Doctoral thesis (Dissertations and theses)
Study of the multi-decadal evolution of the Black Sea hydrodynamics and biogeochemistry using mathematical modelling
Capet, Arthur
2014
 

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Keywords :
Black Sea; Modelling; Biogeochemistry; Hydrodynamics; Multidecadal variability; Hypoxia; Benthic/Pelagic Coupling
Abstract :
[en] This thesis addresses the biogeochemical cycles in the Black Sea (BS) during the shifting environmental context that affected the BS during the last decades of the 20th century. The study is based on sophisticated data analysis tools and on the development and implementation of a coupled 3D biogeochemical model on the BS domain. The long term variability of the BS hydrodynamical structure was first examined on the basis of in-situ profiles (1950-2012), satellite imagery (1985-2000) and 3D modelling (1960-2000). Profiles of temperature and salinity were used to derive vertical characteristics of the BS structure: the mixed layer depth and the cold content of the Cold Intermediate Layer. To untangle the spatial and temporal trends from this heterogeneous dataset, a general methodology was proposed and embedded in the data analysis software DIVA. The detrended climatologies and long-term time series provided by this approach were used to assess statistical relationships with local atmospheric conditions. Satellite data (sea surface temperature and altimetry) and model results were then analyzed to relate observable surface dynamics to internal hydrodynamic properties. The main multivariate modes of variability of the BS hydrodynamic structure were highlighted on the basis of Empirical Orthogonal Function analysis. Their temporal evolution was explained by the occurrences of specific atmospheric patterns, identified on the basis of neural algorithm analysis and related to the phases of well known teleconnection systems (i.e. the North Atlantic and East Asia/West Russia oscillations). To study the dynamics of eutrophication in the shallow Black Sea NorthWestern Shelf (BS-NWS), a benthic model component was developed that considers the environmental control on diagenetic processes and the bottom shear stress restriction on organic matter deposition. The model accurately reproduced the seasonal and spatial variability depicted by in-situ estimates of benthic nutrients and oxygen fluxes in the BS-NWS. Outputs were used to review the role of the benthic component in BS biogeochemical cycles. The multi-decadal simulations, enabled by the low computational requirements of the benthic-pelagic coupling approach, revealed an inertial component in the dynamics of eutrophication resulting from the accumulation of organic matter during the years of important nutrient loads. This refined resolution of the BS-NWS biogeochemistry allowed us to study the phenomenon of seasonal hypoxia, which is believed to have played a part in the sudden collapse of the fisheries stocks in the late 80s. An index H, combining the spatial and temporal extension of the seasonal hypoxic event, was proposed to quantify the annual intensity of hypoxia as a pressure on benthic communities. We have shown that hypoxia was first triggered in the late 70s by high nitrogen loads, and sustained by sedimentary organic matter accumulation after a rapid reduction of these loads in the 90s. After 2000, warmer summers again led to a increase of the H-index, by entraining hypoxic events of smaller spatial extension but increased duration. A practical relationship distinguishing the impacts of eutrophication and climatic drivers was proposed to assess the effect of their projected values on the future intensity of hypoxia.
Research center :
GHER
Laboratoire d'Océanologie
Disciplines :
Earth sciences & physical geography
Author, co-author :
Capet, Arthur  ;  Université de Liège - ULiège > Département de Biologie, Ecologie et Evolution > Océanologie
Language :
English
Title :
Study of the multi-decadal evolution of the Black Sea hydrodynamics and biogeochemistry using mathematical modelling
Defense date :
March 2014
Number of pages :
238
Institution :
ULiège - Université de Liège
Degree :
Docteur en Sciences
Promotor :
Grégoire, Marilaure  ;  Université de Liège - ULiège > Freshwater and OCeanic science Unit of reSearch (FOCUS)
Beckers, Jean-Marie  ;  Université de Liège - ULiège > Freshwater and OCeanic science Unit of reSearch (FOCUS)
President :
Bouquegneau, Jean-Marie ;  Université de Liège - ULiège > Département de Biologie, Ecologie et Evolution
Secretary :
Barth, Alexander  ;  Université de Liège - ULiège > Freshwater and OCeanic science Unit of reSearch (FOCUS)
Jury member :
Meysman, Filip
Borges, Alberto  ;  Université de Liège - ULiège > Freshwater and OCeanic science Unit of reSearch (FOCUS)
Friedrich, Jana
Stanev, Emil
European Projects :
FP7 - 287600 - PERSEUS - Policy-oriented marine Environmental Research in the Southern EUropean Seas
Funders :
FRIA - Fonds pour la Formation à la Recherche dans l'Industrie et dans l'Agriculture [BE]
CE - Commission Européenne [BE]
Available on ORBi :
since 24 February 2014

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