Hypoxia (ecology)
Hypoxia occurs when the concentration of dissolved oxygen in water is reduced to such an extent that aquatic life is impaired. The oxygen saturation depends on the salt content ( salinity ) and the temperature.
At 0% saturation or an oxygen content of the water below 2 mg / l there is anoxia , which leads to anaerobic living conditions ( anoxic environment, if nitrate is present). A hypoxic milieu is when the oxygen saturation is reduced to 1 to 30% of the normal value (at least 80%).
causes
In addition to natural effects, hypoxia is mainly the result of eutrophication due to water pollution with plant nutrients such as ammonium , nitrate , nitrite and phosphates . The main sources are intensive agriculture and sewage . It comes to the algae bloom with increasing oxygen saturation in the day and increasingly declining saturation at night. Dead algae are broken down by bacteria and further reduce the oxygen content. Finally, sulfate-breathing bacteria appear, the metabolic products of which settle in black layers on the bottom of the water. The hypoxic state leads to the death of invertebrates living on the ground such as worms and mussels as well as to the death of fish .
Natural hypoxia occurs in shallow estuaries in the ocean. Less dense fresh water pushes itself onto the salt water below without mixing , whereby the oxygen concentration in the lower layers drops. Other causes are “closed water bodies” with little exchange, such as in the Black Sea or in fjords .
Observations and solutions
The oxygen saturation can very quickly drop to zero when offshore winds drive surface water out to sea, while anoxic deep water rises, the temperature drops and the salinity increases (observed by the Longterm Ecological Observatory in the Bay of Kiel ). More recent studies of the oxygen content include fish and zooplankton , whose behavior changes drastically under reduced conditions, even with low water pollution ( EcoSCOPE ). To avoid hypoxia due to eutrophication, it is necessary to reduce the nutrient input into the oceans. In addition to reducing fertilizer consumption, the renaturation of river banks, marshes and mangrove swamps makes sense.
In by peat mosses acidified bogs hypoxia prevents microbial decomposition in the soil, so that bog bodies of the Iron Age , as the wife of Haraldskær or Tollund Man , are well conserved.
Dead zone
Since oceanographic mapping began in the 1970s, dead zones in the seas and large lakes have doubled in number and size in every decade. The "Global Environment Outlook Year Book" (GEO Year Book 2006) published by the United Nations Environment Program reports on 200 dead zones worldwide in which the oxygen saturation is insufficient for marine life. These zones , which occur mainly in bays and marginal seas, are to be distinguished from the naturally occurring, large-scale oxygen minimum zones , which usually occur in greater water depths. These are mainly known from the eastern Pacific Ocean and the northern Indian Ocean , but also occur in other regions.
Some areas exist only temporarily and over a small area, others over long periods in the annual cycle and up to 70,000 km².
After dead zones near the coast were first registered in the Baltic Sea in 2011 , German coastal areas are also potentially affected.
According to the current state of knowledge, dead zones are reversible. For example, after the collapse of the Soviet Union between 1991 and 2001, the phenomenon in the Black Sea largely disappeared due to increased fertilizer prices. Fishing then became the main source of income in the area again.
The first hypoxic zones discovered are:
- New York Bay
- Chesapeake Bay , Maryland
- Parts of the Baltic Sea
- Kattegat
- Parts in the northern Adriatic
- Parts in the Gulf of Mexico (Mississippi Delta)
- Some parts in the Black Sea
The journal Science published in 2008 the existence of 400 dead zones on 245,000 km².
literature
- U. Kils, U. Waller and P. Fischer: The Fish Kill of the Autumn 1988 in Kiel Bay . International Council for the Exploration of the sea CM 1989 / L: 14
- P. Fischer and U. Kils: In situ Investigations on Respiration and Behavior of Stickleback Gasterosteus aculeatus and the Eelpout Zoaraes viviparus During Low Oxygen Stress . International Council for the Exploration of the Sea CM 1990 / F: 23
- P. Fischer, K. Rademacher and U. Kils: In situ investigations on the respiration and behavior of the eelpout Zoarces viviparus under short term hypoxia . Mar Ecol Prog 1992. Ser 88: 181-184
Web links
- Spektrum .de: The ocean is running out of air October 25, 2018
- The oceans struggle for air - dead zones in the oceans , from the series Equilibria: Views of a Living Planet, Part 2: Oceans , Science in Focus , Deutschlandfunk in December 2012
- English
- Aquatic Dead Zones , NASA Earth Observatory, Jan. 1, 2008
- The Gulf of Mexico Dead Zone , Science Museum of Minnesota (multimedia)
Individual evidence
- ↑ William J. Ripple, Christopher Wolf, Thomas M. Newsome, Mauro Galetti, Mohammed Alamgir, Eileen Crist, Mahmoud I. Mahmoud, William F. Laurance and 15,364 life scientists from 184 countries: World Scientists' Warning to Humanity: A Second Notice . In: BioScience . tape 67 , no. 12 , 2017, p. 1026-1028 , doi : 10.1093 / biosci / bix125 .
- ^ Conley, DJ et al .: Hypoxia Is Increasing in the Coastal Zone of the Baltic Sea. In: Environ. Sci. Technol., 2011, 45 (16), pp 6777-6783, doi: 10.1021 / es201212r . See death zones in the Baltic Sea , the daily newspaper, August 25, 2011
- ^ Robert J. Diaz, Rutger Rosenberg: Spreading Dead Zones and Consequences for Marine Ecosystems . In: Science . Vol. 321, August 15, 2008, pp. 926-929 , doi : 10.1126 / science.1156401 .