Published on: 2.12.2019
Information produced by Finnish Environment Institute
When oxygen depletion strikes

Dead fish are floating in the water and the stink is terrible. Oxygen depletion is a hazard that usually lurks at the bottom, but in the worst case it can reach all the way up to the surface.
Oxygen depletion afflicts lakes suffering from eutrophication, in particular. They contain plenty of algae and plants that sink to the bottom when they die. At the bottom, microorganisms decompose the dead organic matter, using up oxygen reserves. If more oxygen is consumed than the lake can replace, oxygen depletion starts in the water layers close to the bottom.
Oxygen depletion strikes especially in winter when ice prevents oxygen from being dissolved from air into water. Problems may also occur in summer when the temperature difference between the surface and bottom waters is so great that the various layers do not get mixed and no oxygen gets carried to the deeper layers of water.
Without oxygen there can be no higher life, and this also applies to aquatic environments. If oxygen runs out at the bottom, there are fewer benthic animals and eventually the bottom becomes deserted. Unless the situation changes, oxygen depletion spreads to the water layers higher up. The problem is fixed when either spring or autumn comes: the so-called lake turnover mixes up the water layers and introduces new, oxygen-rich water close to the bottom.
Oxygen depletion at the lake bottom may spin out of control, however. Phosphorus previously deposited in sediments may start dissolving back into the water in anoxic conditions. This internal loading exacerbates eutrophication further, completing the vicious circle.
Image: © Mikko Suonio, Vastavalo