Heat Wave May Literally Be Suffocating

The current heat wave has been hard on us all, but it also has been troublesome – maybe even deadly – in Long Island Sound.

Stagnant heat stresses the western end of Long Island Sound every summer, by depriving marine animals of the oxygen that they need.  Storms can stir refreshing breaths back into all depths of the Sound, but we haven’t had any rain now for more than a week.

Here’s the how & why:

The problem starts with the geography of Long Island Sound. It’s long and thin, with only a narrow opening to the Atlantic Ocean at its east end, called The Race (off New London), and an even narrower connection to the East River at its west end (near the Throgs Neck Bridge).  There are two tidal cycles in the Sound each day, but most of the flow comes in through the Race and pushes water to the west, where it has nowhere to go and ends up sloshing up like the water in the back of your bathtub.

The effect is: the waters in the eastern Sound get refreshed by new ocean water more often than the waters in the western Sound.

Then there’s this:  The salinity, temperature and concentration of dissolved oxygen at the water’s surface can be much different than the salinity, temperature and dissolved oxygen just 40 to 50 feet below, at the bottom.

The water near the surface of the Sound gets stirred up by wind, waves and boat wakes, and that helps to mix oxygen into the water, just like the bubbler you put in your home fish tank. Also, the algae in the water – the microscopic phytoplankton that makes the water green – are happiest at the surface, where they have the best opportunity to (like all plants) catch the sun’s rays, make food and exhale oxygen.

So there’s usually adequate dissolved oxygen at the surface.  When storms churn the water, that dissolved oxygen is mixed down into the lower cooler saltier depths of the Sound.

But when there are no storms, dissolved oxygen levels start decreasing in the deeper water. If levels get too low, the animals down there that can’t easily move – the smaller fish, sea stars, mollusks, many crabs and other crustaceans – may actually suffocate.

(Excess fertilizer that flows off our lawns and eventually into the Sound makes the problem worse, but we’ll get into all that some other time.)

You can watch how all this plays out – live – thanks to sensors in Long Island Sound displayed at www.mysound.uconn.edu.  Yesterday afternoon, the monitor for the western Sound – off the Greenwich coast – showed a dissolved-oxygen concentration of 9.4 milligrams per liter (mg/L) at the surface. But at the bottom, the dissolved oxygen reading was only 3.1.

A reading of 3.1 is on the threshold of a condition called hypoxia, where dissolved oxygen levels are too low for marine life. 

Even worse, at the same time, a sensor at Execution Rock (near the Throgs Neck Bridge) showed a dissolved oxygen reading of 12.7 mg/L at the surace but only 2.7 mg/L at the bottom.  That’s a deadly level.

A cold front is predicted to come through tomorrow and break the heat wave. If the front just brings a lot of rain, the near-hypoxic conditions in the deeper waters of the western Sound will remain and animals may die.

What the Sound needs is a storm that kicks up waves, to stir some of that precious dissolved oxygen down to the Sound’s deeper levels. Then we’ll all breathe a little easier.


Centerpiece of the South Norwalk (“SoNo”) waterfront neighborhood of Norwalk, The Maritime Aquarium features sharks, seals, sea turtles and other animals of Long Island Sound and its watershed.  A celebrated place for family fun, the Aquarium also features Connecticut’s largest IMAX movie theater, plus animal encounters at sea during our research vessel’s study cruises.

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