Observation Friday: Hypoxia Threatens LIS This Summer

This week’s Observation Friday has Tom Andersen, author of This Fine Piece of Water, providing his thoughts on the severity of this year’s low-oxygen problem:

Water quality in the western end of Long Island Sound this summer is not good – almost shockingly so considering the generally decent conditions of recent years.

In roughly 18 square miles of the western Sound – off Larchmont, Mamaroneck, New Rochelle, Sands Point and Glen Cove – there is almost no dissolved oxygen in the bottom waters, according to the latest water quality data from the Connecticut Department of Energy and Environmental Protection.

Conditions are almost as bad off Rye, Port Chester, Oak Bluff, Greenwich and Norwalk, where dissolved oxygen concentrations are below 2 parts per million. And as far east as Smithtown and the town of Fairfield – an area covering 288 square miles, or 20 percent of the Sound – dissolved oxygen concentrations in the bottom waters are less than 3 milligrams per liter.


Dissolved oxygen is one of the key measures of a healthy estuary. When levels drop below 3, it means the usually abundant bluefish, striped bass, fluke, blackfish and various crustaceans must flee. When it drops below 1, they can’t survive.

What makes this unsettling is that for the last several years conditions seemed to have stabilized and perhaps even improved. This year marks the first time since 2008 that dissolved oxygen has fallen below 1 millgram per liter; and the 288 square miles of the Sound with dissolved oxygen below 3 is the fourth largest recorded area since water quality monitoring started in 1991.

Here are DEEP water quality maps from 2008 to 2011, showing the same period in August as the 2012 map above (you can find other maps here). The black blotch is where conditions are the worst. You can see a significant black area this year but there’s been no black at all since 2008.


Judgments about whether this year’s data are part of a new downward trend or merely an anomaly will have to wait, but in and of itself, the August report is disturbing.

So what’s going on? Several factors contribute to hypoxia. For one thing, warm water naturally holds less dissolved oxygen than cooler water so, even under the best conditions, concentrations fall in summer (although researchers believe that when the only inhabitants of the area were Native Americans, dissolved oxygen never fell below about 5.5 mpl, which is a perfectly healthy amount).

Average annual water temperatures in the Sound have grown warmer over the years. In fact, water temperatures were so warm this summer that federal regulators forced the shutdown of the Millstone nuclear power plant, in eastern Connecticut, because the water got too warm to cool the reactors. (The eastern half of the Sound, which is wider, gets less treated sewage than the western end, and has a big opening through The Race to the ocean, never suffers from hypoxia.)

But the bulk of the problem in the western end of the Sound is nitrogen, which is a component of treated sewage. In warm weather, nitrogen acts as a fertilizer in the Sound, spurring the growth of algae. When the algae die, the decomposition process removes oxygen from the water.

In other words, nitrogen in treated sewage plus unusually warm water equals bad hypoxia.

The federal government and the states of New York and Connecticut agreed about a dozen years ago to require sewage treatment plants to remove 58.5 percent of the nitrogen in their wastewater. The original target date was 2014, and Connecticut – after some serious delays that Save the Sound worked long and hard to overcome – is now close to meeting its goal.

But the overwhelming amount of treated sewage that enters the western end of the Sound comes from Westchester County, Nassau County and New York City. While Connecticut was moving toward its 58.5 percent goal, Westchester County and New York City asked for and received a three-year extension, to 2017.

Upgrades are underway at those treatment plants. Whether they will meet their deadlines is another question.

The Connecticut DEEP crew is out on the Sound again this week, taking more water quality measurements, and they’ll follow up in September. The data they collect will tell us just how bad this summer turns out to be. Whatever the results, we’ll have to wait at least til next summer’s data for a better understanding of the long-term implications.

Posted by Laura McMillan, Outreach Associate for Connecticut Fund for the Environment.

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