|Case Studies||site overview||applying the target criteria||summary of results|
Three sites were chosen to serve as case studies for examining the recommended habitat criteria for the preservation and restoration of eelgrass to Long Island Sound. These sites are all shallow and partially enclosed. They were chosen because of the history of research in the sites and as they are most likely to experience the effect of nitrogen loading from the watersheds, relative to the well-flushed eelgrass beds found in the more open areas of LIS.
Values for recommended habitat guidelines were compared to water quality parameters, eelgrass distribution, and historical data from the sites.
From these analyses, guidelines for setting restoration goals for water quality were developed.
area = 0.5 km2 mean depth = 1.0 m freshwater residence time = 3.5 days N-Load Rate = 12 g N m2estuary y-1
Mumford Cove has a history of changing nutrient inputs and corresponding eutrophication over the last sixty years. Unlike most sites currently under investigation, the anthropogenic nutrient load to Mumford Cove has been greatly reduced, resulting in a shift from an algal dominated community to one dominated by Zostera marina. From 1945 to 1987, a wastewater treatment facility discharged into the cove. Since the facilityís removal in 1987, eelgrass has returned to the cove and maintained a presence, exhibiting some interannual variability.
area = 2.7 km2 mean depth = 2.6 m freshwater residence time = 27 days N-Load Rate = 14 g N m2estuary y-1
The Niantic River shoreline is residentially developed 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. The eelgrass population experienced a decline in the 1980s, but increases in coverage and density have been observed in recent years.
Pawcatuck River (PR) and Little Narragansett Bay (LNB)
area (km2) = 2.7 (PR) / 9.6 (LNB) mean depth (m) = 1.8 (PR) / 2.0 (LNB) residence time (d) = 1.3s - 6.5b (PR) / 3 (LNB) N-Load Rate (g N m2estuary y-1) = 167 (PR) / 58 (LNB)
The Pawcatuck River has supported industry along itís banks since the 1600ís. The river has a history of water quality problems related to the human activities in the watershed. However, conditions have improved as industry has declined.
Little Narragansett Bay, at the receiving end of this watershed heavily influenced by human activity, supported a healthy population of eelgrass until the early-1990's. By 1999, all eelgrass had disappeared from the area.
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The primary requirement used by the Chesapeake Bay water quality standards is the minimum light requirement for eelgrass (as % of surface irradiance). The light extinction coefficient is no longer used as a requirement because the value could not be adjusted to accommodate different tidal ranges or restoration depths.
Long Island Sound will encounter similar problems with the use of Kd as a management criteria. However, the sites in the case studies were shallow enough that the bottom was often viewable, indicating light was plentiful in the sites. For the case studies, the requirement will be discussed in terms of Kd, as this was the value actually measured. For management purposes, looking at the minimum light requirement, which is related to the Kd by the Lambert-Beer equation, takes into account the actual area of an estuary that may potentially be occupied by eelgrass.
For example, the maximum allowable Kd for eelgrass growth at various depths were calculated for a minimum light requirement of 15% and a more conservative estimate of 22% (figure 1). So, at a minimum required light level of 22% of surface irradiance, the required Kd for eelgrass growth at a depth of 1m would be 1.6 m-1 versus 0.6 m-1 in a site with a depth of 2.5 m. Using the minimum light requirement inherently allows for an adjustment based on the physical parameters of the site.
figure 4: Maximum allowable Kd (m-1) for eelgrass growth at the listed depth and minimum light requirement. Calculated using the Lambert-Beer equation.
Water column dissolved inorganic nitrogen concentration (DIN), chlorophyll a concentration (chl a), and sediment percent organics have been identified as secondary habitat requirements. The secondary requirements can be used to explain the absence of eelgrass if light availability is sufficient but one of these factors greatly exceeds the tolerance limits of eelgrass.
The "lower" section of Mumford Cove and Niantic River (table 1) are the only areas of the case study sites which currently support a population of eelgrass. Little Narragnasett Bay (LNB) supported eelgrass as recently as the early 1990's, but eelgrass has been replaced by macroalgae in this site. The data from these three sites supported the guidelines suggested by Yarish et al. (2006) for Long Island Sound. A more detailed treatment of each site is available on the site specific pages.
Table 1: Comparison of recommended habitat guidelines.
Recommended guidelines from Chesapeake Bay (Batiuk et al. 2000) and Long Island Sound (Koch 2001; Yarish et al. 2006) versus average values (Ī standard error) for the three case study sites. The average values included data from the Kremer et al. data set collected between 1999 and 2004. Only data collected between May and September were included in the analysis (2 sample dates were excluded: 10/20/03 and 11/03/03). Stations were divided into sections (see Case Study Report), then averaged into the categories shown in the table. The boxes shaded light gray indicate values which do not meet the recommended LIS criteria. Dark gray indicates values which do not meet either the LIS or Chesapeake Bay criteria. The maximum depth limit for eelgrass in the site was calculated from the Kd shown in the table.
- see Case Study Report for a full list
Branco, A. and J. N. Kremer. in prep. Predicting the nitrogen load to 10 Southern New England estuaries using a modified version of the N-LOAD nitrogen loading model.
Desbonnet, A. and V. Lee. 1996. Rhode Island Coastal System: Databases for Determining Nitrogen Sensitivity. Vol. 1. Text and Statewide Database. Vol2. Bay Profiles: Pawcatuck River to Narrow River. Vol. 3. Bay Profiles: Wickford to Seekonk River. Vol. 4. Bay Profiles: Bristol, Little Compton and The Islands. Final Report to the Rhode Island Department of Environmental Management, Narragansett Bay Project, Providence, R.I. report.
Mullaney, J. R., G. E. Schwarz, and E. C. T. Trench. 2002. Estimation of nitrogen yields and loads from basins to Long Island Sound, 1988-98. U.S. Department of the Interior, U.S. Geological Survey, Prepared in cooperation with the Connecticut Department of Environmental Protection. report.
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