Oxygen minimum zone
In hydrology, an oxygen minimum zone (or oxygen minimum zone ) refers to an area of the water column in a body of water in which the oxygen content is comparatively low. In extreme cases, oxygen-breathing (aerobic) organisms can no longer exist there permanently. The term is mainly used for large areas in the oceans where the phenomenon occurs naturally.
Oxygen deficiency itself, known as hypoxia , also occurs in a large number of bodies of water under other ecological conditions and is often caused by humans.
oceanography
By buoyancy enters particular, distinguished by special geophysical conditions, regions of the oceans increasingly deep water in the exposed (photic or trophogenic ) zone near the surface where it is because of these nutrients unicellular to a highly excited production of algae, the phytoplankton comes, that can lead to algal blooms . After they die, these phytoplanktons, which are usually a little heavier than the surrounding water, slowly sink to the bottom, their biomass being broken down by bacteria. Since this decomposition takes place aerobically , with the consumption of oxygen, the surrounding seawater is depleted of oxygen at a certain depth below the trophogenic zone. This cannot be quickly replenished either through subsequent delivery from the air or through the production of algae, since no more algae can grow below the exposed zone. This creates pronounced zones in free water at medium depths in which the oxygen content is reduced over longer periods of time when high biomass production is combined with other unfavorable factors such as low turbulence and flow velocity and water that is already relatively low in oxygen in the initial state. Oxygen slowly reaches the depths from the surface layers through diffusion, but also through vertical turbulent mixing, e.g. B. by internal waves , this counteracts the process.
Depending on the selected threshold value, areas with oxygen concentrations of 0.5 milligrams (about 7.5 percent saturation, 22 micromoles) or only 0.2 milligrams per liter or more are calculated as minimum zones. Areas in which the oxygen concentration falls below 10 micromoles per kilogram of seawater are called suboxic. Depending on the group of organisms, significant biological effects are to be expected below 60 to 120 micromoles per kilogram; this is referred to as hypoxic. In suboxic regions, the presence of free nitrate leads to nitrate breathing by bacterial species.
If the oxygen-depleted water hits the continental shelf through currents , extensive hypoxic zones can also develop on the sea floor. Measurements have shown that some of these have existed for (at least) centuries. In the oceans, the zones of minimum oxygen saturation are usually between 200 meters and 1,000 meters deep, but in exceptional cases they can also occur in shallow water depths of up to about 20 meters, especially off the South American coast. They can reach or even exceed a maximum height of 1,000 meters. The extent of the affected zones on the sea floor was estimated to be over one million square kilometers.
Occur
Oxygen minimum zones arise primarily in areas in which the thermohaline circulation directs ocean currents towards transverse coasts; these also depend on the prevailing wind directions due to the influence of the wind (via Ekman transport ). Extensive zones are known from the eastern tropical Pacific, off the American west coast, and the northern tropical Indian Ocean in the Arabian Sea and in the Bay of Bengal . Extensive areas of the tropical Atlantic, north of the west coast of Africa and, above all, south of the Gulf of Guinea , are hypoxic, and thus somewhat less severely affected . Its greatest extent in the eastern Pacific is associated with the fact that the water in the middle depths only rarely exchanges here and the inflowing water is already somewhat depleted in oxygen. Due to the consequences of man-made global warming, the expansion, possibly also further into extra-tropical seas, is forecast. There is evidence that it has expanded since 1960 to the present day. In hypoxic zones, due to the delayed degradation under oxygen-poor environmental conditions, more biomass reaches the sea floor, creating anoxic, often black-colored sediments. Since fewer organisms can live in these, which relocate the sediment via bioturbation , it is a self-reinforcing process.
Difference from low-oxygen shallow sea basins
The resulting oxygen minimum zones are essentially a natural phenomenon, even if human influences have a variety of effects on them. However, they are to be clearly distinguished from oxygen minima that can arise in shallow tributaries or bays as a result of human-made (anthropogenic) nutrient input ( eutrophication, for example, of flowing rivers). These are widely known from the Baltic Sea (at times 84,000 square kilometers), the Gulf of Mexico (at times 20,000 square kilometers) and the Black Sea , but they also occur on a smaller scale in many other regions, such as in many Norwegian fjords. The oxygen minimum zones that occur in upwelling regions are much more extensive and also occur in the open ocean.
