Farm pond management: Understanding oxygen depletion

Dissolved oxygen is probably the single most important water-quality factor that pond managers working on farming and hunting land in Texas must understand.

First and foremost, oxygen dissolves in water at low concentrations. Our atmosphere is 20 percent oxygen, or 200,000 parts per million (ppm), but rarely will a pond have more than 10 ppm of concentrated oxygen in its water. Concentrations below 3 ppm are harmful to most warm-water fish and less than 2 ppm will kill some species outright. Fish affected by oxygen concentrations between  2 or 3 ppm often become susceptible to disease.

Water obtains its oxygen from two sources: the atmosphere and vegetation in the water - particularly the latter, due to the presence of phytoplankton, a microscopic algae, or other submerged plants. Sunlight causes phytoplankton to produce oxygen through photosynthesis and release it into the pond water. During daylight hours, plants normally produce more oxygen than they consume, thus providing the element for the fish and other organisms in the pond. At night and on very cloudy days, algae and submerged plants remove oxygen from the water for respiration.

Oxygen depletions are the most common cause of fish kills in ponds. Most oxygen deletions occur in the summer months because warm water holds less dissolved oxygen than cool or cold water, and so it demands more of it.

Fish deaths from oxygen depletions range from "partial" to "total". In the former, dissolved oxygen levels get low enough to suffocate sensitive species and large fish, but smaller and more resilient fish species survive. This is the most common type of aquatic die-off. Total fish kills, meanwhile, are relatively rare in recreational ponds except for those with extremely high fish populations of more than 1,000 pounds per acre.

Pond managers and farmers must understand the most common types of oxygen depletions, so as to avoid fish die-offs that can, in the long run, negatively impact the property's value when you want to put it up as farm and ranch land for sale in Texas. Let's examine a few below:

Panola agriculture industry update: Farm pond managementOxygen depletion in your pond can lead to serious problems.

Excessive phytoplankton
A plurality of planktonic algae in the pond on your recreational property generally correlates to the amount of nutrients in the water. Nutrients can wash into the pond from woods, pastures, fields and human activities in the watershed - or as a result of pond fertilization. Phytoplankton and other aquatic plants grow at a greater rate when water holds more nutrients. Although phytoplankton is good from an oxygen-producing standpoint, it can become excessive.

When phytoplankton are so abundant that water visibility drops below 12 inches, oxygen depletion soon follows. These heavy or dense blooms use large amounts of dissolved oxygen at night and on overcast, windless days, leading to fish deaths. 

Phytoplankton die-off
Phytoplankton populations, or blooms, can grow rapidly, particularly on sunny days when the water is warm and nutrients are available. Alternatively, they can die quickly, especially in the spring and fall as water temperatures change rapidly with weather fronts. However, a bloom die-off can occur at any time of the year with little or no warning.

Typically during a bloom die-off, the color of the water will start to change. Leading up to a bloom die-off, the pond water may have a "streaky" appearance. Streaks of brown or grayish-black through the otherwise green water of the pond is an indication that algae are starting to die. As the die-off progresses, the whole pond will turn from green to gray, brown or clear. The pond water will typically clear after a die-off as the dead algae settle to the bottom.

Plankton die-offs cause rapid oxygen depletion for two reasons:

  • The remaining dissolved oxygen is consumed by aerobic bacteria and fungi in the process of decaying the dead algae.
  • Few live phytoplankton remain to produce more oxygen.

Secchi disks can be used to monitor bloom densities. Any bloom that reduces visibility in the pond to 12 inches or less may cause oxygen problems. 

Probably the least understood but most commonly reputed cause of an oxygen depletion is pond turnover. As ponds warm in the spring they become stratified or layered, with warm water on the surface and cooler water below. This also leads to oxygen stratification, with warm surface water containing dissolved oxygen (and fish) while deeper, cool water becomes depleted of oxygen because of decomposition and lack of sunlight for photosynthesis. This is particularly true in deep ponds (greater than 8 feet). In fact, the deeper the pond, the more likely turnover can occur.

The problem arises when this stratification is broken down quickly, causing the two layers to "turn over" - essentially, to mix. The oxygen-rich surface water blends with the deeper, depleted water. The dissolved oxygen concentration in the mix can be too low to support life in the pond.

Both fish and plankton can die from low dissolved oxygen following a turnover. A turnover takes place only if the surface water is cooled quickly so that it is close to the temperature of the deep water, allowing them to mix. Thus, turnover can happen if a cold rain and wind cools the surface water. Usually this happens during thunderstorms.

Use of aquatic herbicides
Treating a heavy infestation of aquatic weeds with herbicide during summer can cause oxygen depletion. The rapid decomposition of these weeds has the same effect as a bloom die-off.

Treating weed infestations with herbicides in hot weather is risky. The risk of an oxygen depletion can be lowered by treating only part of the pond at a time. Treat 25 percent or less of the pond to start, then wait two weeks so decomposition is complete before applying the next treatment.

The risk of a fish kill caused by an oxygen depletion can be minimized by following the guidelines and recommendations discussed previously. However, even a lightly stocked pond can have an oxygen depletion. Some ponds have a history of fish kills caused by oxygen depletions. Mechanical aeration usually can save fish during an oxygen depletion.

Numerous aerators are available commercially. Tractor PTO-driven paddle-wheel and pump-sprayer aerators can be easily bought - or built by a competent welder. Water pumps and bush hogs (blades just touching the surface) can also be used for emergency aeration. If a pump is used, pull water from near the surface (upper foot), not off the bottom of the pond. Many types of electric aerators are available for ponds with electrical service. Generally 1 horsepower of aeration per surface acre of pond is adequate for recreational ponds.

Optimal pond conditions and fertilization best practices
To support an ideal fish population, a pond should be somewhat green in color, with visibility ranging between 18 and 24 inches deep - so a white Secchi disk can be seen if dropped in. Good fertilization will help maintain the algal bloom.

For proper phytoplankton management, provide nutrients just as you would for the grass that cattle forage. Hardness and alkalinity should remain at 30 ppm or above, so you may need to add some agricultural limestone to the pond every 3 to 7 years.

Use nitrogen high-phosphorous fertilizer shortly after the pond begins to fill - this promotes the growth of planktonic algae and limits unwanted plants. Ideally, do so in early spring when water temperature falls between 55 and 65 degrees Fahrenheit, and then again in late spring or early summer. Ponds typically require 5 to 8 pounds of fertilizer per acre in spring and about half that amount in summer. If using organic fertilizer instead, 150 to 250 pounds of cottonseed meal spread around the edge of the pond is enough to engender good algal bloom - although you'll see results at a slower pace.

Remember: The right amount of fertilization is good, but too much - even a little bit too much - can be very bad. Over-fertilization might lead to very dense blooms that consume too much oxygen at night in the absence of photosynthesis, leading to early-morning fish die-offs and making a negative impact on your ranching and farmhouse plans.