In the environment dissolved Oxygen is one measure of the health of a body of water. O2 sensors and meters are used to determine the oxygen saturation in water. Oxygen saturation is important for the health of fish and other aquatic wildlife and low Oxygen levels, hypoxia, inhibit the growth of aquatic life and can cause fish kills. Oxygen is depleted in a body of water when an excess of plant nutrients enters the aquatic environment; the resulting phytoplankton bloom may temporarily raise the Oxygen level due to photosynthesis but the O2 level is soon depleted at night due to respiration. When the excess phytoplankton die they are consumed by bacteria, a process which further depletes dissolved Oxygen. Oxygen depletion can also occur in an estuary where fresh water enters the ocean. The stratification caused by the less dense fresh water staying on top of the more dense salt water prevents the normal mixing of the ocean resulting in low O2 levels in the lowest levels of highest salinity.
DO probes have become one of the basic tools in monitoring aquatic environments. Oxygen probes can be used with portable meters as a spot check in lakes streams and oceans, or in the laboratory for research purposes. DO probes are also used as a constant monitor in aquaculture. Manufacturers of DO probes and meters include but are not limited to Mettler Toledo, Thermo Scientific, Beckman Coulter, YSI, and Hach.