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Land and Water26•September/October 2009

P O N D S / L A K E M A N A G E M E N T

www.landandwater.com

by Bob Robinson

ALL ponds and lakes should beviewed as individual organisms. Eachone has a host of different water quality parameters that make itunique. THE most important waterquality parameter for lakes and pondsis dissolved oxygen. Oxygen is essen-tial to the metabolism of all aerobic(air breathing), aquatic organisms.Oxygen is measured in parts per mil-lion (ppm). Five ppm is like a drop offood coloring in a 55 gallon drum.

Five ppm is what I’ll term as thethreshold to support a healthy aquaticeco-system to include both fish andother beneficial organisms.

Here are some basics about dis-solved oxygen in fresh water:

1.) The air we breathe is 20.9%oxygen, the rest is essentially nitrogen.

2.) Oxygen concentration is tem-perature dependent. Warmer waterholds less oxygen. And it is in thewarmer months when metabolic activity is the greatest in ponds whenoxygen demand is typically greatest.

Heat is also the reason more oxygenrelated fish kills are likely to occurduring the dog days of summer.

3.) Oxygen is affected by: initialdissolved oxygen levels, barometricpressure, water temperature, waterpurity and elevation.

4.) In Nature, oxygen is producedin water by: a) Wind action. b)Diffusion at the air water interface. c)Photosynthesis from aquatic plants.

5.) Oxygen levels in your pond’swater typically fluctuate throughoutthe day. During clear, bright days

Aeration as a Lake Management Tool

Land and Water September/October 2009•27

plants utilize the sun’s energy to produceoxygen so that peak oxygen levels occurin late afternoon. Typically, the weathertends to be breezier during mid-day sooxygen exchange at water’s surface isalso greater. As the sun sets, the windstend to calm. The same plants that werejust producing oxygen reverse them-selves and are now consuming it.Typically, in the early morning hours justas the sun rises, oxygen levels will be attheir lowest point of the day.

6.) Without oxygen present or withminimal oxygen:

• Ponds can stratify.• The organic muck layers on the bottom degrade much more slowly.

• Phosphorous can stay in solution making it readily available for plantgrowth.• Oxygen breathing – sediment con-suming organisms can be stressed ordie.• Conditions become favorable for the formulation of harmful gasses like ammonia, hydrogen sulfide andiron. • The effective living space for fish

and many of their food choices is reduced.• Lakes which contain a large vol- ume of cold, “bad” bottom water arecandidates for a “turnover.”

7.) During late spring, summer andearly fall many lakes become stratified.This is the horizontal layering of waterwith warmer water at the surface andcooler water on the bottom. Because


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An example of healthy Trout and excellent water clarity in a pond in Northern Wisconsin.

Hydraulics Laboratory at the Engineering Research Center

For more information, contact us at [email protected]

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cold water is denser than warm water, anatural density barrier exists thatrestricts water movement between thelayers. Since most oxygen is produced orenters the water in the top layer it isunable to mix with the colder bottomlayer. When these stratified conditions

get to an extreme state a couple of thingscan happen.

•A summer storm which typically ispreceded by a few days of calm, cloudydays could cause a turnover. Those calmcloudy days have reduced sunlight whichinfluences oxygen production fromplants. Dissolved oxygen levels in thewater tend to drop. Heavy wind action

coupled with colder rainwater deepensthe possibility of being strong enoughand cool enough to mix the entire bodyof water and cause a fish kill. When thecold, stagnant bottom layer mixes withthe healthy warm water on top, the bot-tom water absorbs much of the oxygen

while releasing its harmful gasses. Theconsequence of this event is a rapid fishkill.

•In fall as water surface temperaturedrops naturally, it will eventually equalthe same temperature as the bottomwater. Once the temperatures are equalthere will no longer be a density barrierwhich will allow the whole lake to mix.If the bottom was extremely stratified afish kill could result just as describedabove.

So what is a person to do to preventa fish kill due to low or no oxygen con-dition?

1.) Fertilize carefully as to not causetoo strong of a “bloom.” Specifically,bluegreen algae are more susceptible todie offs than more stable green algae.Manage your pond for secchi disc read-ing of 24” or greater. Managed algae has

a dramatic effect on fish as well as waterquality and plant species composition.Some people prefer clear water in whichthey can see all the way to the bottom.Clear water is better for a swimminghole than a fishery. Fertilizing correctlyresults in a more productive pond interms of carrying capacity expressed inpounds of fish per acre. Keep in mindcertain species of bluegreen algae canadd to fish off-flavor if you are thinkingof eating what you catch.

