Résumé des résultats (Abstract)
(Anglais)
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The Mediterranean Targeted Project MATER was a complex programme which involved 54 partners from European and non-EAA countries. Its aim was to provide a better understanding of the functioning of the Mediterranean Ecosystems through interdisciplinary studies. It considered a large range of time and space scales, from the biotic and abiotic processes responsible for the transformation of the suspended matter to the seasonal and interannual variability of the general circulation. In that programme, Limnoceane was responsible for the Work 210 'SPM characteristics and dynamic in the Alboran sea'. Our approach was focused on studies of the distribution of the suspended matter, and on its characterisation by optical methods. A detailed study of the optical response of several types of suspended matter has been performed in the lab, and served as a basis for conceiving a new, high sensibility optical device. That instrument was developed and tested during two cruises in the western basin, during which it demonstrated its ability to resolve the tiny gradients of suspended matter encountered in the clear deep water O(1mg/m3). The patchiness of the SPM and the associated Chla distribution was evaluated by 3D visualizations from results of a large scale cruise in the western basin. They confirmed that patchiness is highly dependant from local sources and processes (biological production, resuspension on the shelf or erosion from the slope, rivers and urban areas input). Persistent upwelling of deep waters along the coast and seemingly the formation and downwelling of small water volumes from the most productive areas are clearly identified by these optical data. Fluorescence and turbidity act thus as natural markers of both the productive areas and the downwelled waters. The optical properties were also a key parameter in the study of processes in mesoscale eddies of the open sea, where the surface productivity is enhanced, respectively limited by the vertical exchanges. Two different systems have been compared i.e., the oligotrophic region of the Algerian basin and the anoxic system of the northern Black Sea respectively. While in both cases, the influence of the eddies is limited to the upper part of the water column, the concentration of Chla as well as the depth of its peaks are controlled by the general circulation: the Chla concentration is enhanced at the edge of the anticyclonic eddies and depleted in their centre. This is interpreted as a consequence of the vertical circulation which promotes the transfer of nutrients from the subsurface water, leading to an enrichment of the euphotic layer at the edge of such eddies.
Two other applications of the optical methods were carried out during the programme:
a) Located along the eastern slope of the Sicily channel, erosion areas are likely due to strong currents associated with the input of intermediate water from the eastern basin.
b) In the western basin, hydrological data acquired in 1999 evidenced a peculiar dense water formation that occurred in February. Turbidity, as well as potential temperature/salinity diagrams exhibit significant increases in the vicinity of the sea floor, which were not found during previous years of observations (1993-1998 period). Maximum oxygen concentration at the bottom evidenced the arrival of a newly formed deep water. The spreading speed of that deep water at basin scale was estimated to be 0(3 cm/s). Current observations made in the canyon Rech Lacaze-Duthiers in February 1999 support the hypothesis of formation of dense water on the shelf, together with the classical open sea process around 42~ and 50E. The main reason was a rather long episode of very strong wind from 26 January to 23 February 1999. These data may thus be the signature of strong dense water formations on the shelf of the Gulf of Lions.
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