Anoxia Mystery

A five-year look at satellite imagery raises questions about the reason for die-offs of marine creatures at the bottom of Long Island Sound.

For 30 years people have worried about mass die-offs of lobster, crabs and other bottom-dwelling creatures in estuaries and bays of North America.

One reason for the die-offs is known — a lack of oxygen in deep water especially in spring and summer. Less understood is why this condition — called anoxia — has occured in recent decades. It’s been is linked to unnaturally large plankton blooms, caused by runoff from farmland and wastewater treatment plants.

That’s why Martha Gilmore of Wesleyan University examined satellite data for a link between anoxia and plankton volume. She and a student used satellite images of Long Island Sound from 1997 to 2002 to estimate the amount of plankton in the waters — and compared that to measured oxygen levels in bottom waters.

Martha Gilmore: We didn’t find a correlation at all. The amount of plankton in the Sound made no difference in the onset or duration, or severity of the anoxia event for each of those years.

So the link between agricultural and wastewater runoff — and the die-offs of bottom-dwellers in North American bays and estuaries — doesn’t look as strong. Gilmore believes the die-offs might be related instead to a warming of these waters.

Gilmore’s result throws into question the long-held belief that human-induced high nitrogen levels in Long Island Sound are to blame for bottom-water anoxia. What could be to blame, Gilmore now suggests, is thermal stratification of waters in the sound. In other words, warmer air temperatures could be exaggerating the temperature difference between surface and bottom waters, and making it harder for warm, oxygen-rich surface waters to mix and deliver oxygen to the cooler bottom waters.

Gilmore’s observations agree with previous work done by Robert Wilson, a marine scientist at the State University of New York, Stony Brook. Wilson has shown that temperature can explain the majority of anoxic events. Two other Wesleyan University researchers are looking into the question. Johan Varekamp and Ellen Thomas are currently working on building computer models of the thermal stratification of Long Island Sound.

Martha Gilmore’s study of the waters of Long Island Sound was conducted with data from an ocean-viewing satellite known as SeaWiFs.

An instrument aboard the satellite is designed to measure chlorophyll in ocean water, and thus can be used to determine the amount of plankton in any given ocean location. Gilmore and graduate student, Taras Gapotchenko, sought to measure plankton volume in Long Island Sound over five years to see if there was any connection between plankton amount and episodes of anoxic die-off of marine bottom-dwellers there.

They ran into problems, however, because the satellite has trouble measuring chlorophyll in water that isn’t completely pure and clear. Because estuaries and bays are famously mixed with all kinds of sediment and contaminants, Gilmore and Gapotchenko were not able to get the quantitative measurements they wanted. But they were able to get some qualitative measurements of plankton volume by looking at images from a Landsat satellite from 1997 to 2002.

With Landsat, Gilmore and Gapotchenko got good, cloud-free pictures for about 10 days each month, and were able to see the extent of plankton growth in the sound, Gilmore said, “You can see in this data, the plankton color bloom. And so even though it’s not quantitatively correct, you can still, qualititatively see the extent of this area that’s greener than the other portions of the water.”

They then compared the amount of plankton in the sound throughout the study period to measurements of oxygen levels in the bottom waters. They were surprised to find no correlation between the two.

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