Distribution and trophic importance of anthropogenic nitrogen in Narragansett Bay: An assessment using stable isotopes

Narragansett Bay has been heavily influenced by human activities for more than 200 years. In recent decades, it has been one of the snore intensively fertilized estuaries in the USA, with most of the anthropogenic nutrient load originating from sewage treatment plants (STP). This will soon change as tertiary treatment upgrades reduce nitrogen (N) loads by about one third or more during the summer. Before these reductions take place, we sought to characterize the sewage N signature in primary (macroalgae) and secondary (the hard clam, Mercenaria mercenaria) producers in the bay using stable isotopes of N (delta N-15) and carbon (delta C-13). The delta N-15 signatures of the macroalgae show a clear gradient of approximately 4 parts per thousand from north to south, i.e., high to low point source loading. There is also evidence of a west to east gradient of heavy to light values of delta N-15 in the bay consistent with circulation patterns and residual flows. The Providence River Estuary, just north of Narragansett Bay proper, receives 85% of STP inputs to Narragansett Bay, and lower delta N-15 values in macroalgae there reflected preferential uptake of N-14 in this heavily fertilized area. Differences in pH from N stimulated photosynthesis and related shifts in predominance of dissolved C species may control the observed delta C-13 signatures. Unlike the macroalgae, the clams were remarkably uniform in both delta N-15 (13.2 +/- 0.54 parts per thousand SD) and delta C-13 (-16.76 +/- 0.61 parts per thousand SD) throughout the bay, and the delta N-15 values were 2-5 parts per thousand heavier than in clams collected outside the bay. We suggest that this remarkable uniformity reflects a food source of anthropogenically heavy phytoplankton formed in the upper bay and supported by sewage derived N. We estimate that approximately half of the N in the clams throughout Narragansett Bay may be from anthropogenic sources.