Pond Boss Forums AERATION Systems questions Time it takes the Pond to stratify again

As I was turning off my aerator, the thought came to me as to how long would it take the pond to stratify and for the thermalcline to return. Is it almost immediately are does it take a while?

Assuming the system is fully mixed top to bottom then it takes a while as in days not a couple of hours. It depends on a lot of factors like water depth , temps, weather , current water conditions, pond shape,depth and size and time of year.

I've got my circulator on my big pond turned on from 8 pm to 8 am.

At 8 in the morning my pond is one uniform temperature from top to bottom. By mid-afternoon I can go swimming in the pond and sense a clear difference between the temperature of the water at the top, and the temp of the water three to four feet down. The water begins to stratify almost immediately, but to get a true thermocline, which is cool water at the bottom that's actually been at the bottom for a while, and is considerably cooler than the surface layer I would have to clearly agree with ewest that it would take a few days. It's a good question. Cody probably knows even more specifics. In addition to ewest's list of factors, I'd point specifically to wind as a pretty big factor. I think calm water stratifies much more quickly.

I agree with Bruce. Wind is a big factor (weather). I was addressing the DO loss problem associated with a thermocline. No doubt that the cold water (if there is any at that time) will start sinking soon after the mixing stops. A good example of how fast a problem can start is a couple of cloudy days followed by a cold summer heavy rain's (runoff) effect on a pond causing loss of already low O2 as the plankton dies off and the O rich top water is cooled and sinks.

Bruce your situation sounds like water warming during the day. How do your DOs change over that period if at all.

The DO's stay high until you start to reach water that isn't getting sunlight. My Secchi readings are exactly 1 meter for the last four weeks. Once you get deeper than that the DO drops pretty drastically if I'm not circulating.

Bruce I was wondering how fast/much they change during the daylight hours below 3 feet.

Within an hour the DO starts to drop five feet down.

Last weeks I was testing it and the DO's one foot down were 8+ ppm, and the same at 5 feet.

Now when the circulator is turned off the DO drops to 6 ppm 5 feet down within one hour, then to 4 ppm by ten hours.

It All Depends. If you are primarily concerned about dissolved oxygen in the stratification process (I assume that you are); then it depends mostly on the biological oxygen demand (BOD) of the water column and the water transparency (clarity). Other factors (see below) affect it, but those are the two main ones. BOD and oxidation reactions are responsible for the rate of oxygen loss due to decomposition and respiration in the water and TRANSPARENCY is the main factor allowing for oxygen production (plants) in the water.

The clearer the water is the deeper oxygen will be produced and the slower the oxygen will be consumed. Clear water has fewer organic things in it to use up the oxygen. Fertile, green, productive water looses oxygen relatively fast when it receives no light.

There are situations where ponds/lakes are very clear and light will penetrate deep into the depths at or below the thermocline. Oxygen will then be produced in these deep waters; sometimes even below a thermocline. When water is very clear the productivity is low, the BOD (oxygen loss) is very low and oxgen is consumed very, very slowly in the deepest cold layer (even in very low or no light). In these situations it takes a long time to loose all the dissolved oxygen below the thermocline. In very clean clear lakes (oligotrophic) the oxygen losss is very slow and DO levels remain high enough to support fish in the deep zone all summer. These lakes support trout fisheries (cold water fishes) all year. Productive spring fed ponds can also support trout all year. Spring water is often devoid of dissolved oxygen. That is for another discussion.

Basically rate of oxygen loss when you shut off the aerator will be dependant on how clear the water is. Additional factors that will also affect it are as ewest says: "It depends on a lot of factors like water depth, temps, weather, current water conditions, pond shape, depth and size and time of year".

I think that the reason that my DO drops so quickly below the "clarity line" is not oxygen consumption. It's merely the lack of oxygen addition since the top oxygen rich water isn't being churned downwards.

Shhhh, don't tell anybody, but I just figured that out from Bill's post.

Bruce, if the DO below the "clarity line" was not being consumed it would not need to be added (enhanced) by, in your case, churning. Your churning activity keeps the DO from above, mixed into the zone of high oxygen consumption and no oxygen production due to lack of enough light intensity for photosynthesis. Below the light penetration zone, the plants stop producing DO and the consumers primarily bacteria and zooplankton are always using (24/7) the DO very fast. Mixing pushes the DO into their zone. It is similar to an air fan blowing fresh air into a warm room full of many people.

