Oxygen depletion

Oxygen depletion is a phenomenon that occurs in aquatic environments as dissolved oxygen (DO; molecular oxygen dissolved in the water) becomes reduced in concentration to a point detrimental to aquatic organisms living in the system. Dissolved oxygen is typically expressed as a percentage of the oxygen that would dissolve in the water at the prevailing temperature and salinity (both of which affect the solubility of oxygen in water; see oxygen saturation and underwater). An aquatic system lacking dissolved oxygen (0% saturation) is termed anaerobic, reducing, or anoxic; a system with low DO concentration—in the range between 1 and 30% DO saturation—is called hypoxic. Most fishes cannot live below 30% DO saturation. A "healthy" aquatic environment should seldom experience DO less than 80%.

Causes of oxygen depletion

Oxygen depletion could be the result of a number of factors including natural ones, but is of most concern as a consequence of pollution and as a highly detrimental outcome of a process known as eutrophication. Where plant nutrients enter a river, lake, or ocean, phytoplankton blooms are encouraged. While phytoplankton, through photosynthesis, will raise DO saturation during daylight hours, the dense population of a bloom reduces DO saturation during the night. When phytoplankton cells die, they sink towards the bottom and are decomposed by bacteria, a process that further reduces DO in the water column. If oxygen depletion progresses to hypoxia, fish kills can occur and invertebrates like worms and clams on the bottom may be killed as well.

A graphic (below, left) shows oxygen depletion in the open ocean, a consequence of too much input of oxygen consuming chemicals and bioactive nutrients from the Hudson, Raritan, and Mullica rivers into the Atlantic Ocean off New York and Atlantic City, New Jersey, USA.

Natural occurrences of hypoxia have been observed. Water flowing from a river into the sea is less dense than salt water. When this water does not mix with the underlying saline water, the oxygen concentration in the bottom layer may become low enough for hypoxia to occur. Hypoxia is particularly problematic in shallow waters of semi-enclosed bodies of water like the Waddenzee or the Gulf of Mexico where land runoff is substantial. In these areas, a so-called "dead zone" can be created.

Solutions

To combat hypoxia, it is essential to reduce the amount of land-derived nutrients reaching rivers in runoff. Defensively this can be done by improving sewage treatment and by reducing the amount of fertilizers leaching into the rivers. Offensively this can be done by restoring natural environments along a river; marshes are particularly effective in reducing the amount of phosphorus and nitrogen (nutrients) in water.

In a very short time the oxygen saturation can drop to zero when offshore blowing winds drive surface water out and anoxic depthwater rises up. At the same time a decline in temperature and a rise in salinity is observed (from the longterm ecological observatory in the seas at Kiel Fjord, Germany). New approaches of longterm monitoring of oxygen regime in the ocean observe online the behavior of fish and zooplankton, which changes drastically under reduced oxygen saturations (ecoSCOPE) and already at very low levels of water pollution.

References

• Kils, U., U. Waller, and P. Fischer. 1989. The Fish Kill of the Autumn 1988 in Kiel Bay. International Council for the Exploration of the sea C M 1989/L:14
• Fischer P. and U. Kils. 1990. In situ Investigations on Respiration and Behaviour of Stickleback Gasterosteus aculeatus and the Eelpout Zoaraes viviparus During Low Oxygen Stress. International Council for the Exploration of the Sea C M 1990/F:23
• Fischer P., K. Rademacher, and U. Kils. 1992. In situ investigations on the respiration and behaviour of the eelpout Zoarces viviparus under short term hypoxia. Mar Ecol Prog Ser 88: 181-184

POV - The underwater videos of dying fish were released for broadcasting in the primetime TV news TAGESSCHAU and articles in key newspapers in Germany (against the order of the director of the oceanography institute), with the consequences that the tourism in the area declined drastically, and the research group with all students and instrumentation were destroyed by political forces of the that-time quite currupt capitol Kiel.

See also

Anoxic event

External link

• Hypoxia in the Gulf of Mexico