Roark did a bit on water and winter in response to a question and it makes a
pretty interesting read.
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From: Roark7
Subject: Bubbling bottoms and super cooling
Date: 1997/10/31
David Swarbrick wrote:
Bubblers are recommended for maintaining an ice free portion of a pond
over winter, and maintaining good oxygen levels. I have also seen it
suggested here that it should not be placed on the bottom of the pond,
but raised so that the circulation it induces does not disturb the water
at the every bottom because this merely results in the very cold water
by the ice being distributed all over.
I question this.
Yep... And I don't blame you one bit. I had the same doubts and voiced the
same arguments that you shortly share because it *is* counter-intuitive...
until you start looking hard.
First, warm water rises, and cold water sinks. The water at the bottom
will always circulate if there is a temperature difference. Frozen water
floats. I suppose there must be a point just before water freezes where
the situation turns on its head, but I assume also that this is only at
the point very near freezing.
I had major questions about the physics aspect of this whole "winter pond"
thingy as well. I did some asking and a bit of research and came up with three
things which cause the inverse stratification effect. These are, in ascending
order of importance:
- The native heat of the earth
- The physics of water going from a liquid to a solid
- Surface cooling due to winds and cold air.
I'll rattle through each contributing effect in detail below:
- Heat of the Earth:
It turns out that the earthern bottom of a 4-foot deep pond stays right around
34-38 degrees even though the outside air temperature drops much lower. The
simple reason for this is the Earth is slightly exothermic. As you dig down
you hit a point where more heat is being released by the earth than can be
pulled-away by wind, night sky, etc. This is why you bury water pipes below
the "frost line". This heating effect is small in ponds, but it *is* there.
The earth *is* pumping a bit of heat energy into it all the time. The other
effects (below) tend to magnify this effect into something useful by keeping
this slightly warmer water on the bottom.
- Physics of Water:
It turns out that water doesn't move upwards because it is "warm" nor sink when
it becomes "cold". A little thought reveals this behavior is strictly a
function of density. Warmer water *tends* to be less dense so it rises... but
this isn't gospel. An interesting kink in the water density -vs- temperature
curve shows-up just prior to 32 degrees F. At the pre-freezing point (32.8
F), water undergoes a major density change. As it cools it becomes *less*
dense than water which is just a fraction of a degree warmer. This difference
is fairly large. Being lighter than the surrounding water, near-freezing water
*rises*. (This is one reason frozen pipes tend to burst. The density of the
water decreases, the mass stays the same, so the result is volumetric
expansion which splits pipes with ease.) The degree of final bouyancy is
controlled to a large part by the dissolved oxygen content of the water. The
more dissolved O2, the greater the expansion once the freezing point has been
reached. Since water at the *top* of the pond tends to have a greater O2
concentration, this further contributes to stratification. Near-freezing
water moves toward the surface, and then, having reached the surface, freezes
completely.
- Surface Cooling Effects
This one is pretty obvious, but its worth restating to put it in context.
Given a sub-freezing day and a brisk wind, its a simple matter to pull more
heat from the top layer of water than can be replaced by natural convection.
Once the top starts to freeze, heat loss to ambient and basic water physics
insures the top will *stay* cooler than the bottom. If this wasn't true,
you'd never see a thin coat of ice.... the pond would instead just hit a point
where the entire thing suddenly became a solid chunk of ice.
It seems to me that if the air being pumped in is warmer anyway (in my
case, from inside an unheated shed), then the balance will be about
right.
Your idea about pumping warm air into the water isn't a bad one, but it will
take a lot of warm air to make a dent in the ponds temperature.... far more
than you could reasonably produce. Pumping a large amount of air in would
also create currents which the fish would need to fight or at least adjust
for. Hibernating fish are in no position to do this and forcing them into
this situation uses energy they will need during the rest of the winter.
For keeping a hole in the ice however, you could likely use this warm-air idea
to your benefit. Put an airstone a foot under the water and run warm air to
it. Bear in mind that you will lose lots of heat in just a short run between
your shed and the airstone.
Also if the problem with water under ice is the lack of oxygen and build
up of waste products under the ice, then the more chance the bubbler has
to oxygenate the water and take away the foul water the better.
The oxygen demands of fish near the freezing point are very, very low. This is
a good thing because I've got a feeling there isn't much oxygen available once
you near the peak of winter. Fortunately, decay and decomposition of wastes
by bacteria has nearly stopped as well which relieves a decent portion of the
oxygen load. Very little oxygen is needed in a winter pond.
During a really *cold* winter, I think the idea of a full-blown bubble system
would tend to upset a natural balance which Nature clearly went out of her way
to establish. Having said that however, I can see a very definite *benefit*
to using such a system as the air temp starts to push into the upper 30's and
40's. By introducing additional air and inducing water motion, you'd be
putting lots of needed oxygen into the water as well as helping the pond to
absorb ambient heat. Fish coming out of their winter sleep wouldn't be
oxygen-stressed as well as being thin, worn and badly in need in of a shower.
From what I'm given to understand, most fish have no problems during the
actual *wintering*... its the *transition* from hibernation to normal
metabolism which gets them. Your bubbler could be a great tool during that
transition period.
So.... thats my 2 cents on the subject. YMMV.
Roark --- wouldn't know a real winter if it bit him in the rumpus!
Ventura, Ca.
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kathy :-)
algae primer
http://hometown.aol.com/ka30p/myhomepage/garden.html