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| Titel |
Dynamics and distribution of natural and human-caused hypoxia |
| VerfasserIn |
N. N. Rabalais, R. J. Díaz, L. A. Levin, R. E. Turner, D. Gilbert, J. Zhang |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 7, no. 2 ; Nr. 7, no. 2 (2010-02-12), S.585-619 |
| Datensatznummer |
250004491
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| Publikation (Nr.) |
 copernicus.org/bg-7-585-2010.pdf |
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| Zusammenfassung |
Water masses can become undersaturated with oxygen when natural processes
alone or in combination with anthropogenic processes produce enough organic
carbon that is aerobically decomposed faster than the rate of oxygen
re-aeration. The dominant natural processes usually involved are
photosynthetic carbon production and microbial respiration. The re-supply
rate is indirectly related to its isolation from the surface layer. Hypoxic
water masses (<2 mg L−1, or approximately 30% saturation) can form,
therefore, under "natural" conditions, and are more likely to occur in
marine systems when the water residence time is extended, water exchange and
ventilation are minimal, stratification occurs, and where carbon production
and export to the bottom layer are relatively high. Hypoxia has occurred
through geological time and naturally occurs in oxygen minimum zones, deep
basins, eastern boundary upwelling systems, and fjords.
Hypoxia development and continuation in many areas of the world's coastal
ocean is accelerated by human activities, especially where nutrient loading
increased in the Anthropocene. This higher loading set in motion a cascading
set of events related to eutrophication. The formation of hypoxic areas has
been exacerbated by any combination of interactions that increase primary
production and accumulation of organic carbon leading to increased
respiratory demand for oxygen below a seasonal or permanent pycnocline.
Nutrient loading is likely to increase further as population growth and
resource intensification rises, especially with increased dependency on
crops using fertilizers, burning of fossil fuels, urbanization, and waste
water generation. It is likely that the occurrence and persistence of
hypoxia will be even more widespread and have more impacts than presently
observed.
Global climate change will further complicate the causative factors in both
natural and human-caused hypoxia. The likelihood of strengthened
stratification alone, from increased surface water temperature as the global
climate warms, is sufficient to worsen hypoxia where it currently exists and
facilitate its formation in additional waters. Increased precipitation that
increases freshwater discharge and flux of nutrients will result in
increased primary production in the receiving waters up to a point. The
interplay of increased nutrients and stratification where they occur will
aggravate and accelerate hypoxia. Changes in wind fields may expand oxygen
minimum zones onto more continental shelf areas. On the other hand, not all
regions will experience increased precipitation, some oceanic water
temperatures may decrease as currents shift, and frequency and severity of
tropical storms may increase and temporarily disrupt hypoxia more often.
The consequences of global warming and climate change are effectively
uncontrollable at least in the near term. On the other hand, the
consequences of eutrophication-induced hypoxia can be reversed if long-term,
broad-scale, and persistent efforts to reduce substantial nutrient loads are
developed and implemented. In the face of globally expanding hypoxia, there
is a need for water and resource managers to act now to reduce nutrient
loads to maintain, at least, the current status. |
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