Name:   redleg6 - Subject:   Dead Fish Date:   8/9/2017 8:05:05 PM

Here is some pretty good info on why fish kills occur, and I suspect they apply here.

OXYGEN. Usually, it's the lack of oxygen that commonly is the cause of fish kills.  In the summer as surface waters warm, algae proliferate and provide a boost to microscopic life.  That's good until the algae switches from phosynthesis (producing oxygen) to respiration (using oxygen) during low light periods such as at night or during prolonged cloudy periods.  That process leaves less oxygen for fish, which will start dying if they are already under stress from overcrowding, low water levels, or high water temperatures.  

To complicate matters, oxygen gets further depleted when algae starts dying off in large quantities.  Bacteria-driven decomposition then uses up a lot of oxygen, driving down the oxygen concentration in the water even further.

THERMOCLINE AND OXYGEN PROFILES.  An important feature of lakes experiencing seasons is the thermocline.  As the surface waters of a lake warm up in the summer, a temperature gradient is established, with denser colder water near the bottom and warmer water near the top.  That's not at all surprising, except that the temperature change as you go deeper isn't gradual.  Instead, there is a sharp discontinuity a few meters down, with warmer waters above and cold water locked in below.  The dividing line is the thermocline.  This cutting off of the two large masses of water is very significant for fish.

While winds can usually whip around water enough to thoroughly mix it and bring up cold, oxygen-rich water from the depths, the thermocline blocks that process.  Mixing only occurs above the thermocline, keeping the warm waters oxygen-poor and facilitating fish kills.

Dick Bronson/Lake Watch

PS: If anyone out there thinks I'm smart enough to know this stuff, I have the deed to a bridge in Brooklyn for sale.  Cheap.