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| Titel |
Drivers, mechanisms and long-term variability of seasonal hypoxia on the Black Sea northwestern shelf – is there any recovery after eutrophication? |
| VerfasserIn |
A. Capet, J.-M. Beckers, M. Gregoire |
| 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 ; 10, no. 6 ; Nr. 10, no. 6 (2013-06-18), S.3943-3962 |
| Datensatznummer |
250018294
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| Publikation (Nr.) |
 copernicus.org/bg-10-3943-2013.pdf |
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| Zusammenfassung |
The Black Sea northwestern shelf (NWS) is a shallow eutrophic area in which
the seasonal stratification of the water column isolates the bottom waters
from the atmosphere. This prevents ventilation from counterbalancing the large
consumption of oxygen due to respiration in the bottom waters and in the
sediments, and sets the stage for the development of seasonal hypoxia.
A three-dimensional (3-D) coupled physical–biogeochemical model is used to
investigate the dynamics of bottom hypoxia in the Black Sea NWS, first at
seasonal and then at interannual scales (1981–2009), and to differentiate
its driving factors (climatic versus eutrophication).
Model skills are evaluated by a quantitative comparison of the model results
to 14 123 in situ oxygen measurements available in the NOAA World Ocean and
the Black Sea Commission databases, using different error metrics. This
validation exercise shows that the model is able to represent the seasonal
and interannual variability of the oxygen concentration and of the occurrence
of hypoxia, as well as the spatial distribution of oxygen-depleted waters.
During the period 1981–2009, each year exhibits seasonal bottom hypoxia at
the end of summer. This phenomenon essentially covers the northern part of
the NWS – which receives large inputs of nutrients from the Danube, Dniester and
Dnieper rivers – and extends, during the years of severe hypoxia, towards the
Romanian bay of Constanta.
An index H which merges the aspects of the spatial and temporal extension
of the hypoxic event is proposed to quantify, for each year, the intensity of
hypoxia as an environmental stressor.
In order to explain the interannual variability of H and to disentangle its
drivers, we analyze the long time series of model results by means of a
stepwise multiple linear regression. This statistical model gives a general
relationship that links the intensity of hypoxia to eutrophication and
climate-related variables.
A total of 82% of the interannual variability of H is explained by the combination
of four predictors: the annual riverine nitrate load (N), the sea surface
temperature in the month preceding stratification (Ts), the
amount of semi-labile organic matter accumulated in the sediments (C) and
the sea surface temperature during late summer (Tf). Partial
regression indicates that the climatic impact on hypoxia is almost as
important as that of eutrophication.
Accumulation of organic matter in the sediments introduces an important
inertia in the recovery process after eutrophication, with a typical
timescale of 9.3 yr.
Seasonal fluctuations and the heterogeneous spatial distribution complicate
the monitoring of bottom hypoxia, leading to contradictory conclusions when
the interpretation is done from different sets of data. In particular, it
appears that the recovery reported in the literature after 1995 was
overestimated due to the use of observations concentrated in areas and months
not typically affected by hypoxia. This stresses the urgent need for a
dedicated monitoring effort in the Black Sea NWS focused on the areas and
months concerned by recurrent hypoxic events. |
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