CCOM/NOAA-UNH JHC Seminar Series 

Summertime Hypoxia and Water Quality Improvements in Narragansett Bay, RI 

Abstract 
Spatial surveys of dissolved oxygen (DO) and water column properties are used to assess the extent of hypoxia in mid and upper Narragansett Bay, RI. Following a major fish kill in the summer of 2003 attributed to hypoxic conditions driven by excess nutrient discharges from waste water treatment facilities (WWTF’s), the WWTF’s were mandated to decrease total nitrogen (TN) loading by half during summer months. Excess TN fuels a cycle of biological production that then decays, drawing down DO levels in the waters below the pycnocline, especially when the water column is stratified in summer months.  Major decreases in TN occurred between 2004-2006 and 2011-2013 so that currently the WWTF TN contribution to the Bay has decreased by about 70%.  The spatial surveys sample about 70 stations to quantify the area of the bay where bottom water is hypoxic (<2.9 mg/l).  The % hypoxic area is an integrated and reliable indicator because of the geographical distribution of stations and the number of surveys (58) over the period from 2005-2015.  We find that the area of hypoxia varies between about 2% and 60% inter-annually and within seasons but seems to have decreased since 2010.  Much of the seasonal-scale variability is correlated to “wet summers” where above average river flux is correlated with greater areas of hypoxia.