Distribution and Trophic Importance of Anthropogenic Nitrogen in Narragansett Bay: An Assessment Using Stable Isotopes

Abstract

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.

Can mussels be used as sentinel organisms for characterization of pollution in urban water systems?

“…mussels) Cabana and Rasmussen, 1996;Harrington et al, 1998;Oczkowski et al, 2008;Carvalho et al, 2015). In addition and identical to our study, the range of δ 15 N values for nitrate and POM has been shown to be wider than the range for primary producers, indicating a timeaveraging effect in mussels (Gustafson et al, 2007;Wang et al, 2013).…”

“…We also found a positive relationship between food (POM) and mussel δ 15 N, but a negative relationship between nitrate δ 15 N and consumers (mussels), which was strongly affected by site MC. Such negative relationships were previously found in systems with very high nitrogen concentrations (DIN > 40 µM) (Oczkowski et al, 2008), because in these systems primary producers can be choosy and will preferentially uptake lighter NO x , leading to a higher fractionation at higher concentrations (Lake et al, 2001;Oczkowski et al, 2008). Therefore, the residual NO x in those waters retains more 15 N-enriched material, leading to a positive relation-ship between nitrogen concentration and nitrate δ 15 N, while consumers which incorporate primary producers will have a lighter signature.…”

“…Therefore, the residual NO x in those waters retains more 15 N-enriched material, leading to a positive relation-ship between nitrogen concentration and nitrate δ 15 N, while consumers which incorporate primary producers will have a lighter signature. Because such fractionation is unlikely at TDIN concentrations below 1 µM (Oczkowski et al, 2008), this mechanism is unlikely for most of our sites where mean TDIN concentration was < 1.5 µM. This is also supported by the lack of relationship between mussel δ 15 N and TDIN concentration when omitting MC.…”

“…Oczkowski et al, 2008), which can lead to eutrophication. In urban estuaries, tributaries often transport high amounts of nitrogen from the watershed into the estuary, causing water-quality problems including toxic bloom development (Hamilton, 2000;Atkins et al, 2001).…”

Oyster and Macroalgae Bioindicators Detect Elevated δ15N in Maryland’s Coastal Bays

“…Though oyster muscle δ 15 N gradients were slight, homogeneously elevated values implicated septic sources of nitrogen in southern Chincoteague Bay, likely from the town of Chincoteague, VA. In a similar study, spatial homogeneity of elevated δ 15 N among hard clam tissues (Mercenaria mercenaria) along a eutrophication gradient has been attributed to anthropogenic sources, suggesting that elevated δ 15 N in mollusks can still indicate nitrogen source despite a lack of spatial pattern (Oczkowski et al 2008). Furthermore, oyster δ 15 N values in this study were similar, though somewhat lower (8.5±0.1‰), to muscle tissue (9.4±0.2‰ and 16.0± 2.3‰) of an Australian oyster species (Saccostrea glomerata) influenced by wastewater treatment effluent within 50 m (Piola et al 2006).…”