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Biogeochemical fluxes in the Mediteranean water-sediment system

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The distribution of several biogases has been studied in nine European Atlantic estuaries. The project will provide a first estimate on atmospheric biogas emission at the continental scale. These data can be included in global databases in order to refine global budgets. The results will be published in international scientific literature.
Estuaries are extremely dynamic systems usually characterized by strong physico-chemical gradients, enhanced biological activity (both heterotrophic and autotrophic) and intense sedimentation and resuspension. Profound changes are observed in the speciation of organic and inorganic compounds in response to these factors, particularly in the European estuaries of the North Atlantic system which are additionally subject to macro-tidal variations. The tidal regime of these estuaries leads to an increased residence time of the fresh water in the estuarine mixing zone and the generation of a turbidity maximum, often with an associated oxygen-deplete zone within which various anaerobic processes may be stimulated. In addition, as a result of supporting the major population centres within their catchments, European estuaries are subject to intense anthropogenic disturbances reflected in elevated loadings of detrital organic matter, nutrients and toxic trace elements. All these features increase the potential for biogenic gas production within estuaries and have been studied during the project.
Summary: Estuaries are characterized by some physical and chemical properties (e.g. strong physico-chemical gradients, high level of suspended matter) and some usual techniques cannot be applied as 'simply' in seawater. Within the project, some analytical techniques have been developed suitable for studying estuarine biogeochemistry. Methodologies were developed for measuring biogases (equilibrator technique), VHOC and volatile metals.
Modeling efforts have been directed towards the application of the model to the estimation of biogases atmospheric fluxes, with special emphasis on nitrogenous compounds (NH3, N2O). Estimates of N2O production are derived from the nitrification processes in the water column. Concerning the flux estimates, it has been found that the water/air exchange coefficient has to take into account both the wind stress and the periodically varying, current induced turbulence. A diagenetic model has also been developed, which reproduces major trends in biogeochemical cycling in sediment and related sediment-water exchange processes along the whole estuarine gradient. It includes the processes involved in organic matter mineralization and related element cycling (oxygen, nitrogen, manganese, iron, and sulfate) and the two major biogases produced in estuaries (carbon dioxide and methane) are explicitly incorporated. The model may serve as a framework for implementing production, consumption and release processes of other biogases within the sediment.

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