Prepared by the University of Connecticut and the Connecticut DEP, with the support of researchers and organizations throughout the Long Island Sound watershed.
Establishing restoration objectives for eelgrass in Long Island Sound

Niantic River -
Case Study

History

Scallops once thrived in the Niantic River. (photo courtesy of P. Auster, University of Connecticut)

The Niantic River shoreline is well-developed residentially except for the Oswegatchie Hills, along the northwest shore of the upper estuary. While conditions in the estuary are still good enough to support a population of eelgrass, the large bay scallop population (Argopecten irradians) once found in the area is now largely absent from the bay (Short 1988; Marshall 1994). Local residents are interested in preserving and improving the current status of Niantic River and would like to see the return of a flourishing bay scallop population to the estuary, as well as other "desirable" species. However, Niantic River has been experiencing development pressure in the currently undeveloped Oswegatchie Hills. Development plans in this area could further increase the nutrient and sediment load, both potentially harmful to the health of seagrass in the river.

The eelgrass population in Niantic River was decimated during the 1930's outbreak of the wasting disease which affected populations throughout the Atlantic. By the late 1940's, eelgrass was making a "gradual recovery" in the area (Marshall 1947). By the 1970's, the population was healthy and flourishing until starting to decline during the 1980's (Short 1988; DNC 2007).

Beginning in 1987, the Millstone Environmental Lab at Dominion Nuclear Connecticut began monitoring the status of the eelgrass population in the Niantic River and surrounding areas (DNC 2007). The interannual variability was high, with beds appearing and disappearing throughout the years of the monitoring study (see figures). Some of this variability was attributed to changes in seawater temperature, either warmer winter/spring temperatures for some years or warmer August temperatures for other years, e.g. 1999. These temperature effects were compounded by other effects: e.g. blue mussel overgrowth of the eelgrass (1992), increased macroalgae abundance (1999), and increase in the sediment silt/clay component (1999). Since 1999, eelgrass coverage in Niantic River has steadily increased, likely a result of relatively cool summers (DNC 2007).

Biological disturbances from water fowl (Brent geese, Branta bernicla, and swans, Cygnus spp.) and green crabs (Carcinus maenas) may also have contributed to variability in the eelgrass population as periodic increases in the resident populations of these organism have been observed over the last 20 years. Evidence of grazing or disturbance to the plants (cropped blades, exposed rhizomes) have also been observed, coincident with increases in the populations of these higher level organisms (DNC 2007).

For an in depth treatment of the Niantic River estuary and watershed, the reader is referred to the Kleinschmidt report . The annual reports published by Millstone Environmental Lab also provide a wealth of information on the Niantic River estuary.


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Present Status

Data from this analysis included a period when the eelgrass populations were at a low point in areal abundance (1999), extending through the initial stages of a resurgence in areal coverage (2004). The population continued to expand in the years following the end of this dataset (see figures). During the years of this comparison (1999 to 2004), eelgrass was present in the Mid and South sections of the River. In the late 1980's, eelgrass was also present along Sandy Point and in Smith Cove (see the Case Study Paper for more detailed information.)

Primary Requirement - Light

The summertime average (5/15 to 9/15) light extinction coefficient for all of Niantic River hovered around the 0.7 m-1, which allows for a prediction that eelgrass should be able to colonize to depths of 2.3 m, assuming a conservative estimate of minimum light requirement by eelgrass of 20% of surface light (figure 1). The average Kd over the 6 year summertime sampling period was 0.57 0.02 m-1 in the mid and lower sections combined versus 0.79 0.02 m-1 in the upper sections of the estuary. Eelgrass was found only in the mid and lower sections of the estuary, in areas of the estuary where depth was less than 2.3 m.

Figure 1. Light extinction coefficient data from Niantic River. Data presented were from the Kremer et al. data set. The black line was the summertime average (May 15 to Sep 15). The dashed lines represent the recommended seagrass habitat requirements: 1.5 m-1 (Batiuk et al. 2000) and 0.7 m-1 (Yarish et al. 2006). Dry weight biomass of Zostera marina at two reference "eelgrass" stations (indicated by the white and black circles with drop lines to the x-axis) were plotted to indicate annual success of the macrophyte for comparison to the annual variability of Kd.