How much of an effect then, does air's buoyancy have on the equation? Is dissolved oxygen light enough in that form to begin to lift out of the hypolimnion? Dumb question, maybe?

....and also, when I turn my circulator off do the microscopic plants trapped below the clarity line begin to respire, hence dropping my DO so quickly in the deeper waters?

I think this is such a good question. I don't remember this specific topic, so it's kind of fun when we talk about something new. In my mind the above posts are staying on topic, because my pond may be a really good test for how quickly thermoclines form.

To make a full analysis, I guess we'd have to once again define thermocline. How much of what defines a thermocline is temperature, and how much is DO and other water quality parameters? Must the hypolimnion be mostly devoid of oxygen?

Bruce asks -
1. "How much of an effect then, does air's buoyancy have on the equation?"
As far as I know about physics of gas, air bouyancy has almost no practical affect on this topic.

2. "and also, when I turn my circulator off do the microscopic plants trapped below the clarity line begin to respire, hence dropping my DO so quickly in the deeper waters?"
I assume that whenever the microscopic plants do not get enough light for photosynthesis, then photosynthesis stops or slows down dramatically. I think plants continually respire (24/7) and during daylight oxygen production exceeds amount of oxygen consumed. So when plants are not producing oxygen they are using oxygen along with the bacteria and zooplankton. Remember, bacteria numbers are at tremdous densities in the water column if there are a lot of dead organic particles present, and there usually are if the water is cloudy.

Bruce says -
"because my pond may be a really good test for how quickly thermoclines form". Your pond would be a very good candidate for a test pond for that particular water clarity. You have all the tools needed to conduct the tests.

Remember each pond is different and the rate of formation of the termocline will be dependent on water clarity. A thermocline will form faster in a pond with 2ft visibility than a pond with 4 ft of visibility. A pond with 6 ft visibility will take even longer for a thermocline to form.

Stain in the water, such as Aquashade, will also affect how fast the thermocline forms. Stains inhibit light penetration thus they will accelerate formation of thermoclines.

Working on Bruce's question of -
"To make a full analysis, I guess we'd have to once again define thermocline. How much of what defines a thermocline is temperature, and how much is DO and other water quality parameters? Must the hypolimnion be mostly devoid of oxygen?"

This involves some academic items and it will take a few hours, maybe a day or two.

Thanks for the time you're spending on this Bill. You're right, if I have too much time on my hands I like to ask lots of questions. Bill (Dr. Perca) says "You can learn buy asking questions". Bruce - Keep asking those questions.

This question lies right at the core of what it takes for pondmeisters to push the envelope when it comes to raising species that naturally occur in cooler climates. i.e. yellow perch in Nebraska, or rainbow trout in South Dakota, etc.

It is at the core for anyone who has fish!!!!

If you have high densities or are feeding or have a fertile pond it is critical to understand this concept which is the key to pond biology.

Bill a big thanks for expounding on the subject.

Has anyone developed any device that can be permanently monitor a pond at some point, maybe the deepest point. Maybe a set of thermometers or other instruments staged at different depths that plug into an black berry. This is a really good topic even if I did start it.

heybud it is an excellent topic !!! There are systems to monitor temps and DOs which hook to remote computers. They are expensive and are used in the commercial catfish business.

Yes. Really, really good topic.

I've used such a device, and they're incredible. Only problem, as ewest states, is cost. Way expensive! Luckily I was able to borrow one for a while. A company called ISCO in Lincoln, NE makes them for municipalities and, once again as mentioned by ewest there are units available from Aquatic Eco-sytems for catfish farmers, etc.

Okay I am back with some more info; maybe too much info for some causal readers. This stuff is for the diehard pondowners.

Bruce asks:
"I guess we'd have to once again define thermocline. How much of what defines a thermocline is temperature, and how much is DO and other water quality parameters? Must the hypolimnion be mostly devoid of oxygen?"

Thermocline , also called metalimnion , buy definition involves solely temperature. Distribution of dissolved oxygen (DO) and other chemical concentrations throughout the water column are separate features or items from the definition of thermocline.

Presence and or persistence of oxygen in the hypolimnion (deepest coldest zone) is almost always related to the productivity or eutrophy of the pond/lake. If a thermocline or the deeper hypolimnion develops in most fish ponds it will usually loose the DO in a few hours, days or weeks because the pond is eutrophic – rich in nutrients, has low water visibility and very abundant plankton that includes bacteria and high amounts of suspended organic particulates that when they decompose consume dissolved oxygen.

