-y“
a
Figure 12.
1986).
Seagrass species distribution in the Big Bend area (after lverson and Bittaker
31
various mixtures. Halophila engelmanni
was common in this orassbed. and was often
mixed with Thalassia and Syringodium.
Halophila engelmanni was also abundant
outside the major seagrass bed to depths
of at least 20 m where it occurred in
monotypic stands (Continental Shelf Asso-ciates.
1 9 8 5 ) . Halophila decipiens
occasionallv occurred in small monotvoic
stands or mixed with sparse Halodule'or
Caulerpa populations in northern offshore
areas deeper than 5 m, as well as in some
of the shallowest areas (Continental Shelf
Associates 1985). Ruppia was primarily
restricted to low salinity areas such as
the mouths of the Econfina and Suwannee
Rivers.
a. Depth distribution control. Iverson
and Bittaker (1986) showed that the
major seagrass species were distri-buted
throughout the seagrass beds
in mixed associations (Figure 12),
in contrast to south Florida, where
large monospecific beds are far more
common. Thalassia and Syringodium
comprised most of the biomass which
extended to about 5 m water depth.
Halodule wrightii and Halophila
engelmanni contributed very little
to total seagrass leaf biomass.
A transect taken across a grassbed
near the Florida State University
Marine Laboratory showed that Tha-lassia
was present in greatest leaf
biomass at depths between 0.5 and
2 m, while Syringodium reached
greatest leaf biomass at 2.5 m
(Figure 13). Halodule occurred at
both ends of the transect (Iverson
and Bittaker 1986). The general
pattern in fine-scale depth distri-bution
of seagrass species appears
to be similar among the various
American tropical seagrass beds for
which observations have been
reported. Strawn (1961) described
the cross-bed, seagrass distribution
near Cedar Key in the northeast Gulf
of Mexico. Halodule occurred in
monotypic stands on shoals exposed
to the atmosphere at low tide and
was distributed throughout the sea-grass
bed. Thalassia grew only in
subtidal areas and did not extend to
the deepest limits of the bed which
contained Syringodium. This depth
Thalassia Bqi Ha-lodule
Svrinaodium
I 2 3 4
DEPTH (m)
5oor T
DEPTH (m)
Figure 13. Depth distribution of seagrass biomass
in Apalachee Bay (after lverson and Bittaker 1986).
distribution pattern was evident in
several diverse areas: in the grass
bed samples in northern Apalachee
Bay (Iverson and Bittaker 1986), in
the northwest Cuban seagrass bed
(Buesa 1974), in a Nicaraguan sea-grass
bed (Phillips et al. 1982),
and in a seagrass bed near Buck
Island, St. Croix (Wiginton and
McMillan 1979).
32
For many kilometers along the
outer limit of the Big Bend seagrass
bed between Tarpon Springs and
Crystal River, an observer on the
waters' surface notices a distinct
transition from the dark green outer
edge of the seagrass bed and the
light sediment bottom seaward of the
bed edge. The outer edge of the
grassbed is deeper north of Tarpon
Springs, in the Big Bend bed, com-pared
with the part between St.
Marks and the Crystal River. This
variation is a consequence of
increased water clarity in the
southern part of the Big Bend sea-grass
bed, as indicated by the
extinction coefficients for light
energy in the water column measured
in those areas. In addition, sub-jective
observations made over a
period of years suggest that the
relative differences in water
clarity from the two areas are con-sistent
(R.L. Iverson, unpubl.
data). The nearshore waters of the
Big Bend area receive river runoff
colored
(Bittaker $75) o$?~~c
compounds
in addition
to particulates, contr\butes to the
increased turbidity and higher
extinction coefficients observed in
that area (Zimmerman and Livingston
1979).
