URI scientists report Greenwich Bay fish kill not unexpected

The fish kill of this past summer again brought Greenwich Bay’s water quality problems to the surface along with the dead fish. However, as one scientist puts it, “This is not necessarily an unnatural or unexpected event. And I do not think there will be any long-term ecological impact,” says Scott Nixon, URI oceanography professor. “It is certainly an unpleasant event for the folks around the affected areas, but neither Greenwich Bay nor Narragansett Bay is dying.”

So what caused the anoxic conditions that resulted in the death of
marine organisms from the Providence River to Greenwich Bay? “It is a transient condition arising from an unfortunate combination of circumstances— large freshwater inputs from storms added lower-density fresh water to the system, where it remained in the surface layers,” explains Nixon. “The very hot, windless days following the rain intensified the density stratification since solar warming of the surface water also makes it less dense and there had been little wind to mix the water column.”

In addition, “Greenwich Bay is characterized by generally weak circulation, particularly in the isolated coves,” says Malcolm Spaulding, URI ocean engineering professor. “And the tidal range and time lag for high-to-low tide are small across the bay,” thereby exacerbating the situation.

“The bay is a shallow, very productive area. That is why there are so many shellfish and other benthic critters living there—and the bottom communities and the bottom water have a high rate of oxygen demand. There is not a large volume of water below the fresher water-saltier water density change layer—called the pycnocline—so there is not a large store of oxygen available to support the animals once stratification sets in,” relates Nixon. Furthermore, the water is warm during the summer, thereby holding less oxygen. And because of the proliferation of organisms in the summer, respiration rates are high, so it doesn’t take long to use up the oxygen and turn the bottom anoxic.

Menhaden is a surface-feeding species that is sensitive to low dissolved oxygen. If exposed to hypoxic or anoxic water, these fish commonly die. Nixon suspects that menhaden are one of the species most commonly involved in low-oxygen fish kills along the Atlantic and Gulf coasts—in spite of their being surface fish.

How did they get exposed? “My hypothesis is that the fish were along the western or northern side of Greenwich Bay and we got some wind blowing from west to east or from north to south, which caused a local upwelling of the anoxic bottom water
—in other words, the wind pushed (actually dragged) the surface water away from shore and deeper water came up to replace it. Thus, the fish were quickly hit with a fairly large area of anoxic water at the surface and they died from the anoxia before they could escape,” suggests Nixon.

As for the clam kill, some species are better able to cope with anoxic conditions. “Hard clams can survive quite a while and we didn’t hear about them dying. Mya [soft-shelled clam] is obviously more sensitive. But even though it looks like a lot of dead animals on the beach, there are lots of other steamers in the bay. A good set will easily replace those lost to this summer’s anoxia.”

So what about the reports about pollution from sewage and runoff being to blame for the fish kill? “I don’t believe there is any evidence to suggest a trend of markedly increasing nitrogen input to Greenwich Bay nor to suggest that fish kills will be an every summer thing,” says Nixon. “We had some very unfortunate meteorology this summer. Narragansett Bay—and by association, Greenwich Bay—receives a lot of anthropogenic nitrogen. It has for a century or more. That nitrogencontributes to the great productivity of the Bay.”

Good circulation throughout most of Narragansett Bay usually mitigates the harmful effects of eutrophication. “We don’t usually have such severe bottom-water oxygen problems because we are blessed with a physical setting that favors a relatively well-mixed water column: very low fresh-water input, a north-south alignment of Narragansett Bay that gives a long fetch for the prevailing winds to achieve good mixing, and an energetic tidal circulation over most of the Bay,” explains Nixon.

ut Greenwich Bay is not as well-mixed as Narragansett Bay as a whole. Spaulding found that wind is important to tidal exchange between Greenwich Bay and Narragansett Bay. When there is no wind, the water remains stratified and stays within the confines of Greenwich Bay.

“Every once in a while, our luck runs out and we don’t get to have our cake and eat it too; that is, we don’t get to dump a lot of nitrogen into Narragansett Bay—or Greenwich Bay—get high productivity, and escape from anoxic or hypoxic bottom water,” says Nixon.

Nixon and Spaulding led the research effort as part of the 1996 Greenwich Bay Initiative—a collaborative research effort among Rhode Island Sea Grant, the city of Warwick, and the R.I. Department of Environmental Management (RIDEM) to determine specific means to restore water quality in Greenwich Bay. Sea Grant researchers investigated generation, transport, fate, and impact of contaminants on the Greenwich Bay ecosystem. They also studied pollutant loading, the impact of nutrients on dissolved oxygen levels, eutrophication, and coastal management options. For more information on the Greenwich Bay Initiative’s findings or to learn more about estuaries, visit Sea Grant’s Greenwich Bay Website at: seagrant.gso.uri.edu/G_Bay/.

—Malia Schwartz is Communications
Director for Rhode Island Sea Grant.

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