Eutrophy which is demonstrated by the amount of productivity is the degree of eutrophication or loading of nutrients, i.e. amount of fertility. The degree of nutrient loading (inputs all sources) is very important when discussing eutrophication and productivity. Ponds that are fertilized to increase productivity are definitely eutrophic or even hypereutrophic. Eutrophic lakes usually have water visibilities of 3-8 ft deep. Hypereutrophic is a term used for very enriched ponds/lakes that have nuisance algal blooms and water transparences of usually less than 3 ft. Many fish raising ponds are thus by definition hypereutrophic.

Bruce asks – “Must the hypolimnion be mostly devoid of oxygen?”
Traditionally, eutrophic waters are enriched with nutrients and during summer lose all oxygen in the hypolimnion which is the deepest isolated zone that does not receive light and does not get exposed to the atmospheric air i.e. wave action. Often the middle, thermocline layer also does not receive light from the surface.

However, oligotrophic waters have DO at all depths below the thermocline all summer. Visibilities in oligotrophic waters are usually greater than 12 ft. Some people use the term mesotrophic. Mesotrophic refers to lakes with moderate or a medium amount nutrient enrichment and these lakes have DO in the hypolimnion most of the summer or their waters lose the DO in just part of the hypolimnion during summer. Mesotrophic lakes have visibilities predominantly of 8-12 ft. see link for more trophic info: http://plants.ifas.ufl.edu/guide/trophstate.html

Definitions of thermocline and hypolimnion are not determined by amount of dissolved oxygen present.

The standard textbook definition of thermocline is the depth where the change of temperature is greater than 1 degree centigrade per meter (1.8F per 39” or roughly 3 ft). Thermocline has been defined various ways but it correctly refers to the plane of maximum rate of decrease of temperature with respect to depth – zone of steep thermal gradient (Limnology 3 ed. R.G.Wetzel).

The standard definition of thermocline does not usually apply to ponds because the temperature decrease in ponds often exceeds 1 C degree per meter (Parks et.al 1975). I have observed temperature differences of up to 2.9 C deg (5 F) per one foot in ponds. Boyd(1990) reported a temperature difference of 7 C deg (12.6F) in a thermocline that was just 20 inches thick in a fish pond of maximum depth 8 ft. Temperature differences of 2-3 F deg per foot are common in the thermocline of non-aerated northern ponds during summer stratification. The lowest point of the thermocline or the thermocline’s deepest point occurs when the temperature differences stabilize to less than 1.8F per 3 ft (1C deg/m). In ponds, this varies but will often be displayed by temperatures of less than 1F deg change or decrease per foot.

When ponds stratify or separate into temperature layers, the amount of energy to remix the entire volume of water to a uniform temperature is called stability of stratification . The greater the energy required to completely remix the water column then the more stable the stratification. Stratification is not very stable in small shallow ponds compared to larger deeper ponds. Ponds 5 ft deep in the warm months can thermally stratify daily and destratify nightly when the upper water cools by conduction and wind. Ponds with depths of 8 ft or more will often remain stratified for long periods during summer. Strong winds and or cold dense rain can cause destratification in ponds. This can cause summer fish kills if the deepest water zone has lost all its oxygen.

Thus when a pond thermally stratifies and forms the thermocline , if the deeper water receives no light, photosynthesis will stop and every living thing (aerobic bacteria to fish, except anaerobic bacteria) consumes dissolved oxygen (respiration). Exceptions can occur where oxygen actually increases and very high percent saturations of DO will occur in the thermocline due to dense concentrations of phytoplankton that receive light.

Oxygen is also lost in the areas receiving no light by biochemical and chemical consumption of dissolved oxygen. These are usually referred to as BOD and COD. BOD or Biochemical Oxygen Demand – is the amount of dissolved oxygen necessary by bacteria to decompose organic matter. BOD is also thought of as the amount of DO necessary to oxidize the readily decomposable organic matter. Note - BOD by textbook definition is not biological oxygen demand as I have often casually stated here on this forum. COD or Chemical Oxygen Demand is the amount of oxygen required to oxidize all organic material into carbon dioxide and water.

