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We develop a three dimensional coupled circulation biogeochemical model of the Black Sea in order to assess the status, causes and mechanisms of hypoxia on the north-western shelf (BS-NWS) that was severely impacted by eutrophication in the 80's. Model simulations over the last 3 decades show evidences that hypoxia is still occurring seasonally on a non-negligible area of the bottom waters of the BS-NWS. This important finding (corroborated by the monitoring of local institutes) is in contradiction with the general idea that bottom hypoxia vanished with the reduction after 1992 of riverborne nutrient discharge. We found that the overestimation of recovering was due to the use of observations concentrated in areas and months not typically affected by hypoxia. 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 provide recommendations for the definition of policies aiming to avoid bottom hypoxia and to preserve the GES of the benthic habitat, a simplified statistical model has been derived to link the H index with the level of nutrients loads discharges by the Danube and specific climate drivers of hypoxia. This approach allows establishing a cost of the local warming in terms of its impact on hypoxia. We find that the potential increase of water stratification in a global change context may promote the occurrence of seasonal hypoxia and this stresses that the definition of future management scheme of river discharges have to integrate the impact of climate change.

biogeochemical, biogeochemistry, Black Sea, eutrophication, hypoxia, nutrient discharge, D5, Descriptor 5, modelling

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20/01/2016

Kick-off workshop of the Network of Experts for ReDeveloping Models of the European Marine Environment

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EN

201604192610

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