FACTORS CONTROLLING TROPHIC CONDITIONS IN THE NORTH-WEST ADRIATIC BASIN - SEASONAL VARIABILITY

The North-Adriatic basin shows typical shallow water mass characterist ics which in a first approach, can be considered independent of the Mi ddle and the Southern basins, being more affected by seasonal temperat ure and salinity variability. Primary production estimates represent t he main quantitative assessments of the trophic conditions of a marine system, resulting from the combined effect of a large number of ocean ographic factors. In this paper the results from three EUROMARGE AS (E EC-MAST II-MTP project) field trips carried out in 1994 an presented a s a contribution to the better understanding of the factors controllin g the trophic balance in the Northern Adriatic basin. These results in clude: depth profiles of salinity, nutrients and chlorophyll a concent rations, oxygen saturation, phytoplankton taxonomy and abundance, esti mated biomass and primary production measurements by the C-14 in-situ incubation method. The field trips were carried out in three seasons ( February, July, September 1994) and the results reported belong to thr ee stations in the northern basin, 5 miles off Ravenna, Cesenatico and Ancona, respectively. As expected, the physical situation of the wate r column was different in the three periods: the water was mixed in Fe bruary and stratified in July and September. Nutrient concentrations w ere higher in winter, whilst the maximum of primary production was mea sured in September. The phytoplankton was composed predominantly of di atoms. The correlations between primary production and salinity reflec t a difference in the factors controlling primary production. During F ebruary and September nutrients coming from rivers play an important r ole, although with a decreasing influence from station 1, nearest to t he Po delta, towards station 3. Depth profiles of nutrient concentrati ons and O-2 saturation measured during summer in the water column sugg est that regeneration of nutrients in the water column down to the bot tom boundary layer must play an important role in the nutrient cycling and dynamics in the basin. (C) 1997 Elsevier Science B.V.