Low Oxygen Levels Choke Parts of Bay

By ecoRI News staff

A healthy Narragansett Bay eco­system supports valuable fisheries and a wide variety of other marine life. Marine water quality is a complex issue that is measured in a variety of ways. This year, Water­shed Counts focused on dissolved oxygen, just one of many important metrics of marine water quality.

All aquatic organisms depend on dissolved oxygen that is found throughout the water column. But under certain conditions, this oxygen can be depleted. Low dissolved oxygen conditions, referred to as hypoxia, impacts the bay’s life. Fish, crabs, shrimp and shellfish that can’t flee poor water quality conditions become stressed or die. Additional effects ripple throughout the ecosystem.

The state of Rhode Island evaluates water quality by com­paring measured conditions to standards designed to protect the environment. The state’s assessment of dissolved oxygen looks at the frequency, severity and duration of hypoxia during the period of the year considered important to the reproduction and growth of aquatic organisms — May to Octo­ber.

For example, in the lower portions of the water column, the criteria specify that the dissolved oxygen concentration shouldn’t fall below 2.9 mg/l oxygen for more than a 24-hour period. When the assessment finds hypoxia occurring in a manner that exceeds the criteria, then those coastal waters are designated as “impaired.”

The state assessments conducted by both the state Depart­ment of Environmental Management (DEM) and Massachusetts Department of Environmental Protection (DEP) are summarized in reports to the Environmental Protection Agency (EPA). The 2010 reports were used as the basis of calculating the area of Narragansett Bay that is considered impaired due to low dis­solved oxygen.

Narragansett Bay is 148.6 square miles, with most of the bay (140 square miles) in Rhode Island. A total of 53.8 square miles (47.6 in R.I. and 6.2 in Mass.) or 36 percent of the bay is impaired because of low dissolved oxygen.

The waters of upper Narragansett Bay are the most impacted. Hypoxic events are more frequent and may range from days to weeks. Weather patterns result in variability in the severity of hypoxia from year to year. Generally, the estuarine Seekonk River tends to experience among the most serious and consistently low dissolved oxygen levels.

Conditions improve as you move down the bay along a north to south gradient. Greenwich Bay also is prone to low oxygen conditions during the summer when weather con­ditions (high temperatures and little wind) can intensify stratification.

Among the factors affecting hypoxia, those that we can control the most effectively are the point and non-point discharges that carry pollutants to the bay. Wastewater treat­ment facility discharges are the largest source of nutrients to the estuarine Providence and Seekonk rivers in the upper bay. They deliver about 70 percent of the total nitrogen to the upper bay either through discharges into the estuary or its tributary rivers. Controls on wastewater treatment facili­ties in both Rhode Island and Massachusetts are being pursued in order to abate hypoxia in the upper bay.

As loadings from wastewater sources are controlled, non-point sources will contribute a larger percentage of the total nitrogen to the upper bay. Improving stormwater management through green infrastructure approaches and expanding the use of other appropriate best management practices will need to part of a broader strategy for achieving and sustaining improved marine water quality.