Narragansett Bay is an estuary located southwest of Cape Cod contiguous with the Rhode Island and Long Island sounds. The bay is divided into an upper bay and a lower bay. The upper bay receives flow from the Providence River, which results in a gradient of low-to-high salinity moving from the upper to the lower bay. The mean residence time in Narragansett Bay is 26 days, varying from 10 to 40 days depending on the volume of freshwater input and wind conditions. There is a diverse macroalgae population dominated by red algae in Narragansett Bay.
Several assessment endpoints were reviewed in EPA’s case study of Narragansett Bay presented in the Agency’s Nutrients in Estuaries report. The authors discussed spatial and temporal conditions and trends for the endpoints: DO; turbidity and light; chlorophyll, primary production, algal blooms, and species composition; and benthic primary producers such as seagrass. All the assessment endpoints are affected either directly or indirectly by nitrogen and phosphorus loading and ambient nutrient concentrations in the bay.
The Narragansett Bay is an ecosystem that has been undergoing change since the early 1970s as the result of waste treatment upgrades and watershed management practices that have reduced suspended solid and nutrient pollution, including a 40-percent decline of phosphate concentration between the early 1970s and the late 1990s. Overall decreases in discharges of nutrients and suspended solids from wastewater treatment plans have reduced chlorophyll a levels and increased water clarity in the bay. the Providence River continues, however, to deliver a large amount of anthropogenic nutrients into the upper Narragansett Bay, causing eutrophication symptoms such as algal blooms and low DO in the summer months. Hypoxia-related finfish and shellfish kills caused by the decomposition of poorly grazed algal blooms have been documented.
Nutrient cycles impact primary production and the timing and intensity of algal blooms. Historically, an annual diatom bloom occurred during winter–spring, but recently has become a summer event. In the late spring–early summer, other algal species might bloom depending on nutrient conditions and grazer populations. Analysis of 5 years of weekly sampling in lower Narragansett Bay revealed that minimal concentrations of dissolved inorganic phosphorus (DIP) were observed during the winter–spring bloom, then concentrations rapidly rebounded during the summer before gradually declining through the fall and winter. Dissolved inorganic nitrogen (DIN) generally followed the same pattern except concentrations stayed low through the early summer and then increased in late summer and fall. Silicon (Si), a critical element for diatom growth, was elevated in the fall and early winter, declined during the winter–spring diatom bloom, and then recovered in the summer. Nutrients and Si in upper Narragansett Bay generally follow the same pattern as the lower bay but with a greater seasonal amplitude and elevated nutrients levels. Examining the mean ratios of nitrogen:phosphorus, nitrogen:silicon, and silicon:phosphorus provides insight into nutrient dynamics and influence on phytoplankton species dominance. Collectively, the nutrient ratios indicate that the upper bay is more sensitive to phosphorus and Si availability while the lower bay is more sensitive to nitrogen limitation.
Water quality stations established by various research groups are located throughout Narragansett Bay and are used to collect nutrient, Secchi disk, temperature, DO, and other data. Those data are used to examine spatial and temporal trends and relationships among parameters. For example, the salinity spatial gradient was used to plot spatial gradients for nutrients and other parameters. Linear regression of mean chlorophyll and mean salinity revealed the highest concentrations of chlorophyll a in mid-bay and declining levels both seaward and up-bay (National Estuarine Experts Workgroup 2010).
Reference:
National Estuarine Experts Workgroup. 2010. Nutrients in Estuaries: A Summary Report of the National Estuarine Experts Workgroup 2005–2007. U.S. Environmental Protection Agency. Accessed October 2016. https://www.epa.gov/sites/production/files/documents/nutrients-in-estuaries-november-2010.pdf.