hypoxia

Hypoxia occurs in aquatic environments as dissolved oxygen concentrations are reduced to a point where it becomes detrimental to aquatic organisms living in the system. This oxygen depletion can result from a number of natural factors, but is most often a concern as a consequence of pollution and eutrophication in which plant nutrients enter a river, lake, or ocean, causing phytoplankton and toxic algal blooms.

See eutrophication.