THE LATERAL ADVECTION OF PARTICULATE ORGANIC-MATTER FROM THE PO-DELTA REGION DURING SUMMER STRATIFICATION, AND ITS IMPLICATIONS FOR THE NORTHERN ADRIATIC

The impact of Po River-induced eutrophication on the eastern side of the northern Adriatic was studied during four consecutive summer stratified periods with different river discharge characteristics. The phytoplankton size class structure and composition along a trans-Adriatic trophic gradient was assessed (cell density, biovolume, chlorophyll a, species composition), together with associated environmental parameters. The relative contribution of microdetritus to the particulate organic matter was also assessed. Salinity was a better tracer of eutrophication than extant nutrient concentrations because of concurrent utilization and regeneration processes. While the biomass of all phytoplankton size classes increased under eutrophication, the biomass maxima were due to an increased relative contribution by microplankton suggesting that increased regional eutrophication will increase the relative importance of microplankton in the food web. The summer eastward advection of Po waters and particulate organic matter increased the relative proportion of larger sized particles in primary producer populations in the eastern northern Adriatic, and may be restructuring the size class structure of particle feeding secondary producers. The spatial (west to east) and temporal (interannual) gradients in microplankton biomass distribution could be used as an index of the regional Po River eutrophication impact on the northern Adriatic under summer stratified conditions. © 1992 Academic Press Limited.