WATER MASSES PROPERTIES AND FLUX OF SUSPENDED MATTER IN TWO REPRESENTATIVE SITES OF THE EUROMARGE PROGRAM  <B>WATER MASSES PROPERTIES AND FLUX OF SUSPENDED MATTER IN TWO REPRESENTATIVE SITES OF THE EUROMARGE PROGRAM </B> <HR><P> Nyffeler*, F., Godet*, Ch-H., Casamor**, J. L & Durrieu de Madron***, X. <BR> * LIMNOCEANE / University of Neuchâtel /Switzerland<BR> ** UB / University of Barcelona / Spain<BR> *** LSGM / University of Perpignan / France</B><B><P><P> Introduction</B><P><P> Several cruises of the EUROMARGE program focused on the distribution the suspended matter and its geochemical composition, in relation with the hydrodynamics and the distribution of the water masses. Such surveys were intended to study typical hydrographic seasonal situations as a complement to time series of the particle fluxes obtained by means of sediment traps. The general strategy of the cruises was to cover the areas of investigation by a series of sections, defining thus a network of stations selected on the basis of pre-existing knowledge. A special instrumentation package was designed by Limnoceane for the purpose of linking the physical data and the optical parameters characterising the suspended matter. A CTD probe (ICTD Falmouth Instrument) measuring the usual hydrographic parameters (C, T, P), the dissolved oxygen with a Beckmann polarographic sensor and the pH with standard electrode (Innovative Sensors) has been complemented with optical sensors mounted under the carrousel carrying Niskin samplers for the geochemical analysis. Three attached devices provided companion continuous profiles of the optical parameters, as well as the altitude above the bottom. This combined package allowed thus to extend the measurements down to the last meter of the water column in order to identify the resuspension events which are known to occur on the continental margins. <P><P> Light scattering measurements were used to locate the nepheloid layers in the water column. A semi-quantitative relationship was established between scattering and suspended particulate matter (SPM) concentration by Durrieu de Madron (1994).The conversion of optical data in terms of suspended load is not straightforward as light scattering depends on the nature of the suspended matter. However, in a limited geographic area, one can reasonably assume that similar types of nepheloid layers contains the same type of suspended material. The scattered intensities provides thus an index of the relative particle content in these layers. The real time display on board of all the measured parameters during the casts was used as a basis to drive the sampling for the subsequent geochemical analysis. <P><P> Both forward and backward scattering were measured simultaneously, respectively with a DIP nephelometer (Vangriesheim <I>et al</I>., 1994) and with a Hardt Backscattmeter. In the DIP nephelometer, both direct light and scattered light are continuously compared , which allows to correct the forward scattering measurements from any drift of the light source and pure absorption by the water. Such an absorption (which may result from coloured dissolved matter) can also be evaluated at first order as long as the drift in the light source remains negligible and provides a third optical parameter. The corrected forward scattering is expressed as turbidity in FTU unit (Formazine Turbidity), according to the material used for the nephelometer calibration. As the particle scattering function strongly depends on the optical characteristics of the suspended matter, (shape, material an index of refraction), the comparison between absorption, forward and backward light scattering signals is expected to allow distinguishing between various types of suspended material. This comparison is enhanced by a fourth optical parameter, the Chl-a, measured by fluorescence on a second channel of the Hardt instrument, which indicate the occurrence of biogenic material. <I><P><P> The Cruises</I><BR> Since 1986, several continental margins have been surveyed applying the same strategy (Gulf of Biscaye, Gulf of Lions, Aegean Sea, Adriatic Sea, Black Sea). The results show significant differences in the transport of SPM and resuspension, depending of the type of margin. During the last two years, special emphasis was put on the Northern Balearic and on the Adriatic Sea areas (Fig. 1). Two cruises have been undertaken in the framework of the EUROMARGE program in the Northern Balearic area in summer 1994 and 1995, and one cruise was carried out in summer 1994 within the framework of Adriatic component of the program. We shall illustrate the main outcome of these cruises, while keeping in mind that the data sets are still under interpretation. <B><P><P> Some Key Results</B><I><P><P> North Balearic Basin</I><BR> Being influenced by a series of biogeochemical processes at small scale, the optical properties are obviously less homogeneous than the quasi-steady state of the hydrographic characteristics resulting from long lasting mixing processes. In both areas, the optical parameters show strong horizontal and verticals gradients in the surface and subsurface layers, which make their mapping and quantitative interpretation not so straightforward. Basically however, all these optical parameters display general trends which can be related to the dynamic features. The cruises results confirmed and complemented the main features observed during previous cruises in the Gulf of Lions margin. The variability of the nepheloid structure is strong near the shelf break and the Northern Current limits its extension towards the open sea. <P><P> The comparison between the various optical parameters provides some clue to the respective contribution of the organic and inorganic content to the built up of the nepheloid layers. A more precise characterisation of the suspended material is also expected on the basis of the comparison between the various optical parameters. Advective suspended particulate matter and organic carbon transport was estimated across adjacent regions in the northern Balearic basin.<I><P><P> Adriatic Sea</I><BR> The relative organic and inorganic contribution to the nepheloid structure and their relation with the hydrographic parameters are still more obvious in the Adriatic basin. Significant gradients in the physical properties are induced by the discharges of fresh water by the Po River in the North-West of the basin. The associated nutrients input favours the planktonic development as confirmed by the elevated Chla content (up to 3 microgram(s)/l) in the surface layer. That biogenic component significantly adds to the high turbidity level induced by the Po discharge. Located in the surface in the north, the Chla maximum decreases and moves downwards in the water column while going to south where it stabilises around 50 meters depth. In the southern deep Adriatic basin, the comparison of light scattering and fluorescence clearly allows to distinguish the organic from the inorganic nepheloid layers. Elevated turbidity is also observed near the bottom in the north of the mid-Adriatic trough and is believed to be due to the resuspension of sediment by the internal mid-Adriatic gyre. The pattern of the optical properties complement thus the mapping of the physical parameters and confirm a significant transport southwards of the material discharged or produced in the northern area of the Adriatic basin. While our data set does not highlight any obvious transfer from the shallow to the deep water, one has to keep in mind that such a survey provides an instantaneous view of the spatial SPM distribution. It is not expected to identify special rare events, the identification of which is provided by the time series needed to evaluate the temporal component of fluxes by sediment traps measurements. <B><P><P> References</B><P><P> Durrieu de Madron X. (1995) Calibration du néphélomètre intégrateur profond en terme de charge particulaire. Technical Report 95/1, Université de Perpignan, pp 19<BR> Vangriesheim A., J.P. Gouillou and L. Prieur (1992) A deep-ocean nephelometer to detect bottom and intermediate nepheloid layers. Deep-Sea Research, 39, 1403-1416.<P><P>