Based on the depth-distribution
data obtained in several different
investigations, the light-energy
compensation level for the annual
growth of American tropical seagrass
communities dominated by Thalassia
appears to be about 10% of sea-surface
photosynthetically active
light energy. The depths to which
10% of sea-surface light energy
penetrated, calculated from measured
extinction coefficients, were 7 m
for the part of the seagrass bed
between Tarpon Springs and Crystal
River, and 4.5 m for the portion
north of Crystal River. These
depths,approximate the seaward limit
of the major seaqrass beds comoosed
of Thalassia, -Syringodium, ’ and
Halodule in those respective areas
of the eastern Gulf of Mexico.
Although Thalassia and Syringodium
were distributed to greater depths
in Cuban coastal waters (Buesa 1974)
and in St. Croix waters (Wiginton
and McMillan 1979) compared with the
Big Bend area and Florida Bay, most
of the leaf biomass in the northwest
Cuban and the Buck Island, St.
Croix, seagrass beds was located
shallower than the depth to which
10% of surface light energy pene-trated.
The maximum possible area
in which Thalassia can form well-developed
beds appears to be con-strained
by the slope of the sea
floor and the bottom depth of the
isolume corresponding to 10% of
surface light energy (Iverson and
Bittaker 1986).
b. Salinity and temperature effects.
The nearshore species composition of
seagrass assemblages in the northern
bed is influenced by freshwater dis-charges
entering the northeastern
Gulf of Mexico from several rivers
along the coast. Thalassia testu-dinum
and Syringodium filiforme do
not grow in areas of low salinitv
water in the northeastern Gulf o?
Mexico (Phillips 1960a) and were not
reported in areas with salinities
less than about 17 ppt in the
northern seagrass bed (Zimmerman and
Livingston 1976a).
The seagrasses of the Big Bend
area experience a large temperature
range (8-33 "C) (Goulet and Haynes
1978). Seagrasses from this bed
were more tolerant of very cold tem-peratures
than were seagrasses from
Florida bay (McMillan 1979; McMillan
and Phillips 1979); however, each
winter, leaves of Big Bend sea-grasses
die back to within several
centimeters of the sediment-water
interface (Zimmerman and Livingston
1976b), a phenomenon also observed
in seagrass beds in Texas waters
during cold winters (Phillips 1980).
C. Sediment effects. Thalassia grew in
coarser sediments than did the other
seagrasses of the Big Bend area
(Iverson and Bittaker 1986). Buesa
(1975) reported that Thalassia in
northwest Cuban grassbeds also grew
in coarser sediments than did other
seagrasses.
33
Sediment deposition on leaf sur-faces
significantly interferes with
the growth of both Thalassia testu-dinum
and Halodule wrightii
(Phillips 1980). Water turbidity
was inversely related to distance
from the Econfina River mouth
(Bittaker 1975; Zimmerman and
Livingston 1979), suggesting that
turbidity effects on seagrass growth
occur primarily nearshore as pro-posed
by Humm (1956). Moore (1963a)
reDOrted that hiah-water turbiditv
precluded the griwth of Thalassia
testudinum in Louisiana coastal
waters within the Mississippi River
plume.
3.2 BIOMASS
Seagrass biomass can vary greatly
depending not only on the species but on
such environmental variables as available
light, sediment depth, nutrient avail-ability,
and circulation. The biomass of
Halophila is always low, but Thalassia
biomass can reach values greater than 7 kg
m-2 (Burkholder et al. 1959). Ranges of
biomass values for Thalassia, Syringo-dium,
and Halodule are presented in Table
5. The results of many of these studies
have been summarized by various authors
(McRoy and McMillan 1977; Zieman and
Wetzel 1980; Zieman 1982; Thayer et al.
1984b; Lewis et al. 1985a). Since the
studies involve a wide range of experi-mental
conditions, including differences
in habitat, sampling times and seasons,
and sample replication, attempts to com-pare
or generalize based on the cumulative
data are of questionable value.
The majority of seagrass biomass,
particularly in the larger species, is
below the sediment surface. Ordinarily,
15%-20% of Thalassia's biomass is in the
leaves (although reported values range
from 10% to 45%) with the rest made up by
roots, rhizomes, short shoots, and
sheathing leaves (Zieman 1975, 1982).
