Pond Boss Forums AERATION Should I aerate? Temp Tests&Stratification

INSTRUCTIONS FOR WATER TEMPERATURE TESTS ARE IN A POST BELOW
Questions were asked in a separate post about:
1. How deep does something (pond/lake) have to be to stratify?
2. What are some other factors in stratification?

Introduction. This description of stratification is limited to thermal stratification. Chemical stratification sometimes occurs in unique situations. Comments describe general patterns and numerous exceptions to these descriptions can occur.

Discussion. 1. Small shallow (4'to5' deep) ponds can stratify where you have a shallow warm layer floating on a thin cooler water layer below. This type of straitification is usually not too stable and a windy day or cold rain event that produces decent waves will mix the water. Warm layer may be one to two feet thick. A more stable stratification will occur in ponds around 6 to 8 ft deep. However the cool bottom layer will not be very thick if the pond is only 7'to 9'deep.

2. Factors causing stratification. Firstly we must keep in mind that water changes density (weight) as the water temperature changes. Maximum weight or density of water is at 4 deg C (39.2 degF). When water gets warmer OR colder than this it gets lighter or less dense. For example, water near 80degF is quite a bit lighter compared to when it was 39 or 40 deg.

When the pond comes out of winter or ice cover the surface layer quickly warms from 32 to 39 degF. At 39 deg the pond is uniform temperature and little density difference occurs throughout the entire water column and small amounts of wind can mix the entire water column. Cooling of surface water at night or spring weather events (cloudy/rain) also produce minor convection currents (down to abt 9ft deep). In spring wind mixes the entire water column.

Over one half of the sun's heat is absorbed in the upper 6 ft of water. Dissolved and suspended particles in the water (reduced transparency) cause more of the heat to be absorbed near the surface. Most of the heat energy is distributed into the depths by the wind. So ponds that are wind sheltered do not get the surface heat mixed very deep. In wind sheltered ponds mixing of the entire pond to the bottom only occurs for a few days in spring and again in fall compared to wind exposed ponds or small lakes that may mix completely for several weeks during these periods.

As the surface waters warm, resistance to mixing quickly becomes greater esp during several warm calm days. When surface water warms only a few degrees, now the wind cannot mix the water very deep because the water is a lot lighter and wants to float of top of heavier cooler water below. It takes quite a bit of energy to force the warmer water down into the cooler more dense water below. As day time temperatures keep increasing the thermal resistance to mixing increases. Now even strong winds cannot mix the water very deep. Wave height primarily dictates the mixing depth. Many small ponds develop a warm layer about 6 to 8 feet deep, some even shallower. In large wind exposed ponds and sm lakes the mixing layer is deeper. Thickness of this layer tends to be the same from year to year. A temperature profile where one measures the temperature every foot or two will determine the depth of the thermal layers. Sometimes as the summer progresses the warm layer will migrate deeper and the thermocline will be deeper in September than it was in May or June.

In all nutrient rich ponds and many mesotrophic
ponds the oxygen is lost/consumed in the deep colder zone by early to late summer. In many nutrient rich warm water ponds with shallow light penetration the oxygen is gone/consumed in the cool water deep zone in a few weeks. Length of time for this to occur depends on several factors such as water visibility, and amount of dead organic matter (dead &dying bacteria / phytoplankton/ zooplankton/ plus organic particulates) dropping out of the warm layer into the cold layer. Bacterial decompostion of this dead material is responsible for consuming almost all the oxygen that is lost.

The upper warm layer is called epilimnion.
The deep bottom cold layer is called hypolimnion.
The transition layer in between is called thermocline or metalimnion.

Cooling of the water in fall reverses the above process until the whole water body is 39 deg again and the wind can easily mix the entire water column. Cooling below 39 deg again lowers the water density and water cooler than 39 deg wants to float on top of the 39 deg water below. Water freezes on top when it gets 32degF (O degC) and leaves a big zone of water below the ice that is 39 deg clear to the bottom.

I know that summer kill is a result of oxeygen depleation in the hypolimnion and high tempetures in the eplimnion. How sensitive are LMB to this situation? And what are the best and the cheapest ways to correct or avoid summer kill?

LMB as with most all fish, avoid water with low oxygen. Lower DO concentrations tolerated by LMB: LMB quit eating at around 3ppm DO. AT 1.5ppm DO LMB will beach themselves; at 1ppm DO, mature to med sized LMB will die in abt 11 hrs. LMB will display strong avoidance of water with 1.5 ppm DO.
Temperature tolerance of LMB in epilimnion as asked above. LMB like warm to relatively warm water. Young LMB have been reported to feed readily at 95 degF which is near their upper leathal limit. Their prefered temp if available is 86 to 89.6 deg F. Northern (Ontario) LMB prefer water temps somewhat lower than mid to deep south LMB. Ontario LMB become sluggish at 82 to 86 deg. Upper lethal temp limit of LMB is 94 to 97.5 deg F depending on fish size, brood stock and acclimated temperature.

