Wednesday, October 21, 2009

A New Report of Water Quality Trends in Long Island Sound Show Things Getting Worse

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.



Anonymous Bryan said...

One question that I hope the report would answer (when it's available on line) is how do they determine the hypoxic areas. There are charts available for 2009 on the web site. There are 40 or so monitoring stations and 400 square miles of potentially hypoxic area. I wonder how DEP resolves the boundary between the stations to determine one DO range vs. another. I also wonder how the estimated areas are sensitive to this granularity.

1:59 PM  
Blogger Sam said...

Good questions that I can't answer ... but the idea is to have certain GPS points and you lower a grab sample rube or use a real-time electronic instrument to a few feet above the bottom. Anoxic conditions only exist on the bottom where the worms, crabs, and bottom dwellers live (the fish usually swim away, needing oxygen for their gills). So really, you're looking at the bottom of the Sound, not the entire water column.

In the shallow waters with less current flushing, of course the hypoxia is going to be much, much worse. The water stratifies with oxygen depleted water at the bottom. For example in Corpus Christi Bay (TX), the bay waters were hypoxic the bottom two feet when sampled, but the top 6 feet were fine.

There is a reason for this "bottom of the Sound" problem. Algae grows throughout the water column, actually adding oxygen to the waters via photosynthesis. But when the cells die they fall to the muddy bottom where bacteria consume the dead cells. Bacteria respires, meaning it takes in oxygen and puts out CO2. In bad situations, the bacteria can form a powerful zone at the bottom in which nothing can really live.

What we are finding is that the bottom of the Sound, the sands and muds, are truly alive with microscopic creatures. Usually, the bottoms are teaming with all kinds of life including nasty sandworms. However, when bacteria takes hold and hypoxia happens, little can live because of lack of oxygen.

Sorry I can't answer your question, but those stations and numbers have nothing to do with the water found in the top layer, which is usually highly oxygenated from wind and wave action.

7:03 PM  

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