Nutrients and carbon budgets for the Gulf of Lion during the Moogli cruises

Shelf-slope exchanges and budgets of organic and inorganic nutrients are calculated for the Gulf of Lion continental margin in the northwestern Mediterranean. Computations are based on data from three seasonal marine surveys performed in March 1998, June 1998 and January 1999 in the framework of the French Programme National d’Environnement Côtier. A Land-Ocean Interactions in the Coastal Zone type box model is used to calculate the advective exchanges between the shelf and the adjacent open sea and to estimate the budgets of non-conservative elements (DIP, DIN, DOP, DON, DOC, POC, PON, POP). These budgets consider river discharges, urban sewage supply, atmospheric deposition, and fluxes at the water-sediment and shelf-slope interfaces. Uncertainties on the fluxes and budgets are estimated to assess the robustness of the results with respect to the spatial variability of the system. Results indicate that shelf-slope exchanges by mixing predominate with respect to the major input terms (river discharge and sediment release). Budgets for inorganic nutrients, that show a strong concentration gradient between the shelf and the slope waters, are significantly different from zero and indicate an excess of these elements in the shelf water. For all surveys, these surpluses suggest (i) that the whole shelf system is autotrophic (1.6–4.3 × 10³ mol C s^–1) and acts as a sink of CO₂, and (ii) that it is a site of net denitrification (119 mol N s^–1) and acts as a sink of N₂. Average shelf-slope fluxes of dissolved and particulate organic elements generally indicate an export to the open sea. However, the large uncertainty on these fluxes yields budgets for the shelf not significantly different from zero.