Sediment type can affect the relative
amount of biomass above and below the
surface: the ratio of leaf to root and
rhizome biomass in Thalassia increased
from 1:3 in fine mud to 1:5 in mud and 1:7
in coarse sand (Burkholder et al. 1959).
It is unclear whether this reflects
enhanced leaf production in nutrient-richer
fine sediments or the need for
greater root development for increased
nutrient absorption in the aenerallv
poorer coarse sediments. Thalassia has
the most robust root and rhizome svstem of
the seagrasses of Florida. Halodule and
Syringodium have shallower, less well-developed
roots and rhizomes, and tend to
have a greater portion of their total
biomass, 50% to 60%, in leaves (Zieman
1982). However, Pulich (1985) reported
that Halodule from Redfish Bay, Texas had
66% of total biomass below the sedjment
surface, compared to 3l.% for Ruppia.
Reported values for the relative portions
of' above- and below-ground biomass in
Florida west coast species are shown in
Table 6.
3.2.1 Seagrass Biomass in Tampa Bay
Both above- and below-ground biomass of
the seagrasses of the bay were determined
by Lewis and Phillips (1980). Ruppia had
the lowest biomass, both for standing crop
(portion of plant above sediment surface)
and root and rhizome (below sediment sur-face).
Thalassia had the highest below-ground
biomass, but its leaf standing crop
was similar to that of Syringodium.
In nearby Boca Ciega Bay, Thalassia leaf
standing crop exhibited seasonal varia-tion,
reflecting temperature extremes.
Dry weights peaked in spring and early
summer, declined during mid-summer tem-perature
maxima, and dropped to the lowest
values during winter months (Phillips
1960a). Durako and Moffler (1985c)
observed a similar seasonal pattern in
maximum leaf lengths of Thalassia in Tampa
Bay. Seagrass biomass for the Tampa Bay
area, as reported by Lewis et al. (1985a),
is given in Table 7.
3.2.2 Seagrass Biomass in
the Big Bend Area
Thalassia and Syringodium comprised 84%
of total leaf biomass in the Big Bend
area; Thalassia alone accounted for 58% of
leaf biomass compared to 64% for grass-beds
in Florida Bay (Iverson and Bittaker
1986). Thalassia leaf biomass reached a
seasonal maximum during August and then
declined rapidly at stations located near
the Florida State University Marine
34
Table 5. Representative values of seagrass biomass (g dry weight m-*1.
Species Biomass Location Source
mRuappirai t i m a
Halodule wrightii
60-160
10-400
22-208
10-300
Syringodium filiforme
15-1100
30-70
Thalassia testudinum
30-500
60-718 Puerto Rico
60-250 Texas
20-1800 Florida (east
coast)
57-6,400 Florida (west
coast)
Texas
Texas
North Carolina
South Florida
South Florida
Texas
Cuba
Pulich 1985
McMahan 1968;
McRoy 1974;
Pulich 1985
Kenworthy 1981
Zieman unpubl.
Zieman unpubl.
McMahan 1968
Buesa 1972,1974;
Buesa and
Oleachea 1970
Burkholder et al.
1959; Margalef
and River0 1958
Odum 1963;
MCRoy 1974
Odum 1963; Jones
1968 ;
Zieman 1975b
Bauersfeld et al.
1969; Phillips
1960a; Taylor et
al. 1973a
Laboratory and in St. Joseph Bay (Figure with significant blade densities, the
14). The seasonal effect is related to density decreased by over 50% at 7 of 11
cycles of light intensity and water tem- stations in the winter months. Most
perature (Iverson and Bittaker 1986). The stations that showed no difference or a
ratios of seasonal maximum to seasonal slight increase had only sparse seagrass
minimum values at these sites were between cover.
