What is Dissolved Oxygen

A major component of successful pond management is dissolved oxygen (DO). Dissolved oxygen is gas that is dissolved in water and is available for uptake by fish. Unlike mammals, which use their lungs to absorb oxygen, fish use water flowing through their gills. Because fish are primarily obtaining oxygen from the water, it is imperative that DO is measured, monitored, and managed for. Oxygen is incorporated in the water (1) directly from the atmosphere, (2) by wind and wave action, and (3) photosynthesis.

What Causes Low Dissolved Oxygen?

Oxygen depletion is the loss of oxygen from the aquatic system to a point that causes fish death. A concentration of 5 mg/L of DO is recommended for fish health, and concentrations below this can cause problems. Larger fish that require more oxygen will be affected first. Many things lead to oxygen loss, such as increased consumption, too many plants, pond ‘turnover’, and increased organic waste. These events happen most often in the summer months due to cloudy weather and plant die-off that inhibit photosynthesis.

Identifying Low DO as the Problem

Extension agents can help collect a sample and perform an analysis to quantify the DO level. Other things to look for are fish swimming near the surface and gulping, fish stop eating, pond water color change to black or grey, or n odor from the pond.

Preventing Oxygen Depletion

Low oxygen events can be predicted, and monitoring for these events is the best plan to avoid a fish kill. Electronic oxygen meters are the most reliable, but can be cost prohibitive. Chemical tests are also available, but can be troublesome to perform. Aeration systems can be installed, and used especially when the DO falls below 4 mg/L (depending on the fish species).

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