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Why are deeper ponds less prone to fish kills?
#514480 11/30/19 11:46 AM
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I keep reading that deeper ponds are less prone to fish kills. This doesn't make a lot of sense to me. Wouldn't a deeper pond have a greater difference in temperature between surface and bottom that might make it more prone to turning over? And wouldn't it have more anoxic water that would overwhelm the surface when such a turnover happened?

Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514482 11/30/19 12:35 PM
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I believe it has to do with the fact that warm water is more dense and sinks to the bottom in the winter. Shallower water fluctuates temperatures more rapidly and doesn’t hold heat on the bottom like a deeper BOW does.

Not sure about turnover though.

Last edited by Steve_; 11/30/19 12:36 PM.
Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514488 11/30/19 07:57 PM
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Quote:
I keep reading that deeper ponds are less prone to fish kills.

This is not always true and as usual - it all depends. When you read something you have to consider the source, the author's knowledge, don't take things out of context and don't assume.

The statement "deeper less prone to fish kills" is much more true for ponds in ice cover conditions and winter fish kills rather than during mid-summer conditions. Numerous things affect fish kills in summer such as surface to depth ratio, wind exposure, age of the pond, productivity of the pond, water clarity, cloudy weather (lack of sunshine), cold rain, pond water retention time, amount of submerged vegetation, on and on with added factors.

Steve in his post above has his information backwards. Steve said ""I believe it has to do with the fact that warm water is more dense and sinks to the bottom in the winter."
Warm water is less dense NOT more dense than cold water. In reality,, Less dense "lighter" warm water "floats" on the heavier colder more dense water. As the upper warm water cools it becomes heavier, it sinks and is more easily mixed downward by wind, cooling, and convection currents. This can cause deeper less DO water often with dissolved toxic hydrogen sulfide (H2S) to circulate upward (mix) into the "good" water above. H2S is very toxic to fish. This mixing with a double threat of no DO & elevated H2S can degrade the entire water column enough to suffocate and kill vulnerable fish - invertebrates. H2S concentrations vary from 0 to high due to the duration of lack of dissolved oxygen. If DO is present - H2S will not form. H2S forms only in areas where no DO is present.

Continuing - In winter, ponds especially those with ice cover, can have a THIN warmer layer at the bottom (see later). In winter ponds that do not develop ice cover, these ponds often will regularly or often completely circulate and not maintain a warm bottom layer due the water column of 39F-42F, convection currents, and wind action mixing. Thus the thin warmer bottom layer can frequently get disrupted and diluted into the upper colder water. The winter thin warm layer in non-iced ponds is often very temporary or non-existent depending on the individual pond conditions.

This warm bottom layer during ice cover is not very thick as in a maximum of 2" to maybe 5" thick above the bottom & mostly 2"-3" thick as by my measurements. This thin warmer layer is due to a mild geothermal affect of the bottom mud "giving back" some of the bottom layer heat that was absorbed during summer. As the winter progresses the geothermal affect is lessened. This warmer layer by my measurements is usually only 1F to 2F warmer than the 39F water above it. Keep in mind that this warmer water is lighter less dense so it "wants" to rise into the 39F water, and it quickly gets diluted and cooled by the massive amount of 39F water above plus the geothermal affect is not intense, but mild.

The warm thin layer can get easily mixed, diluted, and somewhat blended into the upper 39F water by bottom oriented fish activity. In some ponds with sizable underground spring fed water flow the warmer layer can be thicker and larger due to the constant flow or influence of spring ground water often being near 50F.

saint-a says
Quote:
Wouldn't a deeper pond have a greater difference in temperature between surface and bottom that might make it more prone to turning over?
No, No to more prone to turning over, And Yes possibly more prone to a fish kill due to a larger volume of bad deep water compared to upper warm layer volume. Here is where surface to volume ratio comes into play.

The greater the difference in temperature between surface and bottom the greater the resistance is to mixing between the two layers. In other words the greater the difference in temperature the less the two layers will "want" to mix together. As the temperature difference increases the more energy it takes to combine or mix the two temperature layers together. Energy as in sun (heat, thermal), wind, mechanical.

Last edited by Bill Cody; 11/30/19 09:24 PM.

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Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514489 11/30/19 08:06 PM
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As far as I understand it, the only thing that causes a turnover is the surface water reaching 39 degrees. Whether than happens due to a cold snap, a cold rain or winter moving in it doesn't matter. It's the 39 degree surface water that sinks down to the bottom and turns things over. I will actually go out on a limb here and make a wild guess. I bet a really cold rain stands more of a chance to cause a turnover in a 10' deep pond than a 20' deep pond.

A deeper pond is more than likely going to have less vegetation than a shallow pond. Dying/decomposing vegetation sucks up O2 under the ice.

Deeper lake SHOULD be more resistant to temperature fluctuations.

I don't think you can always assume a deeper pond has MORE anoxic water. Each body of water is different.

EDIT: Bill Cody posted while I was typing. I defer to what he says!

Last edited by wbuffetjr; 11/30/19 08:07 PM.

