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Tropical Storm Isaias spreads dead zones in Severn River, killing thousands of fish

Dead menhaden at Lake Ogleton August 5. The Maryland Department of Environment said Tropical Storm Isaias mixed oxygen-depleted water through the water column, causing a fish kill. - Original Credit: Chesapeake Bay Foundation
Dead menhaden at Lake Ogleton August 5. The Maryland Department of Environment said Tropical Storm Isaias mixed oxygen-depleted water through the water column, causing a fish kill. - Original Credit: Chesapeake Bay Foundation (Chesapeake Bay Foundation / HANDOUT)

Strong winds from Tropical Storm Isaias caused oxygen-depleted water at the bottom of the Severn River to mix with the rest of the water column, killing at least 10,000 fish Tuesday, the Maryland Department of Environment said in a statement.

Chesapeake Bay Foundation Maryland Fisheries Scientist Allison Colden said to think of oxygen-depleted bottom water like vinegar, and oxygen-laden surface water like oil. The warmer water is, the faster molecules move and the less dense it becomes. Salt makes water more dense –– it adds molecules.

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Cold, salty bottom water is dense and won’t mix with warm, fresh surface water heated by the air temperature, so when oxygen at the bottom is depleted, it doesn’t have a chance to mix with re-oxygenated surface water, Colden said.

Until things get shaken up. That’s what happened Tuesday during high winds brought by Tropical Storm Isaias.

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Maryland Department of Environment spokesman Jay Apperson said wind from Tropical Storm Isaias caused a “seiche,” or a wave oscillating in a body of water. The National Ocean Service compares the phenomena to water sloshing back and forth in a swimming pool.

“Strong winds can cause this type of inversion, in which the oxygen-depleted water rises and becomes closer to the surface and causes conditions in which the fish cannot survive,” Apperson said in an email.

Unfortunately, when the dead zone in the river sloshes around, death follows. On Tuesday, the department investigated between 2,000 and 3,000 dead fish, mainly menhaden, found at Sullivan Cove.

“Crabs were actively attempting to reach the shoreline to evade suffocation, and other species of fish (American eels, spot and killifish) were milling at the surface,” Apperson wrote.

The Maryland Department of Environment Wednesday investigated 7,500 dead fish found in Lake Ogleton, which is farther down the Severn River close to where it meets the Chesapeake. The water in Lake Ogleton was fine, and therefore the kill likely originated in the Severn, Apperson wrote. At Chesapeake Harbour Marina, the department investigated a fish kill Thursday, Apperson said.

They suspect all three events are the result of wind caused by Tropical Storm Isaias mixing the dead zone throughout water.

Sometimes it is called a “crab jubilee,” when crustaceans fleeing the water to prevent suffocation come ashore, to the delight of predators.

The Chesapeake Bay Program estimated in June that the dead zone in the bay this year would be 9% lower than average, based on the amount of nitrogen pollution in January through May.

Colden said stormwater runoff flushes nutrients into the water, which fuel algal blooms, which then die off, sink to the bottom and decompose, with bacteria eating up oxygen in the process, exacerbating the dead zone problem.

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