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Reversal of the net dinitrogen gas flux in coastal marine sediments


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 consumption1,2. 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 ocean3,4. Nitrogen fixation, however, is thought to be a negligible process in sub-tidal heterotrophic marine systems5. 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 production6,7, 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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Figure 1: Multi-decadal mean annual water column chlorophyll a concentrations.
Figure 2: Mean summer (17–23 °C) N 2 fluxes in mid-bay.
Figure 3: Net N 2 fluxes measured across a gradient of organic matter loading in mesocosms at the Graduate School of Oceanography.


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This material is based on work supported by the National Science Foundation to the Rhode Island EPSCoR programme, as well as the Rhode Island Sea Grant College Program (NOAA), the Rhode Island Coastal Resource Management Council, and the Switzer Foundation. We acknowledge and appreciate the assistance of A. Giblin, J. Tucker and K. Koop-Jacobsen of the Marine Biological Laboratory and J. Cornwell, M. Owens, T. Kana, and J. Seabrease of the University of Maryland. We also thank H. Paerl and K. Rossignol of the University of North Carolina. In addition we thank M. Hayn at Cornell University and L. Cole at the University of Virginia. This manuscript was improved with comments from A. Giblin, M. E. Q. Pilson and B. Jenkins. For field assistance we thank P. DiMilla, K. Hanks, K. Henry, M. Horn, J. Krumholz, C. Mueller, A. Oczkowski, A. Pimenta, M. Traber, and N. Hovey. We thank L. Harris for assistance with the primary production model.

Author Contributions R.W.F. was responsible for sediment collection as well as sample and data analysis for net N2 measurements. R.W.F. and S.W.N. co-wrote this manuscript. B.A.B. was responsible for statistical analysis. B.A.B and S.L.G. supervised the large-scale mesocosm experiment.

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Correspondence to R. W. Fulweiler.

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Fulweiler, R., Nixon, S., Buckley, B. et al. Reversal of the net dinitrogen gas flux in coastal marine sediments. Nature 448, 180–182 (2007).

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