Salt marsh trends in selected estuaries of southwestern Connecticut

              ! U.S. Fish & Wildlife Service . …... …. ……
SALT MARSH TRENDS IN SELECTED
ESTUARIES OF SOUTHWESTERN
CONNECTICUT
APRIL 2006
U.S. Fish and Wildlife Service
Northeast Region
National Wetlands Inventory
Program
300 Westgate Center Drive
Hadley, MA 01035
http://northeast.fws.gov/wetlands
Salt Marsh Trends in Selected Estuaries of Southwestern Connecticut
Ralph W. Tiner
U.S. Fish and Wildlife Service
National Wetlands Inventory Program
300 Westgate Center Drive
Hadley, MA 01035
Irene J. Huber, Todd Nuerminger, and Eric Marshall
Department of Plant and Soil Sciences
Stockbridge Hall
University of Massachusetts
Amherst, MA 01003
April 2006
National Wetlands Inventory Cooperative Report
Prepared for the Long Island Studies Program, Connecticut Department of Environmental
Protection, Hartford, CT
This report should be cited as:
Tiner, R.W., I.J. Huber, T. Nuerminger, and E. Marshall. 2006. Salt Marsh Trends in Selected
Estuaries of Southwestern Connecticut. U.S. Fish and Wildlife Service, National Wetlands
Inventory Program, Northeast Region, Hadley, MA. Prepared for the Long Island Studies Program,
Connecticut Department of Environmental Protection, Hartford, CT. NWI Cooperative Report.
20 pp.
TABLE OF CONTENTS
Page
Introduction 1
Study Objectives 1
Study Areas 1
Methods 3
Results 4
Appendices 10
A. Maps showing location of study areas. 11
B. Aerial photos for Canfield Island Cove and
Grays Creek (1974 vs. 2000). 18
LIST OF FIGURES
Figure Page
1. General location of study estuaries along the Connecticut coast. 2
2. Trends for Canfield Island Cove. 7
3. Trends for Cos Cob Harbor. 7
4. Trends for Five Mile River. 8
5. Trends for Grays Creek. 8
6. Trends for Greenwich Cove. 9
7. Trends for Scott Cove. 9
LIST OF TABLES
Table Page
1. Acreage changes in study salt marshes from 1974 to 2004. 5
1
INTRODUCTION
Recent investigations have shown that rising sea levels are having a significant impact on
tidal wetlands in many areas of the United States. Higher water levels are inundating
lower portions of these marshes and converting them to tidal flats, while portions of the
high marsh are being converted to low marsh. The importance of coastal marshes to
marine and estuarine ecosystems and migratory waterfowl is widely recognized. Because
of these and other values (e.g., storm surge detention), most coastal states have adopted
specific legislation to protect these highly valued natural resources. The State of
Connecticut was among the first states to pass such legislation and has been protecting its
tidal wetlands since 1970. While this law has virtually eliminated the once-widespread
dredging and filling of tidal wetlands, nature’s forces (i.e., rising sea level) continues to
impact these wetlands.
The Long Island Studies (LIS) Program of the Connecticut Department of Environmental
Protection (DEP) has noticed the habitat changes indicative of sea-level rise in many
coastal wetlands. In 2005, DEP provided funds to the U.S. Fish and Wildlife Service to
conduct a trends analysis of selected salt marshes along the southwestern coast of the
state to document habitat changes. The Natural Resources Assessment Group (NRAG) at
University of Massachusetts provides technical support to the U.S. Fish and Wildlife
Service’s National Wetlands Inventory (NWI) Program and assisted with this trends
analysis project.
Study Objectives
To document changes in marsh vegetation zones (low marsh and high marsh) in six salt
marsh areas in southwestern Connecticut since 1974. The following time periods would
be evaluated: 1) 1974-1981, 2) 1981-1986, 3) 1986-1990, 4) 1990-1995, 5) 1995-2000,
and 6) 2000-2004. Photointerpretation of vegetation changes would document the total
changes between intervals and over the past quarter century as well as identify any
differences in the rate of change during the entire study period.
Study Areas
The six study areas were located along the western shore of Long Island Sound in
southwestern Connecticut (Figure 1): 1) Cos Cob Harbor (Greenwich; 126-acre intertidal
area), 2) Grays Creek (Westport; 35-acre intertidal zone), 3) Scott Cove (Darien; 121-
acre intertidal area), 4) Five Mile River (Darien/Norwalk; 18-acre intertidal zone), 5)
Greenwich Cove (100-acre intertidal area), and 6) Canfield Island Cove (Norwalk; 109-
acre intertidal area). The spring tide range in this area is about 8.0 feet (i.e., 8.3 feet in
Cos Cob Harbor; http://co-ops.nos.noaa.gov/tide_pred.html), which is more than twice the
range as in Long Island Sound estuaries east of the Connecticut River. The mean tide range is
approximately 7.0 feet (i.e., 7.2 feet for Cos Cob Harbor).