6:l and 8:1, showing the difficulty of
comparing sites on the basis of biomass The seagrass beds of St. Joseph Bay are
data, particularly in higher latitudes primarily composed of Thalassia testudinum
where seasonal patterns are more pro- growing in monospecific stands. McNulty
nounced. Continental Shelf Associates et al.
(1985) found that for offshore stations
(1972) estimated 2,560 ha of
seagrass coverage within St. Joseph Bay.
35
Species
Table 6. Biomass partitioning In seagrasses.
Biomass % of
Component (g dry wt m-*> Total Reference
Ruppia
maritima
Above ground
Below ground
Above ground
Below ground
Halodule
wrightii
Above ground
Below ground
Above ground
Below ground
Syringodium Above ground
filiforme Below ground
Thalassia
testudinum
Above ground
Below ground
110
50
48
48
150
290
5-54 11-33
10-200 67-89
28-102 16-47
31-521 53-84
58-267 11-15
321-2,346 85-90
69
31
50
50
34
66
Pulich 1985a
Lewis and
Phillips 1980
Pulich 1985
Zieman 1982
Zieman 1982
Zieman 1982
aPeak seasonal biomass values.
Table 7. Seagrass biomass of the Tampa Bay area (g dry wt mm*) (from Lewis et al. 1985a).
Species
Biomass
Location Above ground Below ground Reference
Ruppia maritima Tampa Bay
Halodule wrightii Tampa Bay
Syringodium filiforme Tampa Bay 50-170
Thalassia testudinum Boca Ciega
Bay
Bird Key
Cat's Point
Boca Ciega
Bay
32.4
325
98
636
320-1,198
601-819
25-180
Tarpon Springs
Tampa Bay
1.48 18-48
38-50 60-140
160-400 Lewis and
Phillips 1980
48.6
Bauersfeld et
al. 1969
Taylor and
Saloman 1969
Dawes et al.
1979
600-900 Lewis and
Phillis 1980
Lewis and
Phillips 1980
Lewis and
Phillips 1980
Pomeroy 1960
Phillips 1960a
Phillips 1960a
A M J J A S O N D
MONTH
Figure 14. Seasonal cycle of Thalassia at two
stations in Apalachee Bay(after lverson and Bittaker
1986).
Another estimate of St. Joseph Bay
seagrass coverage obtained during 1978 was
2,300-2,400 ha of coverage, suggesting
that seagrass beds are a stable feature of
the benthos of St. Joseph Bay and are not
markedly affected in spatial coverage by
seasonal cycles in leaf biomass density
(Savastano et al. 1984). Iverson and
Bittaker (1986) found that short-shoot
densities did not change significantly
throughout the year, and suggested that,
during the fall of the year, the use of
shoot densities
comparisons would
this area.
for interbed biomass
be more appropriate for
3.3 PRODUCTIVITY
The high rates of primary productivity
of seagrasses is well recognized. Studies
of biomass literature have reported a wide
spectrum of productivity measurements
(Table 8). Past studies have focused on
Table 8. Seagrass productivity measurements.
Species
Productivity
(g C m-2 day-l) Site Reference
Halodule
wrightii 0.5- 2.0 North Carolina
1.1 Florida (east
coast)
Syringodium
filiforme 0.8- 3.0
0.6- 9.0
Thalassia
testudinum 0.6- 7.2 Cuba
2.5- 4.5 Puerto Rico
1.9- 3.0 Jamaica
0.5- 3.0 Barbados
Florida
Texas
0.9-16.0 Florida (east
coast
Dillon 1971
Virnstein 198Za
Zieman unpubl.
Odum and Hoskin
1958; McRoy 1974
Buesa 1972,1974
Odum et al. 1960
Greenway 1974
Patriquin 1972b;
1973
Odum 1957,1963;
Jones 1968; Zieman
1975a
aCalculated as 38% of reported dry weight.
37
Thalassia, but more recently Halodule and
Syringodium have been studied.
