Determinination of the 3-Dimensional structure of the marine coastal front and quantitative estimation of nutrient budgets in a seawater volume

The northern Adriatic Sea is one of the more productive areas of the Mediterranean Sea, where eutrophic and mucilaginous phenomena have also occurred. Although the causes of some of these phenomena are not yet well known, it is commonly accepted that the strong freshwater inputs, and their nutrient contribution, play an important role in both the general behaviour and the eutrophic phenomena of this ecosystem. The main freshwater input comes, in the western part of the basin, from the Po River and it generates a coastal front which separates the lower salinity mixed waters from the offshore higher salinity ones, which are less influenced by the freshwater advection. The well known high variability of this coastal front depends not only by the amount of freshwater inputs but also by the meteorological conditions and hydrological circulation. This latter is characterised by (a) the West Adriatic Current, stronger in winter and flowing southwards along the Italian coast and by (b) the seasonal alternation of horizontal and vertical thermoaline patterns. All these factors influence diffusion, availability and fate of the nutrients in the basin, and their role in the tropic chain of this ecosystem. From this point of view, a detailed description of this area of the western North Adriatic Sea can provide a useful and profitable insight into the global phenomenology of this basin. This paper presents a method for (a) a description of the physical structure of the coastal front, through the reconstruction of the salinity 3-Dimensional field, and (b) a quantitative estimation of the major nutrients and of the inorganic nitrogen uptake in the inner portion of the front. The method was applied in an area South of the Po River Delta, using the CTD data acquired by means of the undulating tow-fish "SARAGO" and bottle samples collected few days later. These data were collected during the cruise of February 1997 of the PRISMA 2 "Biogeochemical Cycles" Research Project and they were calibrated and elaborated in an appropriate way. A total seawater volume of 27.42 Km3 was considered, whereas the coastal portion of the front, defined by salinity values ranging from 29.00 to 37.00, had a volume of 12.00 Km3. The comparison between the nutrient budgets, which represent the quantity of matter made available for the ecosystem, and the inorganic nitrogen uptake budget, which basically represents the inorganic nitrogen demand of the phytoplankton community due to primary production, defines the role of freshwater inputs and of the biological processes in regulating this coastal marine environment. In these lower salinity mixed waters we estimated 1417 t of dissolved inorganic nitrogen (DIN) and 8 t of dissolved inorganic phosphorus (DIP). The amount of dissolved organic nitrogen (DON) and phosphorus (DOP), 1226 and 31 t respectively, represent important fractions of these elements in the marine environment. In the same period, a inorganic nitrogen demand of 34 t·d-1 was calculated for primary production.