2
Figure 1. General location of study estuaries along the Connecticut coast (see Appendix A for
detailed maps).
3
METHODS
Digital images of aerial photography for the study areas were provided by the LIS
Program for the following years: 1974, 1981, 1986, 1990, 1995, 2000, and 2004. These
images were georeferenced in Connecticut State Plane Coordinates (NAD 83). The
boundaries of salt marsh complexes for each of the sites were delineated and saved as a
shapefile in Connecticut State Plane Coordinates (NAD 83) for each of the study sites
and for each year of photography.
For each study site, aerial photographs were interpreted on-screen and the following
features were delineated for each era: low marsh, high marsh, and tidal flat. The area of
each feature was then calculated for each study area and for each time period. In most
cases, the aerial photographs were captured at low tide, so the limits of tidal flats could be
detected. In eras where the aerial photos were not low-tide synchronized, the tidal flats
from the other time periods were assumed to be present. Where human-induced changes
were detected, the extent and nature of the change was recorded.
A geospatial data base was created to store data on wetland changes. This data base was
used to generate maps and statistics of salt marsh trends. Statistics were generated to
reflect the area of low marsh, area of high marsh, area of tidal flat (depending on stage of
tide), the overall area of tidal wetland for each time period, and the extent of human-induced
changes. These data were then used to demonstrate wetland changes between
years (at approximately five-year intervals). A series of maps showing the changes for
each salt marsh complex was generated. A metadata file for this project was also created
to document source data and other pertinent information about the project and
interpretations.
4
RESULTS
All study areas experienced a decline in low marsh from 1974 to 2004 and a gain in tidal
flats, while all areas, except Cos Cob Harbor, also experienced a loss in high marsh
(Table 1). Figures 2 through 7 show the changes in low marsh, high marsh, and tidal flat
at various intervals over the past 30 years.
Canfield Island Cove was unique in that it had a small gain in open water (0.22 acres) and
a gain in palustrine tidal wetland (0.31 acres). Over the 30-year study period, it
experienced a 26% gain in tidal flat, while losing 27% of its low marsh and about 4% of
its high marsh. Aquatic beds appeared to decline by nearly 40%. Aerial photos for 1974
and 2000 are provided in Appendix B to illustrate the changes in these wetlands.
Cos Cob Harbor was the only study estuary to show a gain in high marsh from 1974-
2004, with a negligible 0.4-acre gain (2.5% increase). Tidal flat acreage increased by
about 5 acres (30% gain), largely at the expense of low marsh which declined by 30%.
Five Mile River gained nearly 4 acres (67%) of tidal flat that formed in areas of former
marsh. Nearly half of the low marsh was converted to tidal flat as was nearly one-fifth of
its high marsh.
Similarly, Grays Creek estuary lost low marsh and high marsh to tidal flat which
increased in acreage by 37%. Over half of the low marsh and about one-third of the high
marsh acreage declined over the 30-year study period. Aerial photos for 1974 and 2000
are provided in Appendix B to illustrate the changes in these wetlands.
Greenwich Cove lost nearly 50% of its low marsh and only 8% of its high marsh from
1974-2004. These losses were countered by a nearly 11-acre gain (19%) in tidal flat.
Scott Cove, like the other areas in this study, experienced a gain in tidal flat at the
expense of salt marsh. The nearly 17-acre gain in the former was the result of losses of
16 acres of low marsh and about 1 acre of high marsh. This was the largest acreage loss
of low marsh among the six study areas.
5
Table 1. Acreage changes in study salt marshes from 1974 to 2004.
