PARENT SESSION
Oral Session #35: Nitrogen cycling: Response to inputs, variation in time and space. Presiding: M. Fenn.
Tuesday, August 7, 2001. 1:00 PM to 5:00 PM. Hall of Ideas G.

The fate of nitrogen inputs to coastal lagoons of Narragansett Bay, Rhode Island.

Tartowski, Sandy¹, Marino, Roxanne¹, Howarth, Robert¹, Chan, Francis¹, ¹

ABSTRACT- Human activities have substantially increased the input of nitrogen to coastal estuaries in the Northeastern US, but the processing and fate of this additional N is poorly known. We added ¹⁵N-labeled dissolved organic nitrogen (DIN) to mesocosms containing water (1.1 m depth), sediments (0.3 m depth), eelgrass, drift macroalgae, plankton, epiphytes, fish, shellfish, invertebrates, and other organisms characteristic of coastal lagoons of RI. Mesocosms were flushed 10% vol./da. with Bay water and experienced light, temperature, precipitation, mixing, salinity and oxygen conditions similar to the normal conditions in nearby coastal lagoons. Nitrogen was added to the water of 7 mesocosms during the summer growing season at an average loading of 1.5 mmol/m2/da. Phosphorus was added to produce inputs with N:P=12). We measured ¹⁵N and N pools and fluxes in the components of mesocosms. Added N was removed rapidly from the water column by phytoplankton, epiphytes, macroalgae and eelgrass, producing delta ¹⁵N labels of 2000-4000 per mil. Seagrass effectively accumulated N in actively growing leaves typically with delta 15N near 1000 per mil. The rapid accumulation and regeneration of N in the sediments was particularly dynamic, suggesting strong coupling between water column and sediment processing of nitrogen.

KEY WORDS: nitrogen, estuary, coastal, 15N