The flux of nitrogen from land and atmosphere to estuaries and the coastal ocean has increased substantially in recent decades. The observed increase in nitrogen loading is caused by population growth, urbanization, expanding water and sewer infrastructure, fossil fuel combustion and synthetic fertilizer consumption. Most of the nitrogen is removed by denitrification in the sediments of estuaries and the continental shelf, leading to a reduction in both cultural eutrophication and nitrogen pollution of the open ocean. Nitrogen fixation, however, is thought to be a negligible process in sub-tidal heterotrophic marine systems. Here we report sediment core data from Narragansett Bay, USA, which demonstrate that heterotrophic marine sediments can switch from being a net sink to being a net source of nitrogen. Mesocosm and core incubation experiments, together with a historic data set of mean annual chlorophyll production, support the idea that a climate-induced decrease in primary production has led to a decrease in organic matter deposition to the benthos and the observed reversal of the net sediment nitrogen flux. Our results suggest that some estuaries may no longer remove nitrogen from the water column. Instead, nitrogen could be exported to the continental shelf and the open ocean and could shift the effect of anthropogenic nitrogen loading beyond the immediate coastal zone.

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An ecosystem-level experiment was conducted to identify the nutrient most limiting to productivity and biomass in the marine lagoons of the northeast United States. Mesocosms containing a complex of species characteristic of shallow coastal marine environments were enriched with P alone, N alone, or combined N plus P, at loadings typical of highly enriched natural lagoons. The mesocosms showed significant responses to ennchment with N alone but not P alone, indicating limitation by N. Enrichment with N alone caused increased water column concentrations of chlorophyll a and particulate nitrogen [PN), increased water column daytime net production (NP), and increased rates of growth of juvenile winter flounder. It also caused eelgrass beds and mats of drift macroalgae to decline, apparently in response to phytoplankton shading. Comparison of the N-alone and combined N + P treatments indicated that when enriched with N alone, the limitation of the systems shifted to P limitation of total system metabolism and of phytoplankton production and standing crop, and to light limitation of eelgrass and macroalgal growth. In the combined N + P mesocosms, water column concentrations of chlorophyll a, PN, and particulate P, rates of total system and water column NP and night-time respiration, and growth rates of juvenile winter flounder and killlfish were all increased relative to the N-alone mesocosms. Declines of eelgrass and macroalgae were also more severe.

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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