Acreage Overall Acreage
Salt Marsh System Marsh Zone 1974 1981 1986 1990 1995 2000 2004 Change (% Change)
Canfield Island Cove Tidal Flat 32.15 32.72 34.58 37.58 38.16 39.87 40.51 +8.36 (26.0)
Low Marsh 27.61 27.08 25.36 22.53 21.90 20.62 20.06 -7.55 (27.3)
High Marsh 48.13 47.71 47.25 47.07 46.71 46.36 46.42 -1.71 (3.5)
Open Water 14.95 14.95 14.95 14.87 14.89 14.95 15.17 +0.22 (1.5)
Aquatic Bed 0.80 0.75 0.78 0.78 0.78 0.81 0.49 -0.31 (38.8)
Beaches 0.50 0.41 0.45 0.43 0.47 0.47 0.47 -0.03 (6.0)
Palustrine Tidal 0.00 0.00 0.45 0.53 0.84 0.31 0.31 +0.31 (na)
Cos Cob Harbor Tidal Flat 17.41 19.33 21.68 22.40 22.27 22.70 22.67 +5.26 (30.2)
Beach 73.57 73.57 73.57 73.57 73.57 73.57 73.57 0.00 (0.0)
Rocky Shore 0.24 0.24 0.24 0.24 0.24 0.24 0.24 0.00 (0.0)
Low Marsh 19.38 17.09 14.77 13.88 13.90 13.56 13.58 -5.80 (29.9)
High Marsh 15.85 16.15 15.94 16.19 16.30 16.25 16.25 +0.40 (2.5)
Aquatic Bed 16.55 16.61 16.69 16.61 16.61 16.58 16.58 +0.03 (0.0)
Five Mile River Tidal Flat 5.78 6.79 7.22 8.59 9.54 9.63 9.63 +3.85 (66.6)
Low Marsh 5.75 5.38 5.33 4.23 2.97 2.93 3.04 -2.71 (47.1)
High Marsh 6.56 5.92 5.53 5.27 5.53 5.48 5.37 -1.19 (18.1)
Grays Creek Tidal Flat 18.36 20.15 21.48 23.31 24.17 24.95 25.21 +6.85 (37.3)
Beach 0.41 0.37 0.31 0.32 0.35 0.35 0.32 -0.09 (22.0)
Low Marsh 7.57 5.99 6.05 4.94 4.09 3.66 3.52 -4.05 (53.5)
High Marsh 8.22 8.06 6.73 5.99 5.89 5.52 5.52 -2.70 (32.8)
Aquatic Bed 0.07 0.07 0.07 0.07 0.13 0.15 0.07 0.00 (0.0)
Open Water 0.58 0.58 0.58 0.58 0.58 0.58 0.58 0.00 (0.0)
6
Greenwich Cove Tidal Flat 56.39 58.95 62.84 64.35 66.11 67.50 67.12 +10.73 (19.0)
Beach 2.92 2.90 3.10 2.84 2.98 2.42 2.91 -0.01 (0.0)
Low Marsh 19.62 17.09 13.48 12.35 11.04 10.50 10.40 -9.22 (47.0)
High Marsh 21.30 21.01 20.52 20.40 19.81 19.52 19.52 -1.78 (8.4)
Open Water 14.79 14.79 14.79 14.79 14.79 14.79 14.79 0.00 (0.0)
Aquatic Bed 3.41 3.41 3.41 3.41 3.41 3.41 3.41 0.00 (0.0)
Scott Cove Tidal Flat 71.67 83.48 83.93 88.65 88.23 88.46 88.46 +16.79 (23.4)
Low Marsh 33.39 21.87 21.76 17.01 17.44 17.22 17.22 -16.17 (48.4)
High Marsh 16.39 16.09 15.75 15.79 15.72 15.72 15.72 -0.67 (4.1)
Open Water 3.03 3.03 3.03 3.03 3.03 3.03 3.03 0.00 (0.0)
7
Canfield Island Cove
0
10
20
30
40
50
60
1974 1981 1986 1990 1995 2000 2004
Acres
Low Marsh High Marsh Flat
Figure 2. Trends for Canfield Island Cove.
Cos Cob Harbor 1974-2004
0
5
10
15
20
25
1974 1981 1986 1990 1995 2000 2004
Acres
Low Marsh High Marsh Flat
Figure 3. Trends for Cos Cob Harbor.
8
Five Mile River 1974-2004
0
2
4
6
8
10
12
1974 1981 1986 1990 1995 2000 2004
Acres
Low Marsh High Marsh Flat
Figure 4. Trends for Five Mile River.
Grays Creek 1974-2004
0
5
10
15
20
25
30
1974 1981 1986 1990 1995 2000 2004
Acres
Low Marsh High Marsh Flat
Figure 5. Trends for Grays Creek.
9
Greenwich Cove 1974-2004
0
10
20
30
40
50
60
70
80
1974 1981 1986 1990 1995 2000 2004
Acres
Low Marsh High Marsh Flat
Figure 6. Trends for Greenwich Cove.
Scott Cove 1974-2004
0
10
20
30
40
50
60
70
80
90
100
1974 1981 1986 1990 1995 2000 2004
Acres
Low Marsh High Marsh Flat
Figure 7. Trends for Scott Cove.
10
APPENDICES.
11
Appendix A. Maps showing location of study areas.
12
13
14
15
16
17
18
Appendix B. Aerial photos for Canfield Island Cove and Grays Creek (1974 vs.
2000).
19
CANFIELD ISLAND COVE – YR 1974 (ABOVE), YR 2000 (BELOW)
20
GRAYS CREEK - YR 1974 (ABOVE), YR 2000 (BELOW)