Article (Scientific journals)
Drivers, mechanisms and long-term variability of seasonal hypoxia on the Black Sea northwestern shelf – is there any recovery after eutrophication?
Capet, Arthur; Beckers, Jean-Marie; Grégoire, Marilaure
2013In Biogeosciences, 10, p. 3943-3962
Peer Reviewed verified by ORBi
 

Files


Full Text
bg-10-3943-2013.pdf
Publisher postprint (2.73 MB)
Request a copy

All documents in ORBi are protected by a user license.

Send to



Details



Abstract :
[en] 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.
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 :
Drivers, mechanisms and long-term variability of seasonal hypoxia on the Black Sea northwestern shelf – is there any recovery after eutrophication?
Publication date :
2013
Journal title :
Biogeosciences
ISSN :
1726-4170
eISSN :
1726-4189
Publisher :
European Geosciences Union, Katlenburg-Lindau, Germany
Volume :
10
Pages :
3943-3962
Peer reviewed :
Peer Reviewed verified by ORBi
Available on ORBi :
since 19 June 2013

Statistics


Number of views
94 (13 by ULiège)
Number of downloads
3 (2 by ULiège)

Scopus citations®
 
43
Scopus citations®
without self-citations
35
OpenCitations
 
41

Bibliography


Similar publications



Contact ORBi