The stratification hypothesis holds up especially if there is a large amount
of organic detritus in the bottom pool without plants intersecting the whole
of the water column. A pseudo thermocline forms because the bottom is kept
more thermally stable by the cooler ground around it, while the upper layer
is heated from outside the system (air temp, solar energy). The thermocline
can happen at only a couple degrees difference and is a common problem in
tropic/subtropic impoundments, floodplain lakes, shallow lakes where the
surface is heated quickly before the heat is distributed through the whole
of the column.
It's probably even more graphic if there are floating plants at the surface,
which again is a problem with hyacinth or coontail choked systems. The
surface is supersaturated with dissolved oxygen from the plants and air at
the surface, but the lower layer has nothing but oxygen consumption from
decomposition, and quickly "breathes" itself out of existence.
Anyway, the two layers become thermally and chemically different (which
would interesting to see if anyone has performed this in microcosms in the
Lit). Without some means of interacting the two (by mechanical mixing,
rooted plants that are distributing O2 throughout the system) the bottom
layer can effectively deplete all of its oxygen due to decomposition.
If you quickly add a large amount of water to the surface layer that cools
that layer below the temperature of the lower layer, what's going to happen?
Remember this is only a couple degrees difference, so it wouldn't take that
much water to wreck the thermocline and mix all that oxygen depleted water
into the other layer where the organisms have been living... Mix half the
volume of 6 mg/L water with half at 1 or 0 mg/L water, and Bada boom bada
bing... Fish kill.
Everything including the algae dying, however, has me totally lost. That
would almost have to be chemical contamination. And I would think that it
would only be a 98% bottleneck at *worst* on phytoplankton population,
leaving some recruitment ready to repopulate, and would result in incredibly
green water with all those nutrients suddenly released? So I don't know how
to answer the original question. "Stuff Happens", I guess...
The Muddy Maumee Madness, Toledo, OH
It's never too late to have a happy childhood.
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