Best and cheapest ways to avoid summer kill?.
In my opinion best way to avoid it is to prevent DO loss in the hypolimnion. So if the pond rapidly or quickly turnsover the upwelling deep water has DO it. Turnover effects will be minimal since the pond is continually or frequently mechanically destratified. This means prevent an expansive hypolimnion deficient in oxygen from forming.

Only way to feasably do this I know of is to destratify with bottom aeration (air injection). This is much, much cheaper and more efficient than trying to pump/move water which weighs 8 lbs/gal. vs air which is almost weightless compared to water. Done properly, I can air lift 250,000 gal per hr with 1/4 hp. Explore how big of an electric water pump it would take to do that.

Destratifying large hypolimnions of larger bodies of water becomes more complex because it takes more hp. more diffusers and longer running times.

Temperature Test Instructions
Calling all helpful pondowners. I have an interesting proposal for a simple informative study many of us can do to check pond stratification all across North America.

Lets get several to numerous pond owners from all over North America without pond aeration to measure the depths of the thermocline(metalimnion) of their pond OR use a friends pond or a favorite fishing pond without aeration.

We can do this now (spring) and again in late summer early fall to see if the thermocline moves lower in a pond once it forms. (NOTE - You might want to wait until the LMB are spawning before you test the water temps to make sure the thermocline has become established). These measurements, hopefully from all pond types will also show us the differences or similarities of pond stratification from all over the country.

For results we should at a minimum post:
1. Date & state, (nearest city or region)
2. pond size (maybe notes such as sheltered or pond in a valley,or pond in a woods, clear vs cloudy water, visibility depth)
3. Max depth
4. Water Temp at surface, water temp at thermocline, and maybe bottom temperature.

Necessary Equipment:
1. An electronic indoor/outdoor temperature guage with a 10 to 20 feet cord attached to the probe/sensor. 20 feet cord will be BEST but a 10 feet cord will find the thermocline in most small ponds. (you will prob. need a 20 feet sensor cable for sm lakes). Mark the cable/wire probe every foot w/ duct tape or similar item. These digital temperature guages are available at Radio Shack, Wal-Mart, Target Dept Stores, Northern Tool & Equipment; Also check on eBay under digital thermometer. I got mine from a boat dealer; an 'Airguide" TempSensor w/ a 23 feet cable, AAA battery, $20.00, good buy.

2. Boat w/ anchor. You need a boat to go to the deep part of the pond, anchor, and take temperature readings every foot until you locate the cold/cooler layer. I add a fish sinker to my probe to help it sink and stay straight down.

3. Come back here and post your results. We will all learn a lot, educate others, and have fun too.

Thermocline layer will be marked or found when the water temperature drops 3 to 5 deg F in one or two feet of additonal depth. The upper layer (epilimnion) is usually only 2 to 3 deg F difference from surf & down to top of thermocline.

AS an example, I will post early spring temperatures of my pond Wednesday Apr 16, 2003. I am interested in how many states/provinces across North America we can get pond temp readings; this will be valuable information. Thermoclines can be quite different among ponds within the same state.

Mr. Cody- would love to help but aerate. Sorry!

Jeff

If interested and you still want to participate you can measure temperatures in a friends pond or a pond or small lake that you fish in. All are welcome and the more measurements we have the better the data base will be. If you aerate your pond you should still have one of these electronic digital thermometers to check how well the aerator is working and for miscellaneous water temp records. After taking temperatures you might want to relocate your bubbler/diffuser based on the temperature results. Digital thermometer will indicate if moving the diffuser improved the circulation or not. Temperature data may indicate that you might need to add a second or third diffuser to the pond.

Cody Pond, NW OHIO, Apr 16, 03
3/4ac, somewhat wind sheltered 3 sides, visibility 4 ft. Max depth 17ft briming full
Sur 62.2 7ft 51.4 13ft 45.5
2ft 60.4 8ft 48.7 14ft 45.3
3ft 60.4 9ft 47.7 15ft 45.3
4ft 59.8 10ft47.0 16ft 45.3
5ft 57.5 11ft46.2 16.5Bott 45.5
6ft 53.2 12ft46.0
Thermocline was not fully formed yet. Largest temp difference of 4.3 degF & occurred between 5'&6'. I will aerate this pond but not until the thermocline fully forms. For the study I will do tests on another pond.