Effects
As long as the oxygen content in the lake water is only reduced, but no hypoxia occurs, higher life can still be expected. At the latest below about 0.1 millimole oxygen per liter, almost only protozoa and organisms of the meiofauna appear, larger ones are missing. These small forms can, however, occur in densities that are significantly higher than normal proportions. Many pelagic species try to increase the uptake of oxygen, for example through breathing movements with an increased diffusion rate, others limit their activity in order to reduce the need. Planktonic species make daily vertical migrations into the exposed zone with higher oxygen contents. However, such migrations can even intensify the process through the breathing of the organisms. Many animals sink to the upper edges of the oxygen minimum zones, where they are safer from many predators, but also consume additional oxygen.
Many adapted species have enlarged gills or blood pigments with a higher affinity for oxygen (known for example from the mussel Gnathophausia ingens or the scorpion fish Sebastolobus alascanus ). Many species are known from sediments that have blood stained red by hemoglobins , even if this does not occur in closely related species from other regions.
Biodiversity is greatly reduced on sediments in shelf areas in the area of oxygen minimum zones . Filamentous (thread-forming) non-autotrophic sulfur bacteria of the genera Thioploca and Beggiatoa can form conspicuous mats or turf-like coatings that can reach 120 grams of biomass per square meter. The giant bacterium Thiomargarita namibiensis occurs frequently off the African coast , with cell diameters of up to 750 micrometers, perhaps the largest bacterial species of all. The jumping crayfish species Pleuroncodes monodon lives in the mats on the Pacific coast and grazes it as food. Species with a particularly thin, worm-like body with enlarged gills, often particularly small species, live in the sediment itself. The most common group of animals are often the nematodes . Within the foraminifera , a significant increase in smaller species has been demonstrated in the oxygen minimum zones.
The marine life that can cope well with a low concentration of oxygen includes the deep-sea crab Phronima sedentaria , the vampire squid and some jellyfish , such as the yellow hair jellyfish, as these jellyfish have enough oxygen for one or two in experiments in the mesogloea (a gelatinous tissue) Could save hours.
Oxygen minima in stagnant fresh water
Metalimnic oxygen minimum
Similar to the oceans, there are also areas in stagnant waters with minimal oxygen concentrations. In a layered lake , a transitional layer of water, the metalimnion, arises between the upper water layer , the epilimnion, and the lower, the hypolimnion . There it can come to a metalimnic oxygen minimum as well as a metalimnic oxygen maximum. The latter occurs when there is an increased presence of oxygen-producing algae in this zone.
See also
Web links
Individual evidence
- ↑ Substance transport in the tropical ocean decoded. on: geomar.de , August 1, 2013.
- ^ John J. Helly, Lisa A. Levin (2004): Global distribution of naturally occurring marine hypoxia on continental margins. Deep-Sea Research I 51 (2004) 1159-1168
- ^ Lothar Stramma, Gregory C. Johnson, Janet Sprintall, Volker Mohrholz (2008): Expanding Oxygen-Minimum Zones in the Tropical Oceans. Science 320: 655-658. doi : 10.1126 / science.1153847
- ↑ Andreas Villwock: Is the ocean running out of air? - Oxygen levels in the tropical oceans decreased in the last 50 years -. Leibniz Institute for Marine Sciences, Kiel, press release from May 1, 2008 from Informationsdienst Wissenschaft (idw-online.de), accessed on September 15, 2015.
- ↑ Animal migrations contribute to marine death zones. on: welt.de June 10, 2013.
- ↑ Lisa A. Levin (2003): Oxygen minimum zone Benthos: adaptation and community response to hypoxia. Oceanography and Marine Biology: an Annual Review 41: 1-45.
- ↑ Climate change could turn oxygen-free seas from blessing to curse for zooplankton. on: sciencedaily.com , July 12, 2011.
- ^ EV Thuesen, LD Rutherford, PL Brommer, K. Garrison, MA Gutowska, T. Towanda: Intragel oxygen promotes hypoxia tolerance of scyphomedusae. In: The Journal of experimental biology. Volume 208, Pt 13 July 2005, pp. 2475-2482, doi : 10.1242 / jeb.01655 , PMID 15961733 .