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Analysis and Synthesis of Eutrophication-Related Conditions in Narragansett Bay (RI/MA USA): Updated Through 2019

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The response of eutrophication-related conditions in Narragansett Bay to managed long-term declines in nitrogen load is investigated, with emphasis on the whole-estuary perspective, using multiple independent monitoring datasets through 2019. Since 2013 when bay-wide load declines were substantially complete, there has been a notable decline, over at least several years, in excess chlorophyll and a decline in hypoxia at least as substantial. Although an upcoming year with summer river flow comparable to the wettest earlier years will be necessary for a more complete assessment, the bay appears to have reached a new state characterized by reduced chlorophyll levels and effectively
free from all but mild hypoxia.

There are clear relationships among nitrogen load, chlorophyll, and hypoxia for both inter-annual variability and long-term declines. Variability of both nitrogen load and stratification closely follows that of river flow, the most important driver of inter-annual changes. Stratification is more tightly correlated to river flow than nitrogen load, due to the long-term decline in the latter, which best explains the longterm chlorophyll and oxygen trends. Investigations of the potential importance of other physical influences (salinity, winds, tidal range, non-tidal sea level gradients) demonstrate that they play minor roles. In this sense, changes in the bay have followed expectations from conventional understanding of eutrophication: long-term load reductions have led to declines in chlorophyll and hypoxia. Until now this has been somewhat obscured by both the large range of inter-annual variability relative to the longterm changes and the difficulties in defining bay-wide indicators given the complex system geometry, numerous river flow and nitrogen load sources, and differing characteristics of available datasets. Similarity of results from two key complementary datasets for chlorophyll and oxygen supports confidence in the findings. To the extent climate-driven long-term warming (see next paragraph) is
causing long-term increases in oxygen consumption through metabolic rates, and reducing the oxygen saturation concentration, the observed declines in hypoxia are slower than they otherwise would be.

Long-term trends in temperature, salinity, and density stratification are evaluated for 2006-2019 using multiple independent monitoring datasets. Statistical strength of results is marginal but key results are consistent across datasets. For the summer-centered May to October period surface waters are warming at rates consistent with prior published results throughout the region, and near-seafloor temperatures are warming about 80% faster than at the surface, likely in association with offshore waters entering the bay as the deep inward limb of estuarine exchange flow. Annual-mean temperatures are warming but more weakly, due to long-term cooling of the winter-centered November to April period. Salinities are increasing, more strongly at the surface, consistent with the influence of relatively low river flow since 2013. Stratification is decreasing, at a rate that is weak in the sense that it would require at least a few decades to cause a substantial reduction to typical stratification strength; the decrease is predominantly due to salinity, which is increasing faster near the surface, but also due to the faster temperature increases at depth.

Although the information in this document has been funded by the EPA, it has not undergone the EPA’s publications review process and therefore, may not reflect the views of EPA and no official endorsement is inferred. The views expressed in this report are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency or the New England Interstate Water Pollution Control Commission. Any mention of trade names, products, or services does not imply an endorsement by the U.S. Government, the U.S. Environmental Protection Agency, or the New England Interstate Water Pollution Control Commission. The EPA and NEIWPCC do not endorse any commercial products, services, or enterprises.


USEPA CE00A00407