|Pawcatuck River and Little Narragansett Bay - Case Study||History||Present Status||Future Outlook||Selected References|
The Pawcatuck River has supported industry along it’s banks since the 1600’s, starting with shipbuilding, then switching to textile manufacturing with the Industrial Revolution in the 1800’s. As industry and population in the area increased, so too did pollution on the river. In the 1950’s water quality started to improve with the collapse of the textile industry and accompanying reduction of industrial wastewater entering the river. Further improvement was provided when sewage treatment plants were constructed in Westerly and Pawcatuck. Then in 1972, the improvement of water quality in the area was greatly aided by the passage of the Clean Water Act (1992 Rhode Island SeaGrant fact sheet).
While this system has been receiving a high level of nutrients for decades, a 1992 Rhode Island SeaGrant fact sheet indicated that eelgrass was still present in Little Narragansett Bay, though the blades did carry a "heavy growth" of epiphytes. This statement was repeated in a management plan for the area published during the same time period which mentioned the beds in LNB were "extensive" (Dillingham et al. 1992). By 1998, eelgrass was found at only 6 of the 134 stations sampled in LNB between 6/23/98 and 8/12/98 (figure 1). By 2001, repeated searches for eelgrass in LNB suggested that eelgrass was absent from the area, or if present, in extremely small patches (Kremer et al. data set and personal communication). The 2002 and 2006 aerial surveys conducted by the U.S. Fish and Wildlife Service, confirmed the absence of eelgrass in LNB (Tiner et al. 2003; Tiner et al. 2007). The Pawcatuck River and LNB provide an example where eelgrass once thrived in the receiving waters at the base of the estuary (LNB), but has since disappeared. The exact cause of the disappearance is unknown.
Figure 1: Eelgrass survey conducted by Alan Banister between 6/23/98 and 8/12/98. Each grey point represents a survey location. The green points marked with flags indicate locations where eelgrass was found. The aerial photograph was from 2008, Google Earth.
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The upper sections of the river exhibit reduced light in the water column (average Kd = 1.8 ± 0.1 m-1 for the 6 year period) compared to the lower portion of the river (average Kd = 0.9 ± 0.1 m-1). While eelgrass was no longer present in LNB, sampling in 2001 through 2003 revealed summertime Kd values averaging 0.7 ± 0.1 m-1, similar to levels found in sites with eelgrass: Mumford Cove and Niantic River. Similar Kd values were seen in all sections of the river and LNB by Doering et al. (1994) in 1989 and 1990, a period when eelgrass was still abundant in LNB. This suggests that water clarity may not be the limiting factor for eelgrass in this site. However, light avaialbility may still limit growth through the shading effects of epiphytes or macroalgae on eelgrass.
Figure 2. Light extinction coefficient data from the Pawcatuck River and Little Narragansett Bay. Stations are listed consecutively, moving down the river from Westerly to Watch Hill and out to LNB. Data presented were from the Kremer et al. data set and the Banister / Pine Point School data set. The light extinction coefficients from the Banister / Pine Point School data set are indicated by hollow points with colors corresponding to the legend. The black line was the summertime average (May 15 to Sep 15) for the Pawcatuck River, Little Narragansett Bay not included. 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). No eelgrass was present in PR or LNB during this time period.
The average summertime water column chlorophyll a for the upper river was 10.3 ± 0.8 mg L-1 and the lower river was 7.9 ± 0.9 mg L-1, both well above the value recommended by Yarish et al. (2006). The average value for LNB was 3.5 ± 1.4 mg L-1, below the conservative recommended criteria. While chlorophyll concentrations in the river were slightly elevated, it appeared that levels in LNB were not high enough to warrant the continued absence of eelgrass (see Case Study Report for figures).
A recent analysis of an annual set of nutrient data coupled with information on nutrient loading to the watershed by Fulweiler and Nixon (2005) indicated the nitrogen load received by LNB made it a "heavily nitrogen enriched system," receiving higher levels of nutrients on a volume specific basis than larger urban estuaries (e.g. Narragansett Bay, Potomac Estuary, Delaware Bay). The data sets presented here are on a much coarser resolution than Fulweiler’s 80 sample dates in one year, but between the Kremer et al. summer samples and Banister’s fall and spring samples, some of the annual variation in the nitrogen signal was captured during the sample years (figure 3).
Higher concentrations of nitrogen were seen in the cooler months of the year (figure 3). The DIN concentration in the upper section of the river was significantly higher than in the lower section or LNB (p-value < 0.001). The upper section averaged 0.025 ± 0.02 mg/L, well above the recommended maximum of 0.15 mg/L used in the Chesapeake Bay region. The lower section of the river averaged 0.07 ± 0.02 mg/L, while LNB averaged 0.02 ± 0.02 mg/L. Only LNB met the recommended maximum limit of 0.03 mg/L for DIN proposed by Yarish et al. (2006) for Long Island Sound.
The high rate of nitrogen loading to LNB was not reflected in the DIN water column concentrations, likely due to uptake of N by primary producers (macroalgae and phytoplankton) and a relatively quick freshwater flushing time for LNB of around 3 days (Fulweiler and Nixon 2005). In fact, the macroalgae biomass levels in the lower river and LNB were similar to the substantial biomass seen in other nutrient enriched systems which ranged from 650 g dry weight m-2 in Hog Island Bay, Virginia to 1800 g dry weight m-2 in Venice Lagoon, Italy (from: Havens et al. 2001 and Sfriso et al. 1992; as cited in Burkholder et al. 2007). The higher biomass in the lower sections of the river corresponded with a drop in the DIN water column concentrations.
