Hypoxia

(Low Dissolved Oxygen)

Dissolved oxygen (DO) in the water is critical in sustaining a diverse and healthy estuarine ecosystem. Low levels of DO signal degraded health due to excessive nutrients which can cause rapid growth (blooms) of microscopic phytoplankton and larger macroalgae (seaweed). As the blooms decompose, bacteria consumes oxygen, reducing DO. Oxygen can reach stressfully low levels (hypoxia), sometimes killing fish and other organisms. In Narragansett Bay, hypoxia occurs during hot, calm summer periods, when the water is “stratified” or layered, preventing oxygen from reaching bottom waters. High river flows can increase both stratification and the level of nutrient loads to the Bay. In some areas of the Bay, events occur during which DO approaches zero (anoxia) in bottom waters, killing all but the hardiest organisms.

Dissolved oxygen should not fall below 2.9 milligrams per liter (mg/l) for more than 24 consecutive hours or 1.4 mg/l for more than one hour (RI criteria). Summers with large June river flows tend to experience more severe hypoxia in July or August of that summer, during periods of hot, calm weather and weak tides. Though hypoxic events occur most frequently in July and August, they can also happen in June and September.

Although severe hypoxia and anoxia can cause large fish kills as occurred on Narragansett Bay in August 2003, a more lasting impact is experienced by permanently-attached benthic (bottom-dwelling) biological communities. Impacted areas tend to have lower diversity and abundance of large benthic organisms, although quahogs, a hardy species, are less affected than most. In areas experiencing low DO (hypoxia) on the bottom, other large burrowing organisms are replaced by small surface-dwelling worms. This affects the way the bottom of the Bay recycles nutrients, further affecting the whole system.

Beginning in 1999, DO in the Bay has been measured through several different federally–funded monitoring programs. The datasets complement each other by providing in combination both the timing and extent of oxygen conditions in much of the Bay, allowing for reasonable estimates of the percentage of Bay bottom waters experiencing hypoxia and violating water quality criteria. Long-term trends are not yet apparent in the data because of year-to-year variability in summer conditions due to changes in river flow, water temperature and winds.

About 33 percent of Rhode Island’s estuarine waters are impaired by hypoxia. The Seekonk River, which has high nutrient loads from both Rhode Island and Massachusetts sources, experiences the most severe hypoxic events (0-1 mg/L), with the western side of Greenwich Bay a close second (see red and orange areas of the map). In the Providence and Seekonk Rivers, these events can last two weeks or more, while in shallow Greenwich Bay, where tides and winds can rapidly mix the water column, up to 12 usually short-lived (1 to 3 day) events occur each summer.

Factors implicated in Greenwich Bay’s poor water quality include the groundwater nutrient load from septic systems and poor circulation in the western side of the embayment. A century or more ago, Greenwich Bay was a high-quality habitat area, with extensive eelgrass beds and significant scallop resources. Its condition today serves as a warning that small embayments are vulnerable to nutrient loads. The Upper Bay area north of Prudence Island experiences events of one day to about a week, with rare longer events. Limited data suggest that the upper West Passage sometimes experiences severe hypoxic events ranging from five to 20 days. Even at Fox Island, two-thirds down the Bay, severe low oxygen has been recorded (less than 2 mg/L in 2006). Mount Hope Bay experiences mild hypoxia, only occasionally dropping below 3.0 mg/L. Only the mouths of the rivers in Mount Hope Bay exhibit significant concentrations of nutrients and chlorophyll, as well as poor oxygen levels.

Continue to Beach and shellfish closures