Re: NANFA-L-- Killer rain?

Mysteryman (bestfish at
Tue, 06 Sep 2005 14:37:50 -0700

Todd D. Crail wrote:

>Finally got a chance to get to this, and I think Geoff is right on track.
>Moon's Dad's experience seems to be the same issue.
>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...
Todd, you have perfectly described the condition of my pools and the
pools of many others in the affected area. Some pools with big filters
and fountains survived, although they were damaged, but the stgnant ones
were all wiped out completely.

This stratification disruption model seems to fit perfectly with the
situation. I should have thought of that since I took Limnology in
college, but I guess that was a long time ago.
My thanks to everyone who has been working on this puzzle. I think I
have concluded that the rain itself was possibly contaminated with
something, but that was only an extra factor in the main cause of
destratification caused by significant & sudden temperature drop of the
uppermost layers brought about by 3 inches of cold rain on what was up
until then a very hot day. Everyone's theories have been very
insightful, and I wouldn't be surprised to find they are all correct in
some ways, or that a combination of events occured to bring about this
rare event.
I guess I'll have to install fountains in all the pools to prevent it
from happening again.
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