Dissolved oxygen (DO) is a water quality parameter that can give insight into the health of a lake, river, or the ocean.  Dissolved oxygen is a measure of gaseous oxygen dissolved in an aqueous solution.  Oxygen can get into solution via a few mechanisms, including diffusion from the surrounding area, aeration or turbulence (e.g. wind mixing, inlet streams, waterfalls, etc.), and as a waste product of photosynthesis.

Dissolved oxygen is important because without enough DO in lake bottom waters, fish kills can occur and nutrients that cause water quality degradation can be released into the water column.  The concentration of DO in a waterbody is highly dependent on the location, stratification, season, and morphology.  Below is a chart that lists concentrations and the influence on fish populations.