A major problem encountered in attempts
to synthesize the results of various pro-ductivity
studies is that the three major
methods of measurement--leaf marking, O2
evolution, and l*C uptake --each yield
somewhat different results. In the
literature, the highest values are
obtained using the O2 method, the lowest
values result from leaf marking, while 14C
measurements provide intermediate values
(Zieman and Wetzel 1980; Kemp et al.
1986). In a carefully developed study,
Bittaker and Iverson (1976) found that 14C
and leaf marking gave essentially identi-cal
results when the 14C results were
corrected for inorganic losses, incubation
chamber light absorption, and differences
in light energy resulting from differences
in experimental design. In a study of
Thalassia in Bimini, Capone et al. (1979)
found, however, that productivity measured
by the 14C method was double the rates
obtained from the leaf marking technique
(Zieman 1974; Fry 1983) which underesti-mates
net productivity since it does not
measure below-ground productivity,
excreted carbon, or herbivory. The 14C
method allows the investigator to deter-mine
the partitioning of photosynthate
within the plant. Figure 15 shows the
location of 14C in Thalassia after a
4-hour incubation period. The leaves
contained 49% of the radiocarbon although
they made up only 13% of the total bio-mass.
Despite the methodological differences,
studies of the productivity of seagrasses
have shown that these are highly produc-tive
systems, especially when growing
under optimal or near-optimal conditions.
3.3.1 Seagrass Productivity in Tampa Bay
Surprisingly little data exist on the
productivity of the seagrasses of this
area. In nearby Boca Ciega Bay, Pomeroy
(1960) estimated that Thalassia and
Syringodium occurring at depths less than
2 m, fixed 500 g C.m-2. yr-I. He con-cluded
that, at these depths, seagrasses,
microflora, and phytoplankton were equally
important primary producers, whereas
sheath
rhizome
roots
% Total 14C uptake % Total weight
.:.
n
49.0
2 3 . 3 34.1
2 6 . 6 5 0 . 3
I ,I 3 . 3
13.3
Figure 15. Location of recently fixed carbon photosynthate in Thalassia after4 hour incubation. The right hand
column shows the typical weight distribution in the plants (after Bittaker and lverson 1976).
38
phytoplankton production dominated in
deeper waters. Johansson et al. (1985)
estimated that phytoplankton productivity
in Tampa Bay was higher in deep waters
(340 g C.m-2. yr-l) than in shallow waters
(50 g C.m-2. yr-l), and concluded that, in
contrast to the results of McNulty (1970),
phytoplankton production was ten times
higher than benthic production in the bay.
Studies of Thalassia leaf growth in
Tampa Bay show that leaf lengths can
increase at a rate of 5 cm per month
during the period of maximum growth in the
spring. Maximum leaf length occurs in
early summer, before high temperatures
cause a mid-summer die-back (Lewis et al.
1985a).
3.3.2 Seagrass Productivity in
the Big Bend Area
A seasonal cycle was evident in
macrophyte carbon-production data obtained
over a period of several years at a
station in the northern part of the Big
Bend area (Figure 16). Thalassia
testudinum contributed most of the carbon
production per unit area (up to
2.2 g C.m-2.d-1 in July), except for a
brief midsummer period when red drift
macroalgae were the largest source of
photosynthetic carbon fixation. Data from
which these composite carbon production
figures were made were taken from Bittaker
(1975), who showed that the annual carbon
production cycle was related to annual
variations in solar radiation and water
temperature.
Near the Anclote River, seagrass
productivity estimated from leaf growth
measurements was reported as 2-15 mg
C.m-2.h-1 for Thalassia, 2-37 mg C.m-2.h-1
and 0.9-1.4 mg C m-L h-l for Syringodium
and Halodule, respectively (Ford 1974 et
al .; Ford and Humm 1975).
I -
THALASSIA
(a)
OTHER SEAGRASSES
I
d!tbkiI
RED MACROALGAE
(cl
I=
I-l-
I
‘ J F M A M J J A S O N
MONTH
Figure 16. Seasonal changes in productivity of
seagrasses and red algae in Apalachee Bay
(unpublished data from Ft. L. Iverson).
