Cold water holds more oxygen than warm water. The fish are not using much
oxygen, in their winter stupor. If you do not freeze, I believe the pond
should be kept running, filters and everything. If you do have heavy
freezes, where the pond is going to freeze over and stay frozen for some
period of time, some means of keeping a hole in the ice is needed to allow
the toxic gasses to escape and oxygen to get into the water. The most
common methods are the use of an air stone, which keeps the water moving
near the surface, and moving water is much harder to get to freeze. The
other is the use of a pump to again disturb the surface of the pond. The
air stone and the pump should be mounted fairly close to the surface.

As for disturbing the bottom water, I don't think our ponds are really deep
enough to get a real thermal layer that could be stirred up, but the heat of
the earth, always around 50 degrees a few feet below the frost line, sends
heat up and through the liner, making the bottom water warmer, though some
of the heat moves upward and escapes. The fish hang close to the liner,
where the heat is coming from.
--
RichToyBox
http://www.geocities.com/richtoybox/pondintro.html


"Iain Miller" wrote in message
...
I was just thinking about one or two things & was wondering what happens
to
the oxygen levels in the pond over the winter if all the
filtration/circulation is switched off. I know the fish go very quiet &
hang
out down the bottom but surely the water down there gets pretty oxygen
depleted? As I understand it water is actually at its most dense at about
4
degrees C (39F) so in the height of winter its quite likely that the pond
is
actually warmer at the bottom than the top. If this is so then its
obviously
not a good idea to circulate the water out the bottom of the pond in mid
winter (assuming filters use a bottom drain etc) which means the filters
really do need to be turned off....

What's the general concensus - how do the fish survive in an oxygen
depleted
environment?

I.