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| Titel |
Drivers, mechanisms and long term variability of bottom seasonal hypoxia in the Black Sea north-western Shelf. Is there any recovery after eutrophication ? |
| VerfasserIn |
Arthur Capet, Jean-Marie Beckers, Marilaure Gregoire |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250075472
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| Zusammenfassung |
| The Black Sea North-western shelf (NWS) is a shallow eutrophic area in which seasonal
stratification of the water column isolates bottom waters from the atmosphere and prevents
ventilation to compensate for the large consumption of oxygen, due to respiration in the
bottom waters and in the sediments.
A 3D coupled physical biogeochemical model is used to investigate the dynamics of
bottom hypoxia in the Black Sea NWS at different temporal scales from seasonal to
interannual (1981-2009) and to differentiate the driving factors (climatic versus
eutrophication) of hypoxic conditions in bottom waters.
Model skills are evaluated by comparison with 14500 in-situ oxygen measurements
available in the NOAA World Ocean Database and the Black Sea Commission data. The
choice of skill metrics and data subselections orientate the validation procedure towards
specific aspects of the oxygen dynamics, and prove the model’s ability to resolve
the seasonal cycle and interannual variability of oxygen concentration as well as
the spatial location of the oxygen depleted waters and the specific threshold of
hypoxia.
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,
receiving large inputs of nutrients from the Danube, Dniestr and Dniepr rivers,
and extends, during the years of severe hypoxia, towards the Romanian Bay of
Constanta.
In order to explain the interannual variability of bottom hypoxia and to disentangle its
drivers, a statistical model (multiple linear regression) is proposed using the long time
series of model results as input variables. This statistical model gives a general
relationships that links the intensity of hypoxia to eutrophication and climate related
variables.
The use of four predictors allows to reproduce 78% of hypoxia interannual variability: the
annual nitrate discharge (N), the sea surface temperature in the month preceding stratification
(T ), the amount of semi-labile organic matter in the sediments (C) and the duration of the
stratification (D). Eutrophication (N,C) and climate (T ,D) predictors explain a similar
amount of variability (~ 35%) when considered separately. A typical timescale of 9.3
years is found to describe the inertia of sediments in the recovering process after
eutrophication.
From this analysis, we find that under standard conditions (i.e. average atmospheric
conditions, sediments in equilibrium with river discharges), the intensity of hypoxia can
be linked to the level of nitrate discharge through a non-linear equation (power
law).
Bottom hypoxia does not affect the whole Black Sea NWS but rather exhibits an
important spatial variability. This heterogeneous distribution, in addition to the seasonal
fluctuations, complicates the monitoring of bottom hypoxia leading to contradictory
conclusions when the interpretation is done from different sets of data. We find that it was the
case after 1995 when the recovery process was overestimated due to the use of observations
concentrated in areas and months not typically affected by hypoxia. This stresses out the
urging need of a dedicated monitoring effort in the NWS of the Black Sea focused
on the areas and the period of the year concerned by recurrent hypoxic events. |
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