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Seasonal hypoxia in the Black Sea north-western shelf. Is there any recovery after eutrophication ?
Capet, Arthur; Beckers, Jean-Marie; Grégoire, Marilaure
2013In 9th EGU General Assembly
Peer reviewed
 

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Keywords :
Black Sea; Hypoxia; biogeochemical modeling
Abstract :
[en] The Black Sea North-western shelf (NWS) is a shallow eutrophic area in which seasonal tratification of the water column isolates bottom waters from the atmosphere and prevents entilation to compensate for the large consumption of oxygen, due to respiration in the bottom aters and in the sediments. A 3D coupled physical biogeochemical model is used to investigate he dynamics of bottom hypoxia in the Black Sea NWS at different temporal scales from seasonal o interannual (1981-2009) and to differentiate the driving factors (climatic versus eutrophication) f hypoxic conditions in bottom waters. Model skills are evaluated by comparison with 14500 in- itu oxygen measurements available in the NOAA World Ocean Database and the Black Sea ommission data. The choice of skill metrics and data subselections orientate the validation rocedure towards specific aspects of the oxygen dynamics, and prove the model’s ability to esolve the seasonal cycle and interannual variability of oxygen concentration as well as the patial location of the oxygen depleted waters and the specific threshold of hypoxia. During the eriod 1981-2009, each year exhibits seasonal bottom hypoxia at the end of summer. This henomenon essentially covers the northern part of the NWS, receiving large inputs of nutrients rom the Danube, Dniestr and Dniepr rivers, and extends, during the years of severe hypoxia, owards the Romanian Bay of Constanta. In order to explain the interannual variability of bottom ypoxia and to disentangle its drivers, a statistical model (multiple linear regression) is proposed sing the long time series of model results as input variables. This statis- tical model gives a eneral 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: he annual nitrate discharge (N ), the sea surface temperature in the month preceding tratification (T ), the amount of semi-labile organic matter in the sediments (C) and the duration f the stratification (D). Eutrophication (N ,C) and climate (T ,D) predictors explain a similar mount 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 his analysis, we find that under standard conditions (i.e. average atmospheric conditions, ediments in equi- librium with river discharges), the intensity of hypoxia can be linked to the evel of nitrate discharge through a non-linear equation (power law). Bottom hypoxia does not ffect the whole Black Sea NWS but rather exhibits an important spatial variability. This heterogeneous distribution, in addition to the seasonal fluctuations, complicates the monitoring f ottom hypoxia leading to contradictory conclusions when the interpretation is done from different ets of data. We find that it was the case after 1995 when the recovery process was verestimated due to the use of observations concentrated in areas and months not typically ffected by hypoxia. This stresses out the urging need of a dedicated monitoring effort in the WS f the Black Sea focused on the areas and the period of the year concerned by recurrent hypoxic events.
Disciplines :
Earth sciences & physical geography
Author, co-author :
Capet, Arthur  ;  Université de Liège - ULiège > Département de Biologie, Ecologie et Evolution > Océanologie
Beckers, Jean-Marie  ;  Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > GeoHydrodynamics and Environment Research (GHER)
Grégoire, Marilaure  ;  Université de Liège - ULiège > Département de Biologie, Ecologie et Evolution > Océanologie
Language :
English
Title :
Seasonal hypoxia in the Black Sea north-western shelf. Is there any recovery after eutrophication ?
Publication date :
11 April 2013
Event name :
EGU general assembly 2013
Event place :
Vienna, Austria
Event date :
07-12 april 2013
Audience :
International
Main work title :
9th EGU General Assembly
Publisher :
European Geosciences Union
Peer reviewed :
Peer reviewed
European Projects :
FP7 - 287600 - PERSEUS - Policy-oriented marine Environmental Research in the Southern EUropean Seas
Name of the research project :
PE
Funders :
CE - Commission Européenne [BE]
Available on ORBi :
since 22 December 2013

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