Secondary Requirements - Chlorophyll, Nitrogen

The summertime chlorophyll a data hovered around the 5.5 mg L-1 value proposed by Yarish et al. (2006) for management criteria in LIS, versus the Chesapeake Bay guideline of 15 mg L-1 (Batiuk et al. 2000). Values from the upper reach were statistically different from the lower section of the estuary (p-value < 0.001). The mean value for the mid and lower regions of the estuary, where eelgrass was found, averaged 4.1 0.5 mg L-1 over the 6 year period examined while the upper reach and Smith Cove averaged 9.5 0.4 mg L-1. During 2000, when eelgrass biomass was low, the chlorophyll biomass was higher than the norm (see Case Study Report for figures).

Dissolved inorganic nitrogen in the mid and lower sections of the estuary averaged 0.017 0.004 mg/L and 0.027 0.004 mg/L for the upper reaches and Smith Cove (figure 2). The two areas of the estuary were not significantly different (p-value 0.122). The summertime DIN values hovered around the 0.03 mg/L value recommended by Yarish et al. (2006) and were well below the 0.15 mg/L value recommended in the Chesapeake Bay guidelines. However, some fall samples taken in 2003 did show elevations in DIN concentration. A finely resolved annual dataset from "Save the River, Save the Hills" and DNC / Millstone Environmental Lab provided further information on the typical annual cycle of nutrients in the estuary, indicating the high value seen in the fall of 2003 were typical for that time of year.

Figure 2. Dissolved Inorganic Nitrogen (NH4+, NO3-, NO2-) data from Niantic River. DIN was determined for filtered water samples from near surface (0.25m) and near bottom (0.5m from bottom). Data presented were from the Kremer et al. data set. The black line was the summertime average (May 15 to Sep 15). The dashed lines represent the recommended seagrass habitat requirements: 0.15 mg/L (Batiuk et al. 2000) and 0.025 mg/L (Yarish et al. 2006). Dry weight biomass of Zostera marina at two reference "eelgrass" stations (indicated by the white and black circles with drop lines to the x-axis) were plotted to indicate annual success of the macrophyte for comparison to the annual variability of Kd.

In general, there were sections of the upper reach of the estuary where DIN concentrations were relatively high. Concentrations in the estuary were lower in the summer months, when nutrients were tied up in biomass. As eelgrass and macroalgae senesce in the fall, nutrients were released to the water column, causing a rise in DIN concentrations, most evident in late September through February. There also appeared to be an increase in DIN in March of 2002 and again in 2003, perhaps in response to the spring freshet. In the eelgrass areas, summertime values were low, but winter time DIN increased up to 0.14 mg/L on occasion. Choice of a management criteria for DIN must clearly state what is to be measured: summertime DIN or annually averaged DIN, as the two values were distinctly different (summertime average of less than 0.014 mg/L versus annual average of 0.056 mg/L)

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Future Outlook

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Selected Web-Links and References

Save the River, Save the Hills

Niantic River Watershed Plan - Overview

Niantic River Watershed Protection Plan - DEP Web Site

Batiuk, R. A., P. Bergstrom, M. Kemp, E. W. Koch, L. Murray, J. C. Stevenson, R. Bartleson, V. Carter, N. B. Rybicki, J. M. Landwehr, C. L. Gallegos, L. Karrh, M. Naylor, D. Wilcox, K. A. Moore, S. Ailstock, and M. Teichberg. 2000. Chesapeake Bay submerged aquatic vegetation water quality and habitat-based requirements and restoration targets: A second technical synthesis. report number CBP/TRS 245/00 EPA 903-R-00-014. United States Environmental Protection Agency.

DNC. 2007. Annual Report 2006: Monitoring the environment of Long Island SOund at Millstone Power Station, Waterford, Connecticut. report. Millstone Environmental Laboratory.

Kleinschmidt.Associates. 2006. Niantic River Watershed Protection Plan. report. Connecticut Department of Environmental Protection, Office of Long Island Sound Programs.

Marshall, N. 1947. An abundance of bay scallops in the absence of eelgrass. Ecology 28:321-322.

Marshall, N. 1994. The Scallop Estuary. Th' Anchorage Publisher.

Short, F. T. 1988. Eelgrass-scallop research in the Niantic River. report. Jackson Estuarine Laboratory, University of New Hampshire.

Yarish, C., R. E. Linden, G. Capriulo, E. W. Koch, S. Beer, J. Rehnberg, R. Troy, E. A. Morales, F. R. Trainor, M. DiGiacomo-Cohen, and R. Lewis. 2006. Environmental monitoring, seagrass mapping and biotechnology as means of fisheries habitat enhancement along the Connecticut coast. report number CWF 314-R. University of Connecticut.

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