Partenaires et organisations internationales
(Anglais)
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Institut de Biogéochimie Marine, Montrouge (F), Groupe de Microbiologie des Milieux Aquatiques-Uni Libre de Bruxelles (B), Max Plank Institut fur Limnologie, Plön (D); Netherlands Institute of Ecology, Yerseke (NL), Observatoire Océanographique de Banuyls (F), Institut Universitaire Européen de la Mer, Brest (F), Laboratoire de Géologie Appliquée, Univ. Pierre et Marie Curie, Paris (F), Institute for Remote Sensing, Ispra (I), Institute of Biology of the Southern Seas, Odessa (Ukraine), Institute of Oceanology, Varna (BG), National Institute of Marine Geology and Geo-ecology, Bucharest, Constantza (RO), Institute of Biogeochemistry and Marine Chemistry, Univ. Hamburg (D), Institute of Microbiology of the Russian Academy of Sciences, Moscow (R); Vernadsky Institute of Geochemistry, Moscow (R)
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Résumé des résultats (Abstract)
(Anglais)
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Anoxic conditions in near-shore waters of the North-Western Black Sea have been observed in recent years during summer. This thread to the coastal ecosystems is a direct consequence of increased phosphorus and nitrogen loads by the rivers Danube, Dniepr and Dniestr. In addition, the construction of large dams (Iron Gates) has decreased the dissolved silica discharge from the Danube river, and thus changed the character of primary production in the Black Sea. In the European River-Ocean Systems (EROS 21) project these problems are addressed. Within this framework our group investigated the rates of benthic nutrient cycling in order to quantify the role of near-shore sediments in the eutrophication process of the Black Sea.
Benthic fluxes of nutrients and metals were measured in the coastal zone of the North Western Black Sea which is influenced by the Danube and Dnjestr deltas. Flux data were obtained with a benthic flux chamber during two EROS-21 cruises in late summer 1995 and in spring 1997. Oxygen concentrations varied strongly between seasons. In spring, anoxic conditions were never observed during the in situ experiments near shore or on the shelf. However, the oxygen consumption rates in spring 1997 (13.4-21.2 mmol m-2 d-1) were as high as in summer 1995. The higher oxygen concentrations in spring are related to a more intense mixing of the water column due to stormy periods and higher river discharge during late winter and spring. The benthic recycling of nutrients differed in both seasons. Areas with highest nutrient concentrations at the sediment water interface are found at the Danube Delta Front and Danube Prodelta. On the Danube Delta Front and the Dnjestr mouth, ammonia fluxes were lower in spring (1.46-1.24 mmol m-2 d-1) than in late summer (2.9 mmol m-2 d-1) based on higher nitrification rates and lower influx of organic matter. On the Danube Prodelta, redox conditions shiffed towards hypoxia since nitrate and nitrite concentrations in spring were still high together with high ammonia flux rates. This may be a result of beginning sedimentation of organic matter which may be caused by spring phytoplankton bloom. The coastal zone of the North-Western Black Sea that is influenced by the Danube river has highest nutrient concentrations and benthic flux rates. In contrast, the Continental Shelf seems to be an area with relatively low nutrient fluxes.
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