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Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514490 11/30/19 09:18 PM
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wbuffetJr has correct statements:
Quote:
I bet a really cold rain stands more of a chance to cause a turnover in a 10' deep pond than a 20' deep pond.

A deeper pond is more than likely going to have less vegetation than a shallow pond. Dying/decomposing vegetation sucks up O2 under the ice.

Deeper lake SHOULD be more resistant to temperature fluctuations.

I don't think you can always assume a deeper pond has MORE anoxic water. Each body of water is different.

Last edited by Bill Cody; 11/30/19 09:19 PM.

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Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514491 11/30/19 09:19 PM
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Just a thought....Just because the water on top drops to 39 degrees, it does not mean it will sink to the bottom. As it begins to drop it will encounter "warmer" water that will transfer heat to bring the temperature up before it ever reaches bottom. IMHO it takes a significant event, like a heavy cold rain, to create a cold water volume significant enough to cause a rapid turnover (possible fish kill). I suspect most turnovers occur slowly in the absence of a significant event. The deeper the pond is, the more "warm" water buffer there is between that 39 degree water on top and the bottom and the more significant the cold water event needs to be to cause a rapid turnover.

...Just my 1 cent

Last edited by Bill Cody; 11/30/19 09:27 PM. Reason: bold words

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Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514492 11/30/19 09:25 PM
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BillD is correct.

In most but not all deep non-bottom aerated ponds in central and northern regions, mid-summer bottom water temperatures in these deep ponds is commonly 50F-55F. In very deep lakes the bottom water can be close to 39F-40F year round. Very deep lakes can be unique. Most water bodies completely mix twice a year, some only once a year and a few of the deepest lakes or smaller deep ones with very little wind action never or very rarely completely mix top to bottom. These types of lakes have special limnological names.

Last edited by Bill Cody; 12/01/19 03:31 PM.

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Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514505 12/01/19 11:09 AM
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Interesting stuff

World's deepest lake shows 8 year turnover

"Lake Baikal, in Siberia, the deepest lake in the world, holds a fifth of the world's supply of fresh water. The researchers found that 12.5 percent of the deep-water inventory of the lake is renewed each year, corresponding to a mean deep-water residence time of eight years. Some water at intermediate depths had a much slower turnover rate of once every 14 to 16 years."

Last edited by wbuffetjr; 12/01/19 11:27 AM.

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Re: Why are deeper ponds less prone to fish kills?
Bill Cody #514510 12/01/19 01:28 PM
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Originally Posted By: Bill Cody
Here is where surface to volume ratio comes into play.


What's the ideal ratio to prevent summer kills?

Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514511 12/01/19 02:26 PM
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Originally Posted By: saint_abyssal
Originally Posted By: Bill Cody
Here is where surface to volume ratio comes into play.


What's the ideal ratio to prevent summer kills?


It's hard to say. Maybe about 1 to 8, surface square feet vs gallons capacity, which would be roughly one to one, square feet to cubic feet.

Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514512 12/01/19 03:55 PM
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Here is interesting information from Clemson Univ. about water depth and turnovers.
https://www.clemson.edu/extension/water/...tion/index.html
How do I prevent a turnover?
Proper pond construction is important. The ratio of surface water (epilimnion) to deep water (hypolimnion) is a major determining factor for the frequency and severity of turnovers. Deep ponds with small surface areas often are more prone to severe turnovers because there is a larger volume of deep, low-oxygen water relative to the amount of oxygenated surface water. Broad, shallow ponds have fewer turnovers because less of the pond's volume is tied up in low oxygen deep water. Most stormwater ponds are shallow (6ft deep or less), but some are deeper. It is important to know the topography of the pond's bottom (bathymetry) as well as the average depth of the pond. Over time, the depth of the pond will change as it fills with sediment, so the bottom contours may not be the same as when the pond was constructed originally. (See section on Pond Construction and Sedimentation)
Prevent aquatic weeds from covering more than 20% of the pond surface. Floating and submersed weeds impede circulation and the diffusion of oxygen into the pond. They also remove oxygen from the water as they decay. Controlling aquatic plants will improve circulation and reduce stagnancy.
Reduce nutrients and algae growth. Loading ponds with nutrients from yard fertilizers, exposed sediment, pet waste, and feeding of fish/ducks/turtles will increase algal growth which can further deplete oxygen.
Mechanical circulation can prevent turnovers. The primary purpose of mechanical circulation is not aeration/oxygenation (injecting oxygen into the water). Its purpose is to prevent stratification and bring water to the surface so that it can be exposed to oxygen in the atmosphere. By mixing the water, circulation systems never allow the pond to form layers, thus they do not develop a low oxygen layer on the bottom. In this way a circulation system becomes an insurance investment to prevent a fish kill.


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Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514613 12/04/19 03:38 AM
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Whoops, I just googled “Why do fish go deep in the winter” and the first hit said this:

“ Fish descend into the bottom of streams and lakes where the warmer water is in winter. Density of warm water is higher, causing it to sink to the bottom in winter. This is the same reason that ice cubes float in a glass of water. ... During the cold of winter fish become less active.”

