SPATIAL AND TEMPORAL VARIABILITY IN BIOGEOCHEMISTRY ON THE CONTINENTAL SHELF OFF RHODE ISLAND

The continental shelf region off the coast of Rhode Island is ecologically and economically important and is likely to be impacted by a changing climate. Using available archived data for Rhode Island Sound (RIS) and Block Island Sound (BIS), decadal patterns in observed biogeochemical data were examined and compared to extensive data sets from adjacent water bodies, including Narragansett Bay, Long Island Sound (LIS), Gulf of Maine, and Nantucket Shoals. Chlorophyll a and nitrate/nitrite concentrations in particular were examined for this study as they are being used in conjunction with development and validation of a coupled physical-ecological-biogeochemical model integrating nutrient input in the form of nitrogen, phytoplankton and zooplankton dynamics, and detrital remineralization of organic matter. Chlorophyll a concentrations varied widely by decade and between waters bodies. In the 1970s decadal average chlorophyll concentrations were similar in BIS and RIS, 1.50 ± 0.27 mg m^-3 and 1.39 ± 0.97 mg m^-3 respectively, and both were lower than in adjacent water bodies by up to a factor of 5. In the period between 2008-2010, BIS average chlorophyll a concentrations were higher than in the 1970s, 2.69 ± 1.28 mg m^-3 in BIS and 2.43 ± 0.86 mg m^-3 in RIS, which were lower than averages in adjacent water bodies by up to a factor of 2. There was a distinct gradient in chlorophyll a concentrations between RIS and BIS observed during summer months during both decades. A similar investigation of nitrate/nitrite demonstrated a relatively stable seasonal cycle with lowest nitrogen in summer months for all regions and in all decades where data were available. The decadal differences in nitrogen concentration were measurable, with the smallest decadal difference between the 1970s and 2000s observed in Narragansett Bay, where the 1970s nitrogen concentration were 34% higher than the average concentration from 2000 to 2009. On the eastern LIS/BIS border the average nitrogen concentration in 1973 was 89% higher than in 2008. The results demonstrate the dynamic biogeochemistry of the region and are being used to further investigate shelf processes, such as exchange with estuarine and sediment boundaries, and potential impacts of a changing climate to the region.