Water Quality

Dissolved Oxygen

The amount of oxygen gas dissolved in lake water, measured in milligrams per liter, essential for fish survival and a key indicator of lake health.

What It Means for Your Lake

Dissolved oxygen (DO) is one of the most important water quality parameters for aquatic life. Fish, invertebrates, and beneficial bacteria all require dissolved oxygen to survive. In healthy lakes, dissolved oxygen levels range from 6 to 12 milligrams per liter (mg/L), with coldwater fish species like lake trout and cisco requiring at least 6 mg/L and warm water species like bass and bluegill tolerating levels as low as 4 mg/L. Oxygen enters lake water through two processes: atmospheric diffusion at the surface (enhanced by wind and wave action) and photosynthesis by algae and aquatic plants. Oxygen is consumed by respiration of all living organisms and by decomposition of organic matter on the lake bottom. In stratified lakes during summer, a critical phenomenon called hypolimnetic anoxia can develop, the deep water layer (hypolimnion) becomes isolated from the surface by the thermocline and its dissolved oxygen is gradually consumed by decomposition without being replenished. When dissolved oxygen drops below 2 mg/L in the hypolimnion, phosphorus stored in bottom sediments is released back into the water column (internal loading), creating a feedback loop that worsens eutrophication. Fish kills occur when dissolved oxygen drops below 1 mg/L, particularly during late summer in eutrophic lakes or during winter ice cover when photosynthesis is limited (winterkill). The Minnesota DNR and Wisconsin DNR use dissolved oxygen profiles to assess lake health, determine fish habitat availability, and identify lakes at risk for fish kills.

Frequently Asked Questions