2.) Create a berm and swale aroundthe bank to allow water to be filteredthrough the soil prior to entering yourpond.

3.) Cleared lots typically mean lackof native plants and most likely will leadto increased erosion and runoff.

4.) Artificial beaches require repeat-ed sand replacement, reduces waterquality, and degrades aquatic habitat.

5.) Try to encourage native vegeta-tion around the perimeter of your pondto assist filtering incoming water.

6.) Rooted plants are more stablethan algae. Algae come in two forms.Planktonic (those that are typicallymicroscopic but tint the water green) or f ilamentous which are typicallyreferred to as “moss” or “horsehair.”Rooted plants are more stable as theywon’t die off as fast as algae on cloudydays.

7.) If you use grass carp for rootedaquatic plant control, be careful to notstock too many. Grass carp preferredchoices of meals are higher order vascular plants. If stocked too heavilythese exotic vegetarians eliminate all

vegetation cover in a pond. Those nutri-ents are then released back into thewater column, increasing the fertilityand leading to a pea green soup ofplanktonic algae. Grass carp won’t eatplanktonic algae. Stocking rates vary bythe amount of vegetation and type in



Wouldn't we all like to catch a fish of this size? Proper aeration should be viewed as aninsurance package to prevent fish kills do to little or no oxygen levels.

Those nutrients are thenreleased back into thewater column, increasingthe fertility and leading toa pea green soup of plank-tonic algae.

When the cold, stagnantbottom layer mixes withthe healthy warm water ontop, the bottom waterabsorbs much of the oxygen while releasing itsharmful gasses. The conse-quence of this event is arapid fish kill.


your pond as well as length of growingseason. Rates from 3-30 fish per acrehave been used successfully but dependon many variables. Ask for help on thisone.

8.) Malfunctioning septic systemscan result in phosphorus and bacteriaload increases.

9.) Do not overfeed your fish withfish food. Use a floating extruded feed.Extruded feed has been researched togive your fish the greatest cost per unitgain. It is the most digestible (hence lit-tle waste to cause oxygen demand).Extruded feed floats on water’s surface.Don’t feed more than f ish consumewithin five minutes. Uneaten feed drops

to the bottom and adds to the oxygendemand of the system.

10.) Supplement natural occurringoxygen with an aeration system of sometype. There are several types of aerationsystems that add oxygen to ponds. Nosingle one of them is going to work forevery type lake or pond. Each pond/lakemust be viewed as its own individual living system and aeration needs aredetermined based on the site specificparameters for the body of water and thegoals of the pond owner. With a properlydesigned aeration system you canexpect:

• Greater densities of fish can be achieved.• Spring and fall turnovers can be eliminated.• Winterkills can be prevented in thenorthern United States and waterfowl can be protected from bird eat-ing predators.• Water quality can be improved.• Organic muck decomposition on the bottom can be accelerated.• The body of water can be chemi-cally and thermally destratified.

• Circulation currents create more favorable conditions for desirable algae to out-compete blue greens.• Algae blooms and die offs should be less severe.• Oxygen insurance for fish you have invested time and money.

For pondmeisters in the Northernpart of North America I would be remissfor not mentioning using aeration to prevent winterkill. A combination ofthick ice, heavy snow, and a high con-centration of decaying plants under theice is a recipe for fish kills in shallowlakes. Heavy ice and snow cover do notallow sunlight to penetrate water to anysignificant depth. As a result there is little or no photosynthetic activity toproduce oxygen. If this condition issevere enough a f ish kill could resultbecause oxygen is used up by decayingplants as well as the living creatures.One solution is to install some type ofaerator/circulator that creates a smallopening at the surface to allow oxygento absorb into the water. Colder waterholds more oxygen and metabolic activity


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Each pond/lake must beviewed as its own individ-ual living system and aera-tion needs are determinedbased on the site specificparameters for the body ofwater and the goals of thepond owner.


is at the yearly low so only a small holeis required. An area of maybe 3% of thesurface should be fine. Do not splash thewater because that will cool the waterand actually thicken the ice. Choices forthe right system follow.

