Towards nutrient budget scenarios for the north-western Black Sea shelf.

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Friedrich, J. , Gregoire, M. , Cociasu, A. and Gilbert, A. (2005): Towards nutrient budget scenarios for the north-western Black Sea shelf. , LOICZ II Inaugural Open Science Meeting, 27-29 June, Egmond aan Zee, The Netherlands. .
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Integrating manifold existing and new data on sources and sinks of nutrients in coastal and shelf ecosystems into budget models can assist in developing ecological management scenarios that provide tools to coastal managers for decision support. Those budgets are even more helpful when different conditions of an ecosystem are considered, e.g. time spans that reflect certain states of an ecosystem.The north western Black Sea shelf, for example, experienced severe changes in the ecosystem structure and functioning during the past decades; suffering from pollution, eutrophication and overfishing from the 1960s to the end of 1980s. After the collapse of the centrally planned economies in the beginning of the 1990s in eastern European countries, the ecosystem now starts to respond to the decrease in anthropogenic pressures.We present here a simple nutrient budget for the north-western Black Sea shelf for the end-nineties, and a strategy towards more detailed budgets for the pre-eutrophication period (before 1960s), the eutrophication period (1960 to 1990) and the post-eutrophication period (1990 to recent). The purpose for developing past budgets is to undertake a causative analysis and to identify key moments levels of pressure, ecosystem state changes in the development of the eutrophication problem.The nutrient budget for the pre-eutrophication period is thought to reflect the sources and sinks during the pristine state of the ecosystem as baseline for management decisions. The budget for the eutrophied period should reflect sources, sinks and pathways during the time of ecosystem collapse and provide information on critical nutrient loads. It may also reflect the future if regulations on both sources and quantities of nutrient discharges fail to reduce them to pristine levels. The nutrient budget for the post-eutrophication period will also provide information on timescales until certain components of the system started to respond to decreasing external pressures.The nutrient budget presented for the mid-nineties; the beginning of the post-eutrophication period, is based on field data of river input, atmospheric fluxes, benthic fluxes and sediment studies during the mid 1990s, and on modeling results of primary production, water column mineralization and transport of nutrients from a GHER 3D eddy-resolving coupled hydro-dynamical-biogeochemical model. According to the estimate about 65% of the shelf primary production is remineralised within the euphotic layer. 2% of the material is transported off the shelf. 33% of the primary production reaches the sediment. Benthic nutrient recycling is a significant internal nutrient source for the pelagic system, sustaining high productivity by the release of phosphorus and nitrogen from the sediment in the same range as river inputs. The shelf sediments release about twice as much silicic acid than is discharged by the Danube. However, the shelf acts also as a sink for nutrients. After benthic decomposition 3% of the model primary production remains buried in the sediment. Model estimated atmospheric nutrient deposition seems to be of minor importance as it amounts to only 4-8% of the river inputs.The International Study Group within the Black Sea Ecosystem Recovery Group, jointly with the Eutrophication Task Team of the EU FP6 project ELME (European Lifestyles and Marine Ecosystems), agreed to focus part of their work on detailed nutrient budgets for the different ecosystem states of the north western Black Sea shelf. The outcome of this work will provide valuable information for the International Commission for the Protection of the Danube River and Black Sea Commission in their joint efforts to control eutrophication through adaptive management. The work is a contribution to LOICZ II theme 4: Biogeochemical cycles in coastal and shelf waters.

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