Figure 3. Dissolved Inorganic Nitrogen (NH4+, NO3-, NO2-) data from Pawcatuck River and Little Narragansett Bay. Stations are listed consecutively, moving down the river from Westerly to Watch Hill and out to LNB. 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 (symbols as shown in legend) and the Banister / Pine Point School data set (indicated by hollow symbols with colors corresponding to the legend). The black line was the summertime average (May 15 to Sep 15) for the Pawcatuck River, Little Narragansett Bay not included. 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). No eelgrass was present in PR or LNB during this time period.
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While the water clarity (Kd) and water column nitrogen in LNB meet the standards necessary for eelgrass growth, eelgrass is still absent from LNB. Other factors are affecting the return of eelgrass to LNB:
Currently, the nitrogen load to the system stimulates relatively heavy macroalgae growth in LNB.
The presence of macroalgae keeps water column DIN concentrations low, giving the appearance of good water quality.
The presence of macroalgae may shade developing eelgrass, inhibiting the natural recolonization of LNB.
Recolonization of eelgrass in LNB may also be hampered by the presence of large numbers of swans (Cygnus spp.), known to graze on or uproot eelgrass seedlings (Johnson et al. 2007; Rivers and Short 2007).
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Batiuk, R. A., R. J. Orth, K. A. Moore, W. C. Dennison, and J. C. Stevenson. 1992. Chesapeake Bay submerged aquatic vegetation habitat requirements and restoration targets: A technical synthesis. Virginia Inst. of Marine Science, Gloucester Point (USA). Report. report number CBP/TRS-83/92.
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 & M. Teichberg. 2000. Chesapeake Bay submerged aquatic vegetation water quality and habitat-based requirements and restoration targets: A second technical synthesis. Annapolis, Maryland: United States Environmental Protection Agency. Report for the Chesapeake Bay Program. report number CBP/TRS 245/00 EPA 903-R-00-014.
Burkholder, J. M., D. A. Tomasko, and B. W. Touchette. 2007. Seagrasses and eutrophication. Journal of Experimental Marine Biology and Ecology 350: 46-72.
Dillingham, T. P., R. Abrams, A. Desbonnet, J. M. Willis, and M. G. Hart. 1992. The Pawcatuck River Estuary and Little Narragansett Bay: an interstate management plan. Rhode Island Coastal Resources Management Council and the Connecticut Department of Environmental Protection, Office of Long Island Sound Programs. report.
Doering, P. H., C. A. Oviatt, J. H. McKenna, and L. W. Reed. 1994. Mixing behavior of dissolved organic carbon and its potential biological significance in the Pawcatuck River estuary. Estuaries 17: 521-536.
Fulweiler, R. W. and S. W. Nixon. 2005. Export of nitrogen, phosphorus, and suspended solids from a southern New England watershed to Little Narragansett Bay. Biogeochemistry 76:567-593.
Havens, K. E. and others 2001. Complex interactions between autotrophs in shallow marine and freshwater ecosystems: implications for community responses to nutrient stress. Environmental Pollution 113: 95-107.
Johnson, M. and others 2007. An assessment of the impacts of commercial and recreational fishing and other activities to eelgrass in Connecticut's waters and recommendations for management. report. Connecticut Department of Environmental Protection and Connecticut Department of Agriculture
Rivers, D. O., and F. T. Short. 2007. Effect of grazing by Canada geese Branta canadensis on an intertidal eelgrass Zostera marina meadow. Marine Ecology Progress Series 333: 271-279.
Rozsa, R. 1994. Long term decline of Zostera marina in Long Island Sound and Fishers Island Sound. Office of Long Island Sound Programs, CT Department of Environmental Protection. report.
Sfriso, A., B. Pavoni, A. Marcomini, and A. A. Orio. 1992. Macroalgae, nutrient cycles, and pollutants in the lagoon of Venice. Estuaries 15: 517-528.
Surabian, D. 2007. Coastal Zone Soil Survey: Little Narragansett Bay, Connecticut and Rhode Island. report. USDA, Natural Resources Conservation Service
Tiner, R., H. Bergquist, T. Halavik, and A. MacLachlan. 2003. Eelgrass survey for Eastern Long Island Sound, Connecticut and New York. U.S. Fish and Wildlife Service, National Wetlands Inventory Program, Northeast Region, Hadely MA. National Wetlands Inventory Report. report.
Tiner, R., H. Bergquist, T. Halavik, and A. MacLachlan. 2007. 2006 Eelgrass Survey for Eastern Long Island Sound, Connecticut and New York. U.S. Fish and Wildlife Service, National Wetlands Inventory Program, Northeast Region, Hadely MA. National Wetlands Inventory Report. report.
2006 maps from Tiner et al. (2007) - Lookup "Eastern Long Island Sound 2006 Inventory Maps"
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 & R. Lewis. 2006. Environmental monitoring, seagrass mapping and biotechnology as means of fisheries habitat enhancement along the Connecticut coast. Stamford, Connecticut: University of Connecticut. Final Report submitted to the Connecticut Department of Environmental Protection, Hartford, CT. report number CWF 314-R.
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