Tuesday, September 25, 2018

Rain Rain, Go Away!

This summer has been a wet one for much of the Chesapeake Bay region. Pennsylvania saw its wettest July and August on record, and both Virginia and Maryland received much more rain than normal, with Maryland chalking up its second-wettest July. All of this led to the Bay receiving unusually high amounts of fresh water.

The Susquehanna River, which begins near Cooperstown, N.Y., flows through Pennsylvania and enters the Bay at Havre De Grace, Md., is the Bay’s largest tributary. Normally, it contributes about half of the Bay’s fresh water. This summer, the Susquehanna has reached record high flows, peaking in July at 375,000 cubic feet per second, the highest flow the river has seen since Tropical Storm Lee in 2011.

Because of the high amount of water flowing down the Susquehanna, Exelon, the operator of the Conowingo Dam which sits on the river, opened the dam’s floodgates multiple times. That’s unusual for summertime, since flows tend to be higher in the late spring and early fall.

Opening the dam released debris that had built up behind its walls, including everything from tree trunks and branches to plastic bags and water bottles. The volume of debris was the largest in 20 years, according to Exelon. In a statement, they noted that so far this summer, they removed 1,800 tons of trash from behind the dam, compared to the usual average of 600 tons. . .

Besides the trash what else does this mean for the Chesapeake Bay?

The Bay is less salty. It sure is.
This year, we’ve seen record freshwater flows from the Susquehanna River and other major Bay tributaries such as the Potomac River, diluting the Bay and making it less salty. Waters in the south are still saltier, but the salinity gradient has been pushed down, and areas in the middle have less salt than usual. . . .Changes in the location of fresh and saltier waters, in turn, could have wide-ranging impacts on the Bay’s plant and animal life.
Shellfish, particularly oysters, could be impaired.
Oysters require salty waters and they can’t move if conditions change. They can close their shells, but only for a short period before they begin to suffer. Also, extra sediment that arrives in the Bay with additional fresh water flows could smother reefs and suffocate oysters.

“We’ve had mortality due to freshwater in the past,” said Michael, “but we won’t know the extent of the mortality until we conduct the surveys.” DNR expects to finish surveying oyster reefs by the end of the year.

An influx of fresh water isn’t always bad for oysters; it could provide a bit of relief from disease. In an interview with the Virginia Institute of Marine Science (VIMS), Ryan Carnegie, a research associate professor at VIMS, noted, “MSX disease may largely be purged from Chesapeake Bay oyster populations as the parasite Haplosporidium nelsoni cannot withstand salinities below 10 [parts per thousand].”

Blue crabs, on the other hand, can move to where conditions are more favorable, although they can tolerate fresher water. In the Bay itself, they seem to be staying farther south and at the edges, where salinity and dissolved oxygen levels are higher. In the rivers, DNR trawls show upriver sites containing more crabs, which is normal for this time of year
Finfish are moving to new places (I expect a dramatic expansion of the Northern Snakehead population)
Just like crabs, when salinity changes, finfish can move to areas better suited for them. This means salt-loving fish move south, and freshwater fish begin to appear in areas they aren’t normally seen.
Underwater grasses will decline, but this year will test their overall resilience.
Luckily, the damage doesn't seem too extensive, although we won’t know the full impact until the aerial surveys are completed in Maryland and Virginia. Brooke Landry, a biologist with DNR, noted how the grasses at the mouth of the Susquehanna River, in an area called the Susquehanna Flats, have been holding up. “After the July [rain] event, the water over the SAV bed was clear. You could see grasses in eight feet of water and fish swimming around as well.” Although there was some impact on the grasses, it was primarily at the edge of the bed where those grasses were buffering the sediment from reaching the grasses in the inner bed.
The dead zone may be smaller than anticipated. Good news?
This summer’s additional rains came as storms, with stronger winds and more overcast days. Strong, frequent winds have helped to mix up the Bay’s water and inhibit algae’s ability to thrive.

“Algae like sunny, calm weather,” said Michael. “Because this summer has been so cloudy and overcast, we haven’t seen a lot of algal blooms this year.” In fact, DNR’s monitoring cruise observed the best ever hypoxia conditions (meaning the smallest amount) in the Bay for the end of July, and near average conditions for August.

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