|Dead menhaden at Lake Ogleton August 5|
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.I think we've been on the edge of having a "hypoxic" event. I see lots of crabs and a few fish in shallow water and that usually means there's not enough oxygen for them deeper in the water. What ever mixing Isaias did here, it wasn't enough to substantially alter the state of the water.
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.
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.
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.