Wednesday, July 26, 2006

Water Quality in the Sound Seems To Be Not Quite As Bad This Year As In Recent Years

Don’t ask why because nobody knows, but water quality in Long Island Sound, as measured by dissolved oxygen concentrations, seems to be better this year than at similar times in the previous four years, and conditions actually improved in mid July when compared to the first 10 days of the month.

I wouldn’t be performing in character if I didn’t look at the bad side of this, so let me say that there’s still plenty of time left this summer for conditions to degrade significantly, and just because water quality is better does not mean it’s good. Nevertheless, here’s what the Connecticut DEP, which checks dissolved oxygen (and lots of other criteria) twice a month, has to say:

During the month’s first research cruise, on July 6, 7 and 10, dissolved oxygen concentrations in the bottom waters of the Sound ranged from 2.5 to 8.8 milligram per liter. The latter figure is excellent, and the former indicates that fish and other marine life will have a hard time living there; 3.5 milligrams per liter is considered to be the threshold of hypoxia, and it’s as low as regulators ever want the Sound to get (although obviously conditions are frequently worse). During those three days, 141 kilometers, or 54 square miles, had dissolved oxygen concentrations below 3.5; the map that accompanied the DEP’s written discussion indicates that the vast majority of those 54 squares miles had DO concentrations between 2 and 3 milligrams per liter.

During the second research cruise, on July 18 and 19, bottom water concentrations ranged from 2.8 to 9.5. The area below 3.5 milligrams per liter is slightly smaller (50 square miles) and the area between 2 and 3 is noticeably smaller, as shown here in orange and yellow.

Katie O-Brien-Clayton, the DEP analyst who sent the report out yesterday, wrote that results from the survey of July 18 and 19

seem to be atypical when compared to data from 2002-2005 …, with much of the sound having bottom water DO concentrations greater than 4.8 mg/L. …

In 2005, 690.7 km2 of bottom water had dissolved oxygen concentrations between 4.0 and 4.99 mg/L, 502.6 km2 had concentrations between 3 and 3.99 mg/L, 107.9 km2 had concentrations between 2 and 2.99 mg/L, and 49.5 km2 had concentrations between 1 and 1.99 mg/L.

In 2004, concentrations were not less than 3.0 mg/L at any station (975 km2 had concentrations between 4.99 and 4.0 mg/L; 428 km2 between 3.99 and 3.0 mg/L).

In 2003, DO concentrations only dropped to 2.0 mg/L; the area with concentrations between 2 and 2.99 mg/L was 175 km2, 3.0 and 3.99 mg/L was 163 km2 and 783 km2 had concentrations between 4.0 and 4.99 mg/L.

Data collected in 2002 indicated dissolved oxygen concentrations dropped below 2.0 mg/L over 92 km2. Concentrations were between 2.0 and 2.99 mg/L over 67 km2, between 3.0 and 3.99 mg/L over 34 km2, and between 4.0 and 4.99 mg/L over 809 km2.

So this year thing conditions in the Sound are a bit better, so far. Recall that last year, in mid-August, the DEP got a reading on 0.5 milligrams per liter, in the Greenwich-Rye area, in mid-August. When DO is that low, it means there are no fish in the area. Let’s keep our fingers crossed that it doesn’t bottom out like that again.


Blogger Sam said...

Hmm, some of this may have to do with water temperature and salinity, speaking of dissolved oxygen (DO).

DO is limited by water temperature, meaning that higher temperature waters can hold less water than colder temperatures. I saw seawater temperatures ranging around 71 degrees at New London CT but at Bridgeport and only in spotty places to the west, more like over 80 degrees [as an aside, the seawater in HOT SOUTH TEXAS is about 78-82 degrees F].

Another principle is that lower salinity can increase oxygen loading, at least theoretically. So you've had maybe 5-12 inches of rain and all that water lowers the salinity, which would boost oxygen levels absent a serious algal bloom.

Could that be? I'm still a student of water quality and am still learning the ropes here. /Sam

6:42 PM  

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