Dissolved oxygen (DO) may be the single most important water quality issue pond owners should monitor.  According to the extension service resource Aquaplant, oxygen dissolves in water at very low concentrations. The atmosphere is 20 percent oxygen or 200,000 ppm, but ponds seldom have more than 10 ppm in the water.  Readings below 3 ppm stress most warm water fish.  Levels below 2 will kill some species.

Oxygen dissolves into water from the atmosphere and aquatic plants.  The primary source for pond oxygen is phytoplankton or submerged plants.  In the presence of sunlight, they produce oxygen through photosynthesis and release it into the water.  At night and on very cloudy days, these sources remove oxygen from the water through respiration.  Highest DO levels usually occur in late afternoon.  Lowest levels may occur during summer months just before dawn.

Oxygen depletion is the most common cause of fish kills.  Warm water holds less DO than cool, or cold water because oxygen demands are greater in warm months.  Fish kills may range from partial to total.  During partial events, DO is low enough to suffocate sensitive species and large fish, but many small fish and hardy species survive.

Planktonic algae can influence DO levels.  Generally, it’s related to the amount of nutrients in water.  Excess nutrients may occur from over fertilizing, overfeeding, runoff from fields with high livestock grazing, or septic lines. When abundant algae is present, water may appear very green or almost the color of pea soup. Although phytoplankton can be very beneficial to pond food chains, in excess, it can be equally damaging. If water visibility drops to less than 12 inches, there is danger of oxygen depletion.  Such dense “blooms” use large amounts of DO at night, plus on overcast and/or windless days.

When plankton “blooms” die-off, water color often changes.  Streaks of brown or gray-black appear in otherwise green water.  As the die-off evolves, the whole pond will turn from green to gray, brown, or clear.  Water columns typically clear after a die-off as dead algae settles to the bottom. Plankton die-offs cause rapid oxygen depletions as DO is consumed by bacteria and fungi decaying dead algae.  Few phytoplankton organisms remain to produce more oxygen.

Visit our on-line store for Secchi disks to measure “bloom” densities, test instruments to monitor DO levels, and custom aeration systems.  Consult with our biologists about an emergency response plan should low DO conditions develop in your pond.