Dissolved oxygen levels depend on temperature, pond depth, productivity and fertility, and water movement. In almost any aquatic environment, fluctuations in natural nutrient cycles can create imbalances which lead to oxygen depletions and fish kills. Generally, these fluctuations are difficult or impossible to predict, but high nutrient levels from feeding or over-fertilizing almost always compound problems with oxygen management.
As nutrients accumulate in the pond system, some end up directly in the bottom mud but most go into solution in the water column first, where they are used by plants and animals. The most abundant plants in a well-managed fishing pond are found in the algal bloom, which gives the pond water a greenish tint. Suspended in the water, these microscopic single-celled plants are sometimes referred to as phytoplankton. Except for situations where excessive vegetation is present, most nutrients dissolved in the water are taken up by the algal bloom.
Algal Blooms and Oxygen
Like all green plants, phytoplankton produce oxygen during the daylight hours as a by-product of photosynthesis. This is usually a major source of oxygen in fish ponds. In darkness, however, all plants consume oxygen, including phytoplankton. Blooms in natural water bodies or fish ponds normally produce much more oxygen in the daylight than they consume during the night, but some situations reduce the amount of oxygen a bloom produces without reducing its nighttime oxygen consumption.
Trace minerals or nutrients needed by the algal bloom are occasionally used up. This usually results in some, or occasionally all, of the phytoplankton dying back temporarily. This is probably the most common cause of phytoplankton dieoffs, especially for heavy blooms with competition for light and nutrients. When a large portion of the bloom dies off at once, bacterial decomposition and the loss of normal oxygen production can lead to oxygen depletions and fish kills. Pond water generally changes from a deep green to black, gray, brown or clear after a phytoplankton dieoff.
Blooms respond to changes in the weather. Photosynthesis slows down under cloudy conditions, and, as a result, oxygen production decreases. Extremely calm days may also reduce photosynthesis and oxygen production, even under sunny conditions, by preventing phytoplankton in the middle layers of the pond from mixing near the brighter surface. In summer, oxygen problems may arise because of a simple physical property of water. The warmer the water, the less dissolved oxygen it can hold. When a dense bloom produces a surplus of oxygen on a summer afternoon, the oxygen will not stay in solution and escapes into the atmosphere. During the night, the bloom attempts to take more oxygen out of the water than what remains from daytime photosynthesis. When this occurs, dissolved oxygen levels approach zero. Fish begin to suffocate in the pond, and aeration must be applied to meet the demand for oxygen and prevent fish losses.
Many ponds also experience oxygen problems in early spring. As the water warms and the amount of sunlight increases, algal species which predominated in the bloom during the winter die back, and other species more suited to summer conditions multiply and replace them. When this process proceeds gradually, conditions remain fairly stable. Occasionally, however, the winter bloom dies off abruptly and insufficient oxygen levels may occur for several days.
This type of oxygen depletion may kill some fish directly or cause sufficient stress to weaken their immune systems. Bacterial infections usually occur within the next several days to two weeks. Various signs, such as color and odor changes or a buildup of foam on the downwind bank, can sometimes be useful in anticipating when a winter bloom will die back.
Other Algal Bloom Impacts
Other problems are associated with dense algal blooms. In low-alkalinity waters, dense algal blooms create wide fluctuations in pH daily. Occasionally, heavy phytoplankton populations cause pH to reach extreme values in the afternoon. Algal die-offs can also result in high ammonia concentrations. Both processes affect fish health, growth and survival adversely. An additional problem caused by dense blooms, especially in excessively deep ponds, is stratification. As mentioned earlier, stratification involves layering of the pond water into warm, oxygen-producing upper zones and cool, oxygen-consuming bottom waters. Shading caused by dense blooms limits photosynthesis and dissolved oxygen levels at the pond bottom, resulting in a buildup of potentially toxic compounds, even in aerated ponds.
This situation can lead to physiological stress, reduced fish growth and even fish kills if bottom waters are mixed too rapidly with the rest of the pond. This type of mixing, referred to as a turnover, occurs when cool rain water or heavy wind on the pond surface breaks down layering patterns. Turnovers are often observed in natural waters and ponds in the fall or spring after severe weather disturbances. The potential for a turnover can be detected by an increasing temperature difference between a pond’s bottom layers and its surface waters. If a turnover is likely, prepare for aeration when necessary.
Response Options - Aeration
Once under way, oxygen depletions are fairly easy to recognize. Partial depletions can be recognized by fish hanging at the water surface during the early morning hours or a loss of appetite in ponds where fish are fed. Lethal oxygen depletions begin with similar symptoms. Fish congregate at the pond surface, gulping air. During the early minutes of the depletion, they may dive when disturbed and return to the surface. As conditions worsen, they will ignore most disturbances and continue gulping air.
Mechanical aeration is required to raise oxygen levels once a depletion has begun. Irrigation or industrial pumps can be used to pull water from a depth of 2 to 3 feet and spray it back into the pond. Successful alternatives to agitate pond water mechanically have included outboard motors, bush hogs and other devices. A bush hog can be backed into the pond to a point where the blades just touch the water. An outboard motor operating in a fixed tilted position (keep boat tied up or pointed into the shore) with the propeller wash directed out into the pond can also provide effective aeration in emergency situations.
Flushing with clean, aerated water will help improve water quality, but this alternative is not available for ponds which depend on runoff as a water source. Emergency aeration will be most effective in smaller ponds, and the success of any aeration practice will depend on the severity of oxygen depletion.
Fish Kills from Chemical Contamination
Fish kills resulting from chemical contamination generally take one of two forms: direct poisoning of the fish or oxygen depletion resulting from poisoning of the algal bloom. Application of agricultural chemicals to croplands which run off into the pond must be practiced with great caution. Direct poisoning may be involved if small fish die before larger fish of the same species. If direct poisoning of fish or phytoplankton is suspected, contact the local Extension office and the Louisiana Department of Agriculture and Forestry immediately.