Tuesday, July 17, 2012

Long Hot Summer Good for the Bay

The weirdly mild winter, the dry and toasty spring, and the hottest summer heat wave on record apparently had at least one upside: a cleaner Chesapeake Bay.

Last year around this time, the bay was smothered by one of its largest dead zones — low-oxygen water caused by pollution where fish and plants cannot survive. This year, with so little rain to move pollution from farms and city streets into waterways, the zone “absolutely is much smaller,” said Bruce Michael, director of the resource assessment service for the Maryland Department of Natural Resources.
But state monitoring that started last month amid record-breaking heat and abnormally dry conditions showed a dramatic improvement from this time last year — very few low-oxygen areas in the bay’s main stem so far, “better than average,” Michael said. About 12 percent of water volume in Maryland’s portion of the bay was low in oxygen in June, compared with more than 30 percent last year.

“We were kind of predicting this, because we had relatively low flows in winter and spring . . . not much runoff at all,” Michael said.
But, as witnessed even on my own beach this weekend, there is still a mass of low oxygen water sitting in the deep areas of the bay, waiting for the right meteorological conditions to bring up into the shallow waters and produce a crab jubilee, or even a fish kill. Incidentally, I also heard from a fellow fisherman of a similar crab jubilee on the western shore near Point Lookout.

However, the high temperatures can also bring oxygen stress:
Usually, putting crabs or fish in hot water leads to one thing: dinner.

But water temperatures in the Chesapeake Bay hit record highs in some spots this week because of the heat wave and that's causing problems.  "[It] causes what we call the temperature-dissolved oxygen squeeze," says Bruce Michael, with Maryland's Department of Natural Resources.

He says the squeeze happens between hot surface water and the oxygen-deprived deeper water. Surface water temperatures climbed to record highs in many spots this week, hitting the low- to mid-80s and stressing many fish.  "It's probably not going to cause a fish-kill, but it will stress the fish," Michael says.

The fish become more vulnerable to disease when there's less oxygen. Dissolved oxygen is the single most important measure of habitat quality in Chesapeake Bay. Higher water temperatures can be even worse for underwater grasses since the grasses are stationary. The grasses are where crabs hide from predators.
The Bay managers are much like farmers; no matter what the weather, it's never quite right.  Either there's too much rain or too little rain, too hot or too cold.

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