39
CHAPTER 4. COMPONENTS OF THE SEAGRASS COMMUNITY
The distribution and density of seagrass
species are dependent on the physical,
chemical, and geological environment,
while the associated community is the
product of this seagrass composition as
well as the abiotic variables. Along the
west coast of Florida, from Florida Bay to
Apalachicola Bay, there are large
variations in all of the major
physico-chemical parameters. This
environmental gradient is reflected in the
changes of species associations and
community structure within the seagrass
system.
Although it is obvious that large
changes in abiotic variables and plant
composition and density can produce major
changes in the community structure, even
subtle variations apparently can produce
major community differences. At five
sample sites in a single south Florida
estuary with Thalassia blade densities of
over 3,000 m-2 the total number of
macrofaunal taxa'varied from 38 to 80, and
the average density of individuals varied
over two orders of magnitude, from 292 to
10,644 individuals m-2 (Brook 1978).
Organisms found in seagrass communities
can be classified in a number of ways,
depending on the objectives of the
classification. The biota present in a
seagrass ecosystem can be classified in a
scheme that recognizes the central role of
the seagrass canopy in the organization of
the system, and classifies the organisms
according to their position relative to
the canopy. The principal groups are:
(1) epiphytic organisms, (2) epibenthic
organisms, (3) infaunal organisms, (4) the
planktonic organisms, and (5) the nektonic
organisms.
and Zieman (1982) as any sessile organism
growing on a plant (not just a plant
living on a plant). Epibenthic organisms
are those that live on the surface of the
sediment, and include, in the broadest
sense, motile organisms such as large
gastropods and sea urchins, as well as
sessile forms, such as sponges and sea
anemones or macroalgae. Infaunal
organisms are those that live buried in
the sediments, such as sedentary
polychaetes and bivalves, and relatively
mobile infauna, such as irregular urchins.
Organisms that are buried part-time, for
shelter, such as penaeid shrimp or blue
crabs, or while waiting for prey, like
flounders, are considered epibenthic and
not infauna. Planktonic organisms are the
minute plants and animals, such as
diatoms, dinoflagellates, and many
copepods that drift in the water column.
They may show local movement, and
especially may migrate vertically, but are
largely at the mercy of water currents for
their lateral movement. By comparison,
nektonic organisms are highly mobile
organisms living in or above the plant
canopy, such as fishes and squids.
Another classification scheme, first
proposed by Kikuchi (1980), and slightly
modified by Zieman (1982), is based on the
mode of utilization of the seagrass beds
by the associated fauna. This
classification is based on whether
organisms are: (1) permanent residents,
(2) seasonal residents, (3) temporal
migrants, (4) transients, or (5) casual
visitors.
4.1 ALGAL ASSOCIATES
Epiphytic organisms are defined
according to the usage of Harlin (1980)
The major sources of primary production
for coastal and estuarine areas are: (1)
macrophytes (seagrasses, macroalgae, salt
40
marsh plants, and mangroves), (2) benthic
microalgae (benthic and epiphytic diatoms,
dinoflagellates, filamentous green and
bluegreen algae), and (3) phytoplankton.
In estuarine and coastal regions the
relative balance of standing crop and
productivity between the major groups of
primary producers is a function of many
environmental variables, but the major
determinants are water column nutrients,
turbidity, and substrate. In areas of
high water-column nutrients, phytoplankton
and microalgal growth will dominate,
because these small or single-cell algae
rapidly respond to the increased nutrient
supply. Because benthic plants take up
nutrients from the sediments via their
roots, these plants are less able to
exploit increased nutrient levels in the
water column. The turbidity created by
increased algal growth, along with
suspended sediments, will cause
attenuation of the light reaching the
bottom of the water column and thus
decrease the light available to benthic
plants for photosynthesis. Thus,
increased nutrient levels_favor suspended
and epiphytic plants (both of which derive
their nutrients from the water column) at
the expense of the benthic attached forms.
