Structure and photosynthetic properties of phytoplankton assemblages in a highly dynamic system, the Northern Adriatic Sea.

Photosynthetic properties of phytoplankton populations related to physical–chemical variations on small temporal and spatial scales and to phytoplankton size structure and pigment spectra were investigated in the Northern Adriatic Sea off the Po River delta in late winter 