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.
Excessive Phytoplankton
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.