Types of AerationThere are many different types of

methods to artificially introduce oxygeninto water. The following are the mostcommon methods.

Diffused Aeration: This type of aeration for ponds/lakes employs a compressor, air supply line and sometype of diffuser that breaks up the airinto smaller bubbles. This is one of themost efficient systems for deeper lakesand involves no electricity in the water.Diffused aeration creates a gentle rollingaction at the surface and is an excellentdestratifier. However, efficiency dropsoff in water less than eight feet deep.Most bubbles will rise at 1 foot per sec-ond and shallow water simply does nothave enough contact time to either

entrain water to the surface or transferoxygen as the bubbles rise. The com-pressors require cleaning of air filters;bearings and carbon vanes on somestyles need replacing every 12-18months. Some diffusers will requireperiodical cleaning as well. The noise ofshore mounted air compressor might bean issue with some landowners.

Fountains: These units shall bedefined as any device that splashes waterinto the air to create an aestheticallypleasing display. Keep in mind there isno free lunch. Specifically, the higherthe spray the more energy required.Fancy displays require more energy andoxygen transfer rates suffer. You canhave the best of both worlds with someunits: decent oxygen transfer as well as apretty display. Fountains work fairlywell in small lakes/ponds that are rela-tively shallow as well as very good atventing gasses. To also keep in mind,fountains will typically only draw fromthe top 10’ of depth or shallower andsome energy is wasted to create an

aesthetically pleasing display.Wind Powered aerators/circulators:

Wind power units will either drive asmall compressor that pushes air to adiffuser or will be connected to sometype of blade or paddle that enters the

water and moves as the wind blows. Theresultant agitation causes oxygen trans-fer and water movement. In someremote areas, this could be the onlychoice. This option also requires noelectricity. However, they will not workwhen they are needed most, in no-windconditions and are not portable. Wind



An example of a V pattern aerating fountain which gives you the best of both worlds: an aesthetically pleasing display and decent oxygentransfer simply because you are not wasting energy to spray water high in the air.

Wind power units willeither drive a small com-pressor that pushes air to adiffuser or will be connect-ed to some type of blade orpaddle that enters thewater and moves as thewind blows.


powered will not work very well onlakes that are sunken in valleys.

Solar Powered Aerators: The aera-tion technique harnesses the energy ofthe sun and either moves water withblades or supplies energy for compres-sors to push air into the water. Solarpowered could be the only choice insome cases. There is no electricityrequired and no on-land power sourcerequired. Some of these can be very difficult to install and relative costs vs.results is extremely high.

Agitators: These units employ a float, motor, and prop to splash waterat the surface. Ideal for smaller pondsand fairly decent at Oxygen Transfer,agitators are portable and lightweight.They are not, however, very eff icient at moving water at depths greater than8’-10’.

Horizontal Circulators: These unitscan be adjusted underwater to movewater in a horizontal pattern to cause circulation. Some also have angle adjustments to get to greater depths. Youcan position them to point in any direc-tion as well. Excellent at causing water

movement, horizontal circulators can bemounted or pointed at variable waterdepths to reach and circulate water to18’. This choice is not quite as efficientas a surface aerator at adding oxygen butshould be viewed as a more effectivedestratifier.

Vertical Circulators: This newdevice employs an 1800 rpm motor witha gear reduction box that spins a bladevertically (pushes oxygenated surfacewater down). Preliminary test resultsshow that 1 horse power will effectivelydestratify a lake that is 15-20 acres insize to a depth of at least 12’. Since thisis new, the jury is still out on the cons.

In summary, each lake/pond shouldbe viewed as an individual organism.With proper care and precaution youshould never have a fish kill. Aerationshould not be viewed as a panacea for all

of your lake issues. It is one of the toolsin the toolbox. Make sure you purchasethe right system for your pond andreview the following before your pur-chase:

• Safety.• Lead-time from manufacturer.• Dependability of the product as well as warranty.• Minimal maintenance.• Technical support pre- and post purchase.• Simple operation and installation.• Cost- both initial and operational which includes electrical power, maintenance and replacement parts.• How long has the manufacturer been in business? L&WFor more information contact Bob

Robinson, Fisheries Biologist, KascoMarine at [email protected].


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Aeration should not beviewed as a panacea for allof your lake issues. It is oneof the tools in the toolbox.

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