Further Analysis and Synthesis of Narragansett Bay (RI/MA USA) Oxygen, Chlorophyll, and Temperature

Several related analyses of Narragansett Bay oxygen, chlorophyll, temperature, and influencing factors (nitrogen load, river flow, salinity, stratification, physical drivers) through 2017 are reported to help improve understanding of hypoxia and eutrophication. They build on and address identified gaps in the findings, on years through 2015, of the State of Narragansett Bay and Its Watershed (Narragansett Bay Estuary Program, 2017). Daily-resolution 2001-2017 bay-wide loads of total nitrogen from wastewater treatment facilities and surrounding watersheds are estimated and interpreted in a companion technical report. The seasonal Hypoxia Index results from fixed-site monitoring network near-bottom oxygen time series are updated to add, to the 10 sites previously treated, the shallow Phillipsdale Landing site in the estuarine headwaters of the tidal Seekonk River. At this location hypoxia is the most severe and occurs not only in years with high river flow but also, unlike other sites, in many dry years which may be related to longer water residence times. For both the relatively dry year 2016 and the relatively wet year 2017 hypoxia was among the least severe to date bay-wide. Weaker hypoxia in 2017 than prior years with comparable river flow is one of the strongest indications that bay hypoxia has responded to reduced nutrient loads. When river flow is intermediate both stratification and nitrogen load are generally intermediate, but either strong or weak hypoxia has occurred, suggesting that other factors are important under these conditions; temperature and physical drivers including wind speed and direction, tidal range, and non-tidal sea level difference between Providence and Newport are examined and found not to explain the variability. Oxygen and chlorophyll measurements from vessel-based spatial surveys are analyzed together with the fixed site time series to explore their complementary strengths, determine spatial and temporal decorrelation scales, and quantify correlations between spatial survey percent area metrics and time series metrics. The seasonal Chlorophyll Index from time series measurements, refined here to better capture regional patterns and changes, reveals inter-annual variability that is more independent from site to site and less tightly linked to river flow than oxygen. Spatial survey chlorophyll, from all areas of the bay both shallow and deep, shows a stronger decline during the past several years after load decreases than is seen in the Chlorophyll Index from time series observations, which are mostly from deeper locations; however, in 2017 both declined markedly. Longterm trends spanning all years of fixed site time series observations include warming of surface waters at a rate comparable to that seen in independent analyses region-wide, and warming of deep waters at a rate about twice as high. Salinities are decreasing, more strongly near the surface. Stratification is declining, due mainly to salinity and also weakly due to temperature, but at a rate unlikely to strongly impact hypoxia.


NOTE: For Development of Nitrogen Load Time Series (Task 2) companion report and all supporting information see https://figshare.com/s/95abe0296139515ffb88

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