Winter fish kills are rarely caused by oxygen depletion, under ice.
Let's look at what ice does, that is different to water that does not have ice... it seals over a pond. What builds up under ice, hydrogen sulphide (among other things). It is lighter than water, normally it floats to the surface and gases off.
This is a description of hydrogen sulphide, I don't think you need to be a scientist to realise what happens when you seal over a pond:
-------Highly toxic - may be fatal if inhaled. Inhalation of a single breath at a concentration of 1000 ppm ( 0.1% ) may cause coma. Corrosive when moist. Skin contact may cause burns. There is a rapid loss of sense of smell on exposure to gas concentrations above 150 ppm, and this means that the extent of exposure may be underestimated. Perception threshold ranges from 0.5 ppt to 0.1 ppm. Irritant. Asphyxiant.-------
That is just one of the goodies that decomposition in a pond makes, as a by product
Evidence that a single hole in a pond works, folk don't have significant fish kills where a pond is vented, venting works (no great science there) toxic gases, lighter than water, are released...
Now, a body of freshwater on the scale of a lake is different to a garden pond, by size and the scale of decomposition likely to be occurring (folk are more likely to keep the proportion lower on a fishpond)
But, you will see the principle of thermoclines, layered water in both... the risk of fishkills on a lake are likely to be more dramatic because of the greater scale of decomposition going on and the greater proportion of water with low oxygen levels, a lot more deeper water to potentially mix when inversion happens... so, while folk describe it as an 'oxygen' shortage the use of 'aerators' on lakes is quite likely mistaken... the chemical reaction of mixing low oxygen deep water plus inversion mixes toxic low oxygen water into the area where the fish are... under ice
It does not matter how much oxygen you pump under that ice, the likes of hydrogen sulphide is going to be sealed under and the fish have nowhere to hide
This is a different sort of fishkill compared to what a pondkeeper is likely to experience...
Most pondkeepers don't have such deep volumes of toxic water, when inversion happens there is not going to be so much toxic stuff being present under ice, but, it will accumulate...
But, there are several other threats likely to emerge caused by turbulence, using pumps or aerators through freezing conditions:
1) Pumps fail and drain a pond under the ice
2) Super cooled water, colder than freezing but still fluid are circulated into deeper water
3) The layer of ice is accelerated in thickness, a bit lethal where ponds are shallow...
If you really, really want to find out about inversion, first hand, go out there in sub zero conditions and put your arm in a still pond. The wimps may be permitted to drop a thermometer in instead
Assuming the organic debris is a modest low proportion, the pond has a modest vent, two feet down is quite a cosey place on a still pond, compared to a pond with freezing water moving around it... You can actually feel the difference in warmth in the frogs and the mud down there...
While your garden pond may have far less volume, it certainly will have that delicate inversion there through the winter months, sometimes you can see it on a winters morning, a speckly difference just six inches down, where cold brittle clear water is sitting over a gently warmed thermal currents below, with fine traces of clay like particles catching the light...
Regards, andy
http://www.members.aol.com/abdavisnc/swglist.html