Biological utilisation of the major nutrients in the western coastal waters of the North Adriatic Sea

The Northern Adriatic basin is one of the areas of the Mediterranean Sea where eutrophic and mucilaginous phenomena have often occurred. As to the causes of these phenomena, the respective importance of nutrient advection, the general marine circulation and the meteorological conditions on seasonal and interannual time-scale is not yet well known. According to a simplified hydrological scheme, the western part of the basin is characterised by relevant inputs of nutrients, due to land runoff and to fresh water advection, mainly from the Po River, and by the southwardflowing North Adriatic Current along the Italian coast. The continental inputs, as a source of nutrients, are characterised by a high variability and by a disequilibrium between the nitrogen and phosphorus supply. On the other hand, the coastal current and the frontal structures between fresh and sea water strongly influence both basin flushing and the ventilation of the bottom layer. It is noteworthy that these two processes work against the accumulation of inorganic nutrients and of dissolved organic matter produced by biological activity. It seems likely that, depending on the coupling of these physical and biological forcings, temporary ecological imbalances can occur determining, firstly, the nature of the food chain, either classical, based on the phytoplankton and inorganic nutrients, or alternatively the microbial loop, based on the bacterial activity and dissolved and particulate organic matter. As a second consideration, the same imbalances can cause, in each chain, a stress on the living organisms which, according to some hypotheses, could lead to undesirable phenomena like quasi-anoxic conditions in the bottom layer and the production of mucous aggregates. In the context of these considerations, knowledge of the distributions of the inorganic and organic nutrients, their biological pathways and budgets, passing from the fresh waters to the coastal and thereafter to the intermediate offshore marine environment, is fundamental to the understanding of the fate of such nutrients in the Northern Adriatic basin. The aim of the presented work is to show some preliminary results on the distribution and on the calculation of the budgets of silicate, dissolved inorganic and organic nitrogen and phosphorus during the four cruises of the PRISMA 2 "Biogeochemical Cycles" Research Project (June 1996 - February 1998). The sampling stations were located, in each cruise, on transects crossing the frontal system delimiting the coastal zone directly influenced by the freshwater inputs, and the intermediate offshore zone, which is less influenced by these fresh water sources. This has permitted the estimation of the inorganic and organic nutrient budgets in both these environments in early spring and summer during different phases of the evolution of the coastal front. Some differences in the seasonal biological utilisation of the pools of dissolved inorganic and organic nutrients have also been evidenced.