It is very interesting that research has shown the amount of suspended DISSOLVED organic matter can be many times more abundant than the suspended particulate forms of plankton and tiny dead organic particles (Wetzel 2001). The decomposition and direct chemical reactions of dissolved organic matter can be responsible for utilizing lots of dissolved oxygen from the water. Organic matter can be increased also from external sources such as leaf fall, wind blown land derived materials, leaching from soils and runoff.

If you have read this far, good for you, and I will now provide a way for you to fairly easy determine how long it will LIKELY take for your pond to loose the DO in the thermocline when you turn off your aerator. It is not as important to know how fast the pond will stratify when you turn off your aerator as to know how fast the oxygen will be lost in the depths below the zone of light penetration. For our experiment or test, we will use a modification of the standard light-dark bottle BOD method.

For our procedure you will need a way to measure dissolved oxygen (DO) and two similar sized stoppered or capped clear glass bottles. Simple dissolved oxygen test kits are available in larger pet stores and aquaculture supply companies.

First fill both bottles with pond water. Set one bottle outside so it receives daily sunlight similar to light intensities that the pond receives. Take the second bottle of pond water and put it in complete darkness. You can wrap the "dark" bottle in aluminum foil to simulate darkness or put the bottle in a light-tight box. You could also use an opaque bottle for the sample but make sure it it truly dark inside the bottle. This "dark" bottle will simulate water in the dark area of the thermocline or bottom of your pond. Let the dark bottle set in the dark for 24 hrs and then measure the DO. The difference in DO of the two bottles will be the rate oxygen is lost per day from your water that receives no sunlight.

If you want, you can put several bottles of water in the dark and then test one every few hours or days. Checking the DO remaining in each bottle over an extended period will give you a better and more accurate estimate of oxygen loss rate for your particular pond. Correlate the rate of oxygen loss with water visibility and you will have a general idea how long it will take to lose the DO for that particular measurement of water clarity in your pond. Notes – 1. each pond will be different and DO loss will change as water clarity changes, 2. the DO loss rate is just a “ball park” number because of SEVERAL variables (such as cooler water temperature of the thermocline) not taken into account in this simple test.

Example:
For bottle #1 in the ambient natural light cycle, the DO of fresh pondwater tested 8 ppm when collected and it was 10 ppm after 24 hrs. Reason - DO can accumulate in a stoppered bottle when exposed to sunlight. Bottle #2 in complete darkness tested 4 ppm DO after 24 hrs. DO loss is 4ppm/24 hrs or 1ppm per every 6 hrs. If you have complete water column mixing (in this case DO of 8ppm) and you turn off your aerator in this example, you can leave it off for 48 hrs before the DO in the dark part of the water column will drop to 0 - 0.5 ppm (8ppm X 6hrs per ppm). Note that the DO loss will be greater at the very bottom of the pond because decomposition is more rapid at the mud water interface due to the amount of many accumulated organics compared those in the water a foot or more above the bottom.

Why not try this simple test in your pond and let us know your test results?. Remember to include your water clarity measurement with your results. Note the suspended, inorganic mud-dirt- clay in the water column will not consume very much DO. So water with high clay turbidity and low visibility will not be real comparable to water with same clarity and abundant plankton. I have several ponds that have big differences in water clarity from 1 to 15 ft that I can test using this method.

There you go pondmeisters – a long answer to a short but good question. And as always, remember, “it all depends”.

References and additional reading.
Boyd C.E. Water Quality in Ponds for Aquaculture 1990. Birmingham Publishing.

Parks, R.W., E. Scarsbrook, and C.E.Boyd. 1975. Phytoplankton and Water Quality in a Fertilized Fish Pond. Auburn Univ Circ.224.

Wetzel, R.G. 2001. Limnology Lake and River Ecosystems. Third Edition. Academic Press.

Bill,
That was absolutely a great response. I appreciate your research and work on this complicated topic.

Thanks Bill !!!! A great reply and easy to read as well. I need to do more water quality work on our ponds. So many projects so little time.

First of all, I'm now archiving this thread, and in particular, Bill's answer.

Secondly I would like to encourage Mr. Lusk, if he sees this, to publish it as an article in Pond Boss magazine.

Thirdly, if you read this response, and are not a subscriber to the magazine, I strongly encourage you to sit down and write a check to Pond Boss world headquarters, because the magazine and forum are completely intertwined, and each would lose value without the other.

Thanks,

Bruce