Turbidity favors phytoplankton primarily
since they are capable of moving upward in
the water column to intercept the light
necessary for photosynthesis. Sediment
type is also important in determining
benthic communities. - Soft sediments favor
seagrasses and certain rhizophytic green
algae, while rocky substrates favor the
development of macroalgal communities.
While portions of the coastal region of
west Florida are still miraculously
pristine, much of the area is heavily
urbanized or otherwise disturbed. Still,
as late as 1968, Taylor and Saloman
estimated that in Boca Ciega Bay total
production, dominated by macrophytes, was
six times the annual phytoplankton
production.
4.1.1 Phytoplankton
In the coastal and estuarine waters of
west Florida, Steidinger (1973) identified
four phytoplankton assemblages:
estuarine, estuarine-coastal, coastal-open
Gulf of Mexico and open gulf. Within
these areas, diatoms generally dominate
the estuarine and inshore regions, while
dinoflagellates are more diverse and
abundant in the open gulf and in
gulf-influenced areas. The predominant
organisms are ubiquitous, cosmopolitan
species that are coastal residents, but
occasional secondary abundance peaks are
attributed to sporadic visitor species.
Standing crop and productivity are higher
in areas of terrestrial runoff or river
mouths, and are lowest offshore in the
open gulf, except in areas where
divergence or upwelling make more
nutrients available (Steidinger 1972,
1973).
The phytoplankton of Tampa Bay are
typically dominated by nannoplankton (less
than 20 pm), except for periodic blooms of
blue-green algae (Schizothrix)
dinoflagellates (Gonyaulax, Gymnodiniik
nelsonii and others). The dominant
species in the bay is the diatom Skeleto-nema
costatum. The red-tide organism
Ptychodiscus brevis (= Gymnodinium breve),
a toxic coastal species. has invaded the
bay 12 times between 1946 and 1982, domi-nating
once for over three months
(Steidinger and Gardiner 1985).
Johansson et al. (1985) estimated that
phytoplankton in Tampa Bay accounted for
9l% of the submerged vegetative produc-tion.
In deep areas of the bay
phytoplankton production was estimated at
340 8 C.m-2.yr-1; a maximum value of 620 g
C.m- .yr-l was calculated from 14C data
(Johansson et al. 1985). In Boca Ciega
Bay, Pomeroy (1960) estimated that phyto-plankton,
benthic microflora, and
Thalassia production were of equal impor-tance
in depths less than 2 m, which
included 75% of the bay. Phytoplankton
production dominated in deeper areas.
4.1.2 Benthic Algae
The coastal regions and estuaries of
west Florida have a diverse benthic algal
flora, occupying several different
habitats. Although once regarded as
depauperate (Taylor 1954), the flora of
the eastern gulf have been shown to be
quite diverse in numerous subsequent
studies (summarized in Earle 1972; Dawes
1974). In addition to cosmopolitan gulf
and Caribbean species, the region also has
a pronounced seasonal peak of species with
41
a discontinuous Atlantic-northern gulf Mexico: west Florida waters exhibit less
distribution (Earle 1972). Figure 17 variation in algal flora than the waters
shows the relative richness and diversity of south Florida and northern Cuba but are
of the algal flora of the region when more diverse in its algal composition than
compared to other areas of the Gulf of the northern Gulf of Mexico. Table 9
Taxonomlc Group Number of Specfes In Different Areas
Chlorophyta
Chrysophyta
Cryptophyta
Cyanophyta
Phaeophyta
Rhodophyta
Tracheophyta
Xanthophyta
A B C D E F Total
85 42 45 43 97 151
1
6 16 21 21 30 2:
41 33 23 24 52 58
120 121 86 42 171 270
6 5 4 6 6 6
1 1
174
1
31
82
349
Figure 17. Distribution and diversity of benthic marine plants in the Gulf of Mexico. Total is the actual
number of species counted in areas A-F (after Earle 1972).