Sorry for the misinformation, the internet let me down this time!

Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514615 12/04/19 05:28 AM
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Steve - it's a really weird phenomenon if you think about it. 39 degree water is the densest. Anything warmer or colder floats. I still scratch my head about how that process works. It's that one degree that makes the whole thing work. Pretty cool.


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Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514626 12/04/19 01:57 PM
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Fish kills need to be separated into groups based on cause. I would suggest that southern ponds have as many and maybe more water related kills than northern ponds absent kills caused by ice over. Rarely do southern ponds get to 39 F in deep water (colder above - i.e. ice).

While the remedies are often similar the causes vary as do timing.
















Re: Why are deeper ponds less prone to fish kills?
ewest #514637 12/04/19 05:40 PM
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Would the cross-section of a pond ideally resistant to fish kills look something like this? Deep to maximize volume but with the areas shallow enough for plants partly walled in to keep organic matter from making its way down to the anaerobic bottom?

Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514641 12/04/19 08:02 PM
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No, not nearly that extreme. Some shallows at one end, then all around the rest, about a three or four to one slope to the maximum depth of maybe 12 to 15 feet, then a mainly flat bottom. If it's a small pond, maximum depth will necessarily be less.

Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514642 12/04/19 08:04 PM
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This type of deep steep sided pond will still accumulate a "normal" amount of organic matter because a lot of the organic matter is derived from dead plankton. In many instances this accumulation is very close to 1" per year. Dead plankton is primarily phytoplankton and zooplankton. "Normal" used here is a very relative term. Ponds that receive lots of tree leaf or terrestrial organics are not IMO normal ponds. Dead organic plankton matter is very dependent on the productivity of the pond - often determined by water transparency.

Last edited by Bill Cody; 12/04/19 08:08 PM.

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Re: Why are deeper ponds less prone to fish kills?
Bill Cody #514645 12/04/19 10:18 PM
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Originally Posted By: Bill Cody
This type of deep steep sided pond will still accumulate a "normal" amount of organic matter because a lot of the organic matter is derived from dead plankton. In many instances this accumulation is very close to 1" per year.


Ah. I hadn't thought of plankton as a contributing factor. In that case how would one design a pond to minimize risk of fish kills without aeration?

Re: Why are deeper ponds less prone to fish kills?
wbuffetjr #514650 12/05/19 03:57 AM
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Originally Posted By: wbuffetjr
Steve - it's a really weird phenomenon if you think about it. 39 degree water is the densest. Anything warmer or colder floats. I still scratch my head about how that process works. It's that one degree that makes the whole thing work. Pretty cool.


I agree, that is fascinating.

Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514705 12/06/19 07:37 PM
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Quote:
In that case how would one design a pond to minimize risk of fish kills without aeration?


The answer highly depends on location / climate of the pond. Deeper is not all that beneficial in the south. Deeper is better in the north. Both locations keep the pond in an area that receives the maximum wind exposure. Wooded areas are conducive to fish kills in older ponds due to lots of leaves/organics, higher BODs and lack of wind.


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Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514741 12/08/19 04:34 PM
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I didn't remember that 39 degrees was the magic number, but it sounds about right.

Ice has a crystal structure that takes up more space than when it is a liquid. Therefore ice is less dense than water and floats. H2O starts this "expansion" at about 39 degrees when the water molecules are just beginning to position themselves to form ice crystals. With no other outside factors, the colder water sinking to the bottom of a pond should never be lower than 39 degrees. Anything warmer OR colder will rise. However, wind action could stir up and move water colder than 39 degrees to the bottom. The deeper a pond, the more protected the bottom would be from this stirring. And the temperature of the surrounding ground could act as a heat sink to buffer the bottom temperatures, especially in the South.

Also, I'm guessing that it is partly that a sudden change in temperature of the water is hazardous to fish. A deeper pond would have more water in it than a shallow pond of the same surface acres, and the greater water volume would slow down any temperature change. Plus the fish would have a wider choice of places to be to get at a temperature they could tolerate.

Last edited by HaBi Farm; 12/08/19 04:38 PM. Reason: Additional info
Re: Why are deeper ponds less prone to fish kills?
Bill Cody #514785 12/09/19 07:10 PM
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Originally Posted By: Bill Cody
[quote]The answer highly depends on location / climate of the pond. Deeper is not all that beneficial in the south. Deeper is better in the north.


Would West Virginia count as north or south for this purpose?

Re: Why are deeper ponds less prone to fish kills?
saint_abyssal #514787 12/09/19 08:48 PM
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Assuming we are only considering climate, If the pond gets 30+ days of ice cover, I would classify it as northern pond. A refined classification would put a WVA pond and those with similar latitude as "intermediate".

Technically if I was establishing the north vs south pond classification, I would create a scale that was similar to vegetation plant hardiness zones. Incorporate number of growing days with temperatures above 50F.
https://garden.org/nga/zipzone/2012/

Last edited by Bill Cody; 12/09/19 08:53 PM.

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