Thursday, August 11, 2005

Why the Western Sound is in Such Bad Shape

The far western end of Long Island is in bad shape this week. In the bottom waters between Westchester and Nassau counties there is so little dissolved oxygen that it's as if a wall has been erected to keep fish out of an area in which they'd normally be abundant. At Execution Rocks, off New Rochelle, DO concentrations are 0.3 milligrams per liter -- that is, just 4 percent of saturation. Essentially nothing can live there. And it's been that way for about a month.

My impression from looking at the Connecticut DEP water quality maps (the link is on the right) is that in the area between Westchester and Nassau, this is one of the worst summers for hypoxia in a long time. How does hypoxia -- a severe shortage in dissolved oxygen -- happen? Here's how I described it:

The tides ... infuse the estuary with oxygen, carrying it from the oxygen-rich ocean or roiling the surface so it mixes in from the atmosphere. The waves and winds that chop the water into whitecaps also add oxygen, as does the mass of vegetation in an estuary, through photosynthesis.

Oxygen concentrations fall naturally in an estuary in summer. As the water grows warmer, its ability to hold oxygen diminishes. But the deeper water stays cooler than the surface, and so the estuary stratifies into two layers, warmer water floating on top of cooler water. The stratification is strong; the two layers do not mix. So when the occasional thunderstorm or cold front moves through and churns in oxygen at the surface, it does not reach the bottom layer.

Yet while fresh oxygen is confined to the surface, the estuary's organisms can move freely from top to bottom. The plants and animals that make up the estuary's plankton still consume nutrients, grow and reproduce, and die and sink, using up oxygen as they decompose in the oxygen-starved bottom.

But the stratification stops nutrients that are released from the decomposing organic material on the bottom from returning to the top, where they would feed more plankton. The nutrient supply gradually drops, and so does the growth of plankton that would continue to suck oxygen from the bottom. It’s a natural shut-off valve that keeps the summertime oxygen depletion under control.

Change comes in late summer and fall. Winds pick up and temperatures drop. The upper layer is cooled by the atmosphere, the lower layer by cold, salty water pushing in from the sea. The temperatures of the layers equalize, and the stratification breaks down. From top to bottom, oxygen-rich water infuses the estuary.

Oxygen-rich is a relative term. Even in the best of times the amount of dissolved oxygen in sea water is minuscule. In winter, when Long Island Sound is saturated with oxygen, concentrations peak at approximately eight milligrams per liter, or 0.08 percent. (For comparison, the air we breathe is about twenty percent oxygen.) In summer, under the burden of man-made nitrogen, oxygen concentrations can plummet to zero....

As long as the amount of nitrogen flowing into the Sound remained relatively low, the mass of plankton remained under control and oxygen levels stayed high enough to sustain the winter flounder and blackfish and lobsters that crowded into the Sound. They all relied on that perfect mechanism of self-regulation that shut off the supply of nitrogen to the deeper waters and prevented oxygen concentrations from plummeting. But as the number of people who lived in the watershed grew, spreading across land that had never supported anything but forests and farms, the amount of nitrogen reaching the Sound grew as well. Four centuries of ever-increasing development and population had swollen the Sound's burden of nitrogen to ninety one thousand three hundred tons a year, a one hundred and twenty eight percent increase over the forty thousand tons that researchers had estimated flowed to the Sound before European settlement. The additional fifty one thousand three hundred tons of nitrogen included twenty nine thousand six hundred tons from sewage plants and factories, and eight thousand eight hundred tons from storm water that ran off into the Sound. That flow subverted the Sound's natural safety-valve, because the supply of nitrogen was steady, year-round, and overwhelming. The system that the Sound had evolved for surviving the summertime oxygen-depletion no longer worked. Nothing remained to stop oxygen concentrations from crashing.

When it happened in the late 1980s, fish died from City Island to Bridgeport, and it was rightly seen as an ecological crisis. I've not heard reports of dead fish this year, and improvements to sewage plants are well underway, but in terms of habitat loss and ecosystem health, the crisis now seems as severe as it was then.


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