Bill-- I'll be glad to add temps from my non-aerated 6.5 acre pond in South Central Michigan.
I'll get a digital thermometer today and post as soon as I get one of the boats in the water. Would total water depth at the point of measurement be useful? I may be at 18 feet depth today but by mid August I may be down to 16 feet or less. I'll post as soon as I get the readings.

Dan

Dan - Slow down, if possible get the Digital guage with the 23 ft cable. It is a better more versatile unit. Do not post temps until the LMB are spawning prob Mid to late May for your location. Otherwise you will get same results I did, not much defined stratification or layering. Let the thermocline fully develop then let's see if it migrates downward during the summer. Your pond should give good info since it is somewhat larger.

Maximum depth at measuring station would be good, better if you could record the temp at or near (within 4") of bottom. BC

Bill- After checking Wal Mart, Meijers, ACO, True Value, Lowes, 4 other independent hardware/home centers and the Northern and Cabelas catalogues all the digital temp. gauges I located had a 10' probe. I'll keep looking but if I splice another 10' wire (of the same size and type)into the existing wire would that affect the reading? I don't know if additional resistance would be aproblem or not.
I printed up a small chart with visibility, surface thru 18 feet lines and total water depth so I can just fill in the blanks.
I'll try the 1st. and 15th. of June, July and Aug. and we'll see what the readings are.

Dan - I will check on the availability of the Airguide Temp Sensor w/ 23ft cable. I have information about splicing in extra cable. I will look it up and post back asap on this post with the edit mode.

Above post of Apr 14 has been edited for LMB DO and temperature tolerances.

Bill, did you come to any conclusions from this study? I aerate and we found a thermocline at about 10 ft. I can certainly put an aerator in the deepest part of the lake. However, I'm worried about warming the lake too much by forcing hot air to the bottom. Is this the same problem as forcing cold water into the lake in the winter?

Norm - My "study" here was basically a flop or failure because almost no one participated and supplied the requested temperature measurements. It was pretty hard for me to go all around to various parts of the country taking measurements in various ponds.

The warming of the pond/lake botom in summer is usually not as much of a problem as it is in winter, UNLESS you are raising cold water species of fish such as trout, whitefish, cisco and similar types. Many or most cool water fish species can with stand or tolerate a fair amount of warming depending on the species and the geographic latitude. If one has critical temperature tolerant species, the bottom temperatures should be monitored and the aerator run time should then be adjusted to obtain the upper desired threshold of temperature. We had to do this in several ponds where y.perch were dying when air temps were in the 90+F for several days straight.

If the bottom water (hypolimnion) or cool water refuge develops low or anoxic oxygen conditions then the fish cannot live there regardless of the preferred temperature. Plus with anoxic condtions overlying the benthic sediments all the invertebrate processors suffocate and their benefit is negated. So I think it is ultimately more important to have oxygen on the bottom with warmed conditions rather than have cold anoxic conditions on the bottom.

Bill, I almost learned the hard way regarding cold water refuge for trout in summer. By June/30 my pond was completely stratified with surface temps of 72 and bottom at 56. It was my pond's first year with fish stocked in May, no aeration. Visibility was over six feet and no bottom buildup and very little vegitation of any kind. I thought everything was fine, until I noticed there were virtually no fish below the thermocline and most were swimming in the upper three feet apparently gasping. My cold water refuge was useless to these fish as you stated.
I quickly installed aeration and gradually extended run times and diffusor depth and within a week I had uniform temps not exceeding 68 in any depths. After two weeks temps levelled off at 64 top to bottom. I was running aerator only when air temps were lower than water temps. I did not lose any fish and learned that trout will handle warmer water if oxygen levels are maintained.
I did not let my pond stratify this year and everything is fine so far.

Wood,

I have only brook trout this year (about 140 in the 1/10th acre pond) and only run the diffuser primarily in the evening when air temps cool down and shut off in the morning as air temps rise. I think there is enough aerated flow coming in along with phytoplankton and macrophytes during the day to make enough oxygen.

Not sure what what the temps are in the deep water but about 4 feet down temps peak at about 64 on really hot days.

So far no mortalities and I hope to put in a floating dock to measure temps in the deep water. Of course I have about 38 gpm flowing in 24/7 at about 10.8 C (51.6 F.)

Unfortunately this strain of brook brout (Lake Nipigon) is a dissappointment. Growth rates are not a fast as the last more domestic strain I had, and growth is variable among individuals. I am fuming at my state DNR for not allowing me to get my brook trout from the last source.

Wood - As you have learned, one can within reason, control the bottom water temps while maintaining adequate deep water oxygen concentrations by varying the benthic aerator run times and monitoring the bottom water temps. It just takes a little knowledgable effort and common sense.