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Low dissolved oxygen events were characterized in Narragansett Bay (NB), a moderate-size (370 km2) temperate estuary with a complex passage/embayment geometry, using time series from 2001 to 2006 at nine fixed-site monitoring stations. Metrics for event intensity and severity were the event-mean deficit relative to a threshold (mg O2 l−1) and the deficit-duration (mg O2 l−1 day; product of deficit and duration [day]). Hypoxia (threshold 2.9 mg O2 l−1) typically occurred intermittently from late June through August at most stations, as multiple (two to five per season) events each 2 to 7 days long with deficit-duration 2 to 5 mg O2 l−1 day. Conditions were more severe to the north and west, a pattern attributed to a north–south nutrient/productivity gradient and east–west structure of residual circulation. Spatial patterns for suboxic and severely hypoxic events (thresholds 4.8 and 1.4 mg O2 l−1) were similar. The view that different processes govern event variability in different regions, each influenced by local hydrodynamics, is supported by both weak spatial synchronicity (quantified using overlap of event times at different sites) and multiple linear regressions of biological and physical parameters against event severity. Interannual changes were prominent and season-cumulative hypoxia severity correlated with June-mean river runoff and June-mean stratification. Benthic ecological implications for areas experiencing events include: NB hypoxia classifies as periodic/episodic on a near-annual basis; highest direct mortality risk is to sensitive and moderately sensitive sessile species in the northern West Passage and western Greenwich Bay, with some risk to Upper Bay; direct risk to mobile species may be ameliorated by weak spatial synchronicity; and indirect impacts, including reduced growth rates and shifts in predator–prey balances, are very likely throughout the sampled area due to observed suboxic and hypoxic conditions.

Narragansett Bay, RI, is considered to be a relatively well-mixed estuary not subject to extensive seasonal stratification and hypoxia. However, results of surveys of dissolved oxygen (DO) for the upper half of Narragansett Bay on August 15, 2001 and on August 6, 2002 have documented evidence of wide-area intermittent subpycnoclinal hypoxia (≤ 3 mg l−1). For the August 2001 survey, severe hypoxic to near-anoxic levels were confined to the Providence River, the western side of Greenwich Bay, and a small area of Mount Hope Bay, but hypoxic levels below 2 mg l−1 were also experienced on the western side of the Upper Bay in an extensive, shallow oxygen minimum. Hypoxic bottom waters (≤ 3 mg l−1) extended from the Upper Bay into the upper West Passage. Hypoxic waters covered approximately 66 km2 (36%) of the survey area for August 15, 2001. A more extensive and severe hypoxic event occurred during the August 2002 survey, when near-bottom waters of the entire Providence River and a large area of the Up...

Coastal ecosystems are affected by ever-increasing natural and human pressures. Because the physical, chemical, and biological characteristics unique to estuarine ecosystems control the ways that biological resources respond to ecosystem stressors, we present a flexible and adaptable biological assessment method for estuaries. The biological condition gradient (BCG) is a scientific framework of biological response to increasing anthropogenic stress that is comprehensive and ecosystem based and evaluates environmental conditions and the status of ecosystem services in order to identify, communicate, and prioritize management action. Using existing data, we constructed the first estuarine BCG framework that examines changes in habitat structure through time. Working in a New England (U.S.) estuary with a long history of human influence, we developed an approach to define a reference level, which we described as a “minimally disturbed” range of conditions for the ecosystem, anchored by observations before 1850 AD. Like many estuaries in the U.S., the relative importance of environmental stressors changed over time, but even qualitative descriptions of the biological indicators’ status provided useful information for defining condition levels. This BCG demonstrated that stressors rarely acted alone and that declines in one biological indicator influenced the declines of others. By documenting the biological responses to cumulative stressors, the BCG inherently suggests an ecosystem-based approach to management. Additionally, the BCG process initiates thinking over long time scales and can be used to inspire scientists, managers, and the public toward environmental action.

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.

Larvae of two fish species, Menidia menidia and Paralichthys dentatus, were subjected to a 15-min thermal shock of 10 C above acchmation temperature, returned to acclimation temperature, and exposed to a predator, Fundulus majalis. Four- and six-week-old M. menidia were significantly more vulnerable to predation after being shocked than control larvae, but younger larvae were not. Shocked larvae of P. dentatus were less susceptible to predation than control larvae.