BILL CODY PROVIDES THE FOLLOWING DEFINITION OF THERMOCLINE
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 are 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, decomposing 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.