42
Table 9. Macroalgae of seagrass communities of the west Florida coast.
Location
Total
Species Cyanophyceae Chlorophyceae Phaeophyceae Rhodophyceae
Anclote Anchoragea 124 18 39 17 50
Apalachee Bayb 34 13 4 17
Seven SitesC 30 11 2 17
Crystal Riverd 106 19 24 63
Southwest Coaste 148 50 28 70
Table modified from Dawes (1987), with additional material. (a) Hamm and Humm
(1976); (b) Zimmerman and Livingston (1976b); (c) Dawes (1985) Dominant species
only; (d) Steidinger and van Breedveld (1971); (e) Dawes et al. (1967). Many
stations in this survey were offshore of developed seagrass beds.
lists the total number of macroalgal taxa
from several sites in Florida and shows
the distribution by division at each area.
Because of the combination of protected
estuaries on the central and southern
portions of the Florida west coast and the
gently sloping shelf and moderate wave
climate to the north of Tampa Bay, the
west coast offers an enormous area for the
colonization of either algae or
seagrasses. The primary substrates
available for algae in the region include:
(1) rocky outcrops and hard bottom (2)
soft sediments (3) seagrass leaves and
mangrove roots and (4) the water column.
Much of the shallow region north of Tampa
Bay consists of rocky outcrops suitable
for algal attachment. Throughout the
area, oyster reefs, mangrove prop roots,
and scattered rocks or shells offer
additional algal substrate, in addition
to: human-made structures like pilings,
bridge supports, and canal walls.
The only marine and estuarine algae able
to consistently utilize sediments as
substrate are the mat-forming algae and
members of the order Caulerpales of the
division Chlorophyta, which possess
creeping rhizoids that provide an anchor
in sediments (Humm 1973; Dawes 1981).
Among the most important genera are
Halimeda, Penicillus, Caulerpa, and
Udotea, which are primary producers of
organic carbon. Halimeda and Penicillus
also deposit rigid skeletons of calcium
carbonate that become a major component of
the sediments upon the death of the plant.
These algae do not have ability to
stabilize the sediments as effectively as
the seagrasses, although they do buffer
currents to some degree, and by their
extremely rapid growth can accomodate
changes in shifting sediments.
Historically, the main utility of their
rhizoidal holdfasts was considered to be
serving as an anchor for the plant in the
substrate, but Williams (1981) has shown
that they can take in nutrients through
their rhizoids and translocate these
throughout the plant in a manner similar
to higher plants.
In many tropical and subtropical seas,
the calcareous green algae are the major
source of sediments. The different genera
produce characteristic particles, with
Halimeda tending to form sand-grain-sized
plates, while Penicillus produces
fine-grained aragonitic mud. At current
growth rates, Penicillus alone could
account for all of the fine mud behind the
Florida reef tract and one third of the
fine mud in northeastern Florida Bay
(Stockman et al. 1967). In addition, the
combination of Rhipocephalus, Udotea, and
43
Acetabularia generates at least as much
mud as Penicillus in the same locations.
In the Bight of Abaco, Bahamas, Neumann
and Land (19751 calculated that the arowth
of Peniciilus,'Rhipocephalus, and Haiimeda
has produced 1.5 to 3 times the amount of
mud and Halimeda sand now in the basin and
that in a typical Bahamian Bank lagoon,
calcareous green algae alone produce more
sediment than can be accomodated. Bach
(1979) measured the rates of organic and
inorganic production of calcareous green
algae in Card Sound, south of Miami.
Organic production was low in this lagoon,
ranging from 8.6 to 38.4 g ash-free dry
weight.m-2.yr-1, and 4.2 to 16.8 g
CaC03.m-2.yr-1 for all the species
combined.
In areas of western Florida with hard
substrate, numerous species of attached
algae are found. Amona the most common
brown algae (Phaeophyta) are Dictyota
dichotom