Summer Bottom Water Dissolved Oxygen in Upper Narragansett Bay

Abstract

Narragansett Bay is a medium-sized (328 km), relatively shallow (average depth 8.6 m), temperate latitude estuary located in the northeastern United States (Chinman and Nixon, 1985). With relatively low fresh water input (Pilson, 1985; Ries, 1990), the majority of Narragansett Bay is considered to be partially to well mixed (Hicks, 1959; Kremer and Nixon, 1978; Pilson, 1985; Nixon et al., 1995) and only moderately susceptible to adverse effects of nutrient loading, including low oxygen (hypoxic) conditions (Bricker et al., 1999). Despite Narragansett Bay’s purportedly low susceptibility to eutrophication, the mid-to-upper bay exhibits many symptoms of excessive nitrogen loading, including hypoxic conditions, macroalgae accumulation, eelgrass loss, and fish kills (RIDEM, 2003; Chapter 12). In response to these symptoms of eutrophication in Narragansett Bay, the Rhode Island Department of Environmental Management (RIDEM) has proposed reducing the summertime (May to October) nitrogen concentrations of wastewater effluent from major treatment plants to 5 mg L , which would be about a 50% reduction (Kerr, 1999; RIDEM, 2005). As part of a diverse effort to assess the effectiveness of nitrogen reduction measures, this study seeks to document the current state of summertime hypoxia in Narragansett Bay, and to assess its variability and predictability on the basis of spatial survey and continuous monitoring buoy data. In this chapter, the term ‘‘upper bay’’ refers to all of Narragansett Bay north of the southern end of Prudence Island, including the Providence River, Greenwich Bay, Mount Hope Bay, and the upper East and West Passages, i.e., the upper half of Narragansett Bay (Fig. 11.1). The more formal term ‘‘Upper Bay’’ is reserved for the area south of Conimicut Point and north of Prudence Island.