The Connecticut DEP just released an excellent summary of water quality conditions in Long Island Sound in 2009 and of trends since 1991. It clearly shows that in many ways water quality in the Sound is getting worse.
I can’t say I know why it’s getting worse. The management committee of the Long Island Sound Study is meeting today and perhaps it will be part of the discussion. But the worst hypoxia – the conditions under which no, or very few, fish can live – is getting worse.
The DEP summary uses 2 milligrams of dissolved oxygen per liter as the threshold of severe hypoxia. When DO drops that low, the deep-water habitat of that part of the Sound is all but unlivable for fish – only 18 percent of the fish that normally live there can be found there.
Using 2 mg/l as the criterion, 2009 was unusually good: only 17 square miles of the Sound had dissolved oxygen concentrations of 2 or below. But 2009 was to be an outlier. In 1998, 48 square miles had DO of 2 or below. Eight of the 11 years since then have been worse. Here’s what the DEP summary said:“It seems that there is an increasing trend towards severe hypoxia in LIS (i.e., hypoxia area at 2.0 mg/L seems to be getting worse).”
Here’s the graph that shows the trend:
For anoxia – DO concentrations below 1 mg/l – the data seems almost as clear, although the report does not say so.
When DO drops below 1, no fish can live there. In nine of the first 11 years of the water quality survey, anoxia affected seven square miles or less. But in five of the last eight years, anoxia affected 28 square miles or more. (And to confuse things just a bit, in two of the last three years, there was no anoxia at all.) The report said:“Prior to 2002, the average area of bottom waters affected by anoxia was 5.92 mi2. From 2002-2009 the average area affected was 28.4 mi2.”
So what’s going on? Hypoxia is caused by pollution, specifically nitrogen that mostly comes from sewage treatment plants, but hypoxia occurs only when the water is warm and is at its worst when the Sound stratifies into two layers, with warmer water on top above slightly cooler water. The stratification prevents oxygen that gets churned into the water at the surface from mixing with deeper waters.
There are a number of charts and maps included in the DEP report that show temperature and temperature differences in the Sound. But I have to pass the buck and say that unless I get help, I won’t be able to understand them.
Perhaps someone who worked on the report will explain the connection between water temperature and hypoxia as depicted in the DEP maps and charts.
The DEP report, by the way, was sent out via email as a PowerPoint document, so I can't provide a link.