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Narragansett Bay has been heavily influenced by human activities for more than 200 years. In recent decades, it has been one of the more 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 (δ 15 N) and carbon (δ 13 C). The δ 15 N signatures of the macroalgae show a clear gradient of approximately 4‰ 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 δ 15 N 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 δ 15 N values in macroalgae there reflected preferential uptake of 14 N 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 δ 13 C signatures. Unlike the macroalgae, the clams were remarkably uniform in both δ 15 N (13.2±0.54‰ SD) and δ 13 C (−16.76±0.61‰ SD) throughout the bay, and the δ 15 N values were 2-5‰ 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.

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During the last several decades, the waters of mid Narragansett Bay, Rhode Island have increased in temperature and decreased in chlorophyll concentration, and it is possible that these changes affected the growth and success of a common benthic filter feeder, the hard clam, Mercenaria mercenaria. We determined recent hard clam growth rates through a sclerochronological analysis and compared them to the rich historical record of Narragansett Bay growth rates in order to understand how these opposing changes influenced hard clam growth. We found no significant differences in short-term growth between 1985 and 2000. Long-term juvenile growth showed a significant decrease between the 1960s and 1990s, while long-term adult (mature) growth showed a significant increase over this same time period. While it is not clear why the changes in juvenile and adult growth rates differ, it appears as though the decrease in chlorophyll concentration, together with a change in phytoplankton community composition, increasing water temperature, and an increase in predator abundance, may all have influenced hard clam growth between the 1960s and the 1990s.