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| Titel |
Effects of natural and human-induced hypoxia on coastal benthos |
| VerfasserIn |
L. A. Levin, W. Ekau, A. J. Gooday, F. Jorissen, J. J. Middelburg, S. W. A. Naqvi, C. Neira, N. N. Rabalais, 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 ; 6, no. 10 ; Nr. 6, no. 10 (2009-10-08), S.2063-2098 |
| Datensatznummer |
250004032
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| Publikation (Nr.) |
 copernicus.org/bg-6-2063-2009.pdf |
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| Zusammenfassung |
Coastal hypoxia (defined here as <1.42 ml L−1; 62.5 μM;
2 mg L−1, approx. 30% oxygen saturation) develops seasonally in
many estuaries, fjords, and along open coasts as a result of natural
upwelling or from anthropogenic eutrophication induced by riverine nutrient
inputs. Permanent hypoxia occurs naturally in some isolated seas and marine
basins as well as in open slope oxygen minimum zones. Responses of benthos to
hypoxia depend on the duration, predictability, and intensity of oxygen
depletion and on whether H2S is formed. Under suboxic conditions, large
mats of filamentous sulfide oxidizing bacteria cover the seabed and consume
sulfide. They are hypothesized to provide a detoxified microhabitat for
eukaryotic benthic communities. Calcareous foraminiferans and nematodes are
particularly tolerant of low oxygen concentrations and may attain high
densities and dominance, often in association with microbial mats. When
oxygen is sufficient to support metazoans, small, soft-bodied invertebrates
(typically annelids), often with short generation times and elaborate
branchial structures, predominate. Large taxa are more sensitive than small
taxa to hypoxia. Crustaceans and echinoderms are typically more sensitive to
hypoxia, with lower oxygen thresholds, than annelids, sipunculans, molluscs
and cnidarians. Mobile fish and shellfish will migrate away from low-oxygen
areas. Within a species, early life stages may be more subject to oxygen
stress than older life stages.
Hypoxia alters both the structure and function of benthic communities, but
effects may differ with regional hypoxia history. Human-caused hypoxia is
generally linked to eutrophication, and occurs adjacent to watersheds with
large populations or agricultural activities. Many occurrences are seasonal,
within estuaries, fjords or enclosed seas of the North Atlantic and the NW
Pacific Oceans. Benthic faunal responses, elicited at oxygen levels below
2 ml L−1, typically involve avoidance or mortality of large species
and elevated abundances of enrichment opportunists, sometimes prior to
population crashes. Areas of low oxygen persist seasonally or continuously
beneath upwelling regions, associated with the upper parts of oxygen minimum
zones (SE Pacific, W Africa, N Indian Ocean). These have a distribution
largely distinct from eutrophic areas and support a resident fauna that is
adapted to survive and reproduce at oxygen concentrations
<0.5 ml L−1. Under both natural and eutrophication-caused hypoxia
there is loss of diversity, through attrition of intolerant species and
elevated dominance, as well as reductions in body size. These shifts in
species composition and diversity yield altered trophic structure, energy
flow pathways, and corresponding ecosystem services such as production,
organic matter cycling and organic C burial. Increasingly the influences of
nature and humans interact to generate or exacerbate hypoxia. A warmer ocean
is more stratified, holds less oxygen, and may experience greater advection
of oxygen-poor source waters, making new regions subject to hypoxia. Future
understanding of benthic responses to hypoxia must be established in the
context of global climate change and other human influences such as
overfishing, pollution, disease, habitat loss, and species invasions. |
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