Dissolved oxygen is probably the single most important water quality factor that pond managers need to understand. Oxygen dissolves in water at very low concentrations. Our atmosphere is 20% oxygen or 200,000 ppm but seldom will a pond have more than 10 ppm oxygen dissolved in its’ water. Dissolved oxygen concentrations below 3 ppm stress most warmwater species of fish and concentrations below 2 ppm will kill some species. Often fish that have been stressed by dissolved oxygen concentrations in the range of 2 or 3 ppm will become susceptible to disease.
Oxygen dissolves into water from two sources: the atmosphere and from plants in the water. The primary source of oxygen for a pond is from microscopic algae (phytoplankton) or submerged plants. In the presence of sunlight, these produce oxygen through photosynthesis and release this oxygen into the pond water. At night and on very cloudy days, algae and submerged plants remove oxygen from the water for respiration. During daylight hours plants normally produce more oxygen than they consume, thus providing oxygen for the fish and other organisms in the pond.
Oxygen depletions are the most common cause of fish kills in ponds. Most oxygen deletions occur in the summer months because 1) warm water holds less dissolved oxygen than cool or cold water, and 2) because the pond’s oxygen demand is greater in warm water than in cold water. Fish kills from oxygen depletions can range from “partial” to “total”. In a partial kill the dissolved oxygen level gets low enough to suffocate sensitive species and large fish, but many small fish and hardy species survive. Most oxygen depletions cause partial fish kills; total fish kills are relatively rare in recreational ponds except for those with extremely high fish populations (>1,000 pounds/acre). The following are descriptions of the most common types of oxygen depletions.
The abundance of planktonic algae (very green water) in a pond is generally related to the amount of nutrients present in the water. Nutrients can wash into the pond from woods, pastures, fields, human activities in the watershed, or come from pond fertilization. Generally, the more nutrients, the more planktonic algae (or other aquatic plants) will grow or bloom. Although phytoplankton is good from an abundance of natural food and oxygen producing standpoint, it can become too abundant or excessive. When phytoplankton become so abundant that water visibility is limited to less than 12 inches there is a danger of an oxygen depletion. These heavy or dense blooms use large amounts of dissolved oxygen at night and on very cloudy/overcast, windless days causing an oxygen depletion and fish kill. This problem is often a consequence of overfertilizing, overfeeding, or excessive nutrients from livestock, fields, or septic lines.
Phytoplankton populations, or blooms, can grow rapidly, particularly on sunny days when the water is warm and nutrients are available. Alternatively, they can die-off 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 gray-black through the otherwise green water of the pond is an indication that the 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 depletions for two reasons: 1) the remaining dissolved oxygen is consumed by aerobic bacteria and fungi in the process of decaying the dead algae and 2) 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 a pond turnover. As ponds warm in the spring they become stratified or layered, with warm water on the surface and cooler water below. This temperature stratification also leads to an oxygen stratification, with the warm surface water containing dissolved oxygen (and fish) while the 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 a turnover can occur.
The problem arises when this stratification is broken down quickly, causing the two layers to mix or “turnover.” The turnover mixes the oxygen rich surface water with the deep oxygen 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 a turnover can happen if a cold rain and wind cools the surface water. Usually this happens during thunderstorms. Many people confuse a turnover with an algae die-off; externally the can look very similar but without a cold rain and wind there can be no turnover.
Use of Aquatic Herbicides and Oxygen Depletions
Treating a heavy infestation of aquatic weeds with a herbicide during the summer can cause an oxygen depletion. The rapid decomposition of a mass of aquatic weeds depletes dissolved oxygen like 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% or less of the pond at a time then wait two weeks so that decomposition is complete before the next treatment is applied.
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. Many types of aerators are available commercially. Tractor PTO-driven paddlewheel and pump-sprayer aerators can be purchased commercially 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 ? to 1 horsepower of aeration per surface acre of pond is adequate for recreational ponds.