BIOGEOCHEMICAL PROCESSES IN THE WATER COLUMN IN THE WESTERN ADRIATIC SEA (EUROMARGE-AS PROJECT)  <B>BIOGEOCHEMICAL PROCESSES IN THE WATER COLUMN IN THE WESTERN ADRIATIC SEA (EUROMARGE-AS PROJECT) </B> <HR><P> Miserocchi</B><SUP>1</SUP><B>, S., Mowbray</B><SUP>2</SUP><B>, S., Statham</B><SUP>3</SUP><B>, P., Tankere</B><SUP>3</SUP><B>, S., Balboni</B><SUP>1</SUP><B>, V., Langone</B><SUP>1</SUP><B>, L., Heussner</B><SUP>4</SUP><B>, S., Monaco</B><SUP>4</SUP><B>, A., Kerhervé</B><SUP>4</SUP><B>, P., Carbonne</B><SUP>4</SUP><B>, J., Charriere</B><SUP>4</SUP><B>, B., Delsaut</B><SUP>4</SUP><B>, N., Nyffeler</B><SUP>5</SUP><B>, F., Godet</B><SUP>5</SUP><B>, C. H. & Faganeli </B><SUP>6</SUP><B>, J. </B><P><P> 1Istituto di Geologia Marina del CNR, Bologna, Italy<BR> 2Department of Geology and Geophysics, University of Edinburgh, UK<BR> 3Department of Oceanography, University of Southampton, UK<BR> 4LSGM CNRS, Université de Perpignan, F<BR> 5Limnoceane, Neuchâtel, CH <BR> 6Marine Biological Station, Piran, Slovenia<B><P><P> Introduction</B><P><P> One of the main objectives of the MAST II EUROMARGE-AS project is to assess the seasonal and interannual variability of biogeochemical exchanges between river outfalls, the western continental margin and the mid and southern basin of the Adriatic Sea. An attempt has been made here to integrate and interpret results from three key areas of research: <P><P> *seasonal variations of dissolved trace metals in the water column; <BR> seasonal variability in the composition and quantity of suspended particulate material; <BR> *seasonal variations of downward particulate fluxes in the deep southern pit of Adriatic Sea. <BR> Shelf waters receive dissolved and particulate inputs from rivers in which trace metal concentrations are generally higher than in coastal sea water. Other factors such as benthic remobilisation or eolian inputs may also be important in determining the distribution of trace metals in shelf waters. The impact on biota as well as the flux to the ocean of these inputs may be controlled by their fate during estuarine mixing. Interactions between dissolved and particulate phases play a major role in the cycling and fate of these elements in a shelf environment. The ocean margin is important in trapping continental detritus and reducing their transport to the deep Adriatic Sea. Recent studies suggest that only about 5-10% or less than 25-30% of the suspended sediment delivered by rivers escapes the ocean margin and as a consequence 75-95% of the trace element flux associated with riverine transport is removed and deposited in the ocean margin.<P><P> Suspended particles that escape the ocean margin are deposited in the deep basin together with particles formed in the overlying water column. The composition of the falling particles reflects their origin. <P><P> A consequence of the settling and deposition of suspended sediment from the water column is the enhancement of primary productivity which is normally light limited in estuarine and near-shore waters. As the primary productivity is enhanced, dissolved trace metals are removed; it has been shown that Fe, Mn, Zn and Ni can be complexed by marine plankton. <B><P><P> Materials & Methods</B><P><P> Samples were collected in three stations in the NW Adriatic shelf (Fig. 1) during six cruises in the period from August 1993 to April 1995.<P><P> Suspended particulate material (SPM) was filtered using pre-weighted Nuclepore membranes, major components were measured using thin-film X-ray fluorescence spectrometry (XRF), whilst trace components were measured by atomic absorption Spectrometry (AAS) on duplicate filters. Techniques for the clean collection and analysis of samples for dissolved trace metals are fully described in Statham<I> et al. </I>(1993).<P><P> Two sediment traps (PPS3 Technicap; 0.125 m<SUP>2</SUP> collecting area; 12 receiving cups) and two Aanderaa RCM7 current meters were deployed in the south Adriatic pit at 1030 m depth. The first trap was located at 35 m above bottom and the second at 500 m. The first deployment lasted six months and concerned only the near-bottom trap. The sampling interval was set at 2 weeks. Samples were processed according to the procedure described by Heussner <I>et al.</I> (1990).<B><P><P> Results & Discussion</B><I><P><P> Dissolved and particulate trace elements</I><BR> As interactions of dissolved metals with particles are important in their geochemical cycles (Morel <I>et al,</I> 1991), and both dissolved and particulate trace metal data exist here, the distribution coefficients (Kd) can be calculated and these values will help in examining the fate of trace metals introduced into to the coastal environment. The distribution coefficient (Kd) between dissolved and particulate trace metals may be defined as: <P><P> Kd = (P x 10<SUP>6</SUP>)/CD <P><P> where P is the particulate metal concentration (mg kg<SUP>-</SUP><SUP>1</SUP>) and CD is the dissolved metal concentration (ng kg<SUP>-</SUP><SUP>1</SUP>) (Duursma <I>et al</I>, 1986). <P><P> In the NW Adriatic, the metal content of particles is strongly influenced by the source of the material, especially in surface waters near the Po River outfall and in bottom waters where strong resuspension of sediments is common. Particulate loadings are also influenced by seasonality effects whereby summer months are characterised by a stable, well developed thermocline and low particulate loadings, at other times of the year loadings can be 3 orders of magnitude higher especially during storm periods when sediments are resuspended throughout the entire water-column. <P><P> Particle reactivity varies between elements with Pb and Mn among the most strongly reactive and Cd as relatively non reactive, Cu and Ni are of intermediate reactivity. Example of Kds for these elements from samples collected in August 1993 are reported in tab. 1. <P><P> Kds tend to be higher for Ni than Cu suggesting that Ni is more particulate reactive than Cu. For both metals, Kds are higher in bottom waters than in surface waters showing that particulate material inputs from resuspension are more rich in trace elements than the particulate material coming from rivers or atmospheric inputs. <I><P><P> Downward particle fluxes</I><BR> The results on particle fluxes presented here only pertain to the first deployment (April to August 1994). Total mass fluxes, at 35 m above bottom, were characterised by a high temporal variability, ranging from 18.6 mg m<SUP>-</SUP><SUP>2</SUP> d<SUP>-</SUP><SUP>1</SUP> to 1577 mg m<SUP>-</SUP><SUP>2</SUP> d<SUP>-</SUP><SUP>1</SUP>. Based on the temporal variations of both fluxes and contents (Fig. 2), two phases can be distinguished : phase I from April 1 to June 15 (samples A1 to A5), and phase II from June 16 to August 31 (samples A6 to A10). Phase I was characterised by slightly higher contents in organic carbon and lower contents in carbonate. This latter constituent significantly increased during phase II and reached its maximum value (25.6%) during the period of maximum mass flux (A8, 16-31 July), whereas organic carbon came to a minimum value of 3.7% (A7, 1-15 July). The carbonates could be related to coccolithophorids and sometimes to foraminifera in the second phase when carbonate were predominant. Silicate particles and coccolithophorids were often embedded in the mucus strands and the fibrillar network of diatoms. The later showed a bad preservation probably due to dissolution during a sedimentary stage (reworking). <P><P> Delta <SUP>1</SUP><SUP>3</SUP>C analyses of settling POM revealed rather high values in the first phase (between -23.09o/oo and -22.07o/oo), and lower values (between -24.05o/oo and -24.6o/oo) during the second phase. Such values, close to those observed for sedimentary organic matter, most probably reflected the marine (autochthonous) or terrigenous (allochthonous) origin rather than temperature-induced differences in isotopic fractionation by plankton or differences in plankton species composition.<BR> Carbohydrate content of settling particles further helped to characterise the origin of the material. The monosaccharide composition of these samples was characterised by a relatively homogenous distribution. Glucose and galactose generally represented the two major monosaccharides, and samples from phase I presented higher glucose contents than those from phase II. Sample A5 (1-15 June) exhibited a slightly different composition and was characterised by the highest total carbohydrate content. This sample, who presented the largest fraction of mucus, was also largely dominated by glucose, a result which is not surprising since phytoplanktonic mucilage is characterised by a high glucose content. Nevertheless, and even if the mucus collected from sample A5 presents a monosaccharide composition close to that of the reminder of the sample, that are some differences which indicates that there are other sources of carbohydrate (e.g., compared to the mucus, total sample A5 has a higher content in xylose and mannose).<P><P> Three main features can be drawn from these preliminary flux results:<P><P> *Fresh organic matter produced in surface layers is exported to depth in a stratified water column; there is, nevertheless, a shift within the organic sources feeding the deeper zone of the Mid-Adriatic trough. The first period of the experiment (from April to mid-June) was characterised by an input of "fresh" organic material mostly composed by siliceous phytoplankton. The second period (Mid-June to end of August) was, on the contrary, characterised by a more degraded and abundant material originating from calcareous phytoplankton and terrigenous remains.<BR> *The material collected by the traps was characterised by the presence, in all ten samples, of a low density material of biological origin, resembling mucus or mucilage; it is not clear, for now, if this material directly relates to the mucilage produced in the northern part of the Adriatic or if it represents a more local production.<BR> *The large terrigenous input observed in July, probably related to a resuspension mechanism on the northern shelf or slope, shows that advective mechanisms occasionally feed the deep basin, as on other continental margins.<B><P><P> References </B><P><P> Duursma E. K. and J. M.Bewers (1986) Applications of Kd's in marine geochemistry and environmental assesment. Scientific seminar on the application of distribution coefficients to radiological assessment models. Elsevier Appl. Sci. publ. London, 138-165. <BR> Heussner S., C. Ratti & J. Carbonne (1990) The PPS3 time-series sediment trap and the trap sample processing techniques used during the ECOMARGE experiment.Cont. Shelf Res., 10, 9-11, 943- 958.<BR> Morel F.M.M., D.A. Dzombak and N.M. Price (1991) Heterogenous Reactions in Coastal Waters. in: Ocean Margin Processes in Global Change. Eds: R.F.C. Mantoura, J-M Martin, R. Wollast, John Wiley & Sons, England. <BR> Statham P.J., Y. Auger, J.D. Burton, P. Choisy, J.C. Fischer, R.H. James, N.H. Morley B. Ouddane, E. Puskaric and M. Wartel (1993) Fluxes of Cd, Co, Cu, Fe, Mn, Ni, Pb, and Zn through the Strait of Dover into the Southern North Sea. Oceanologica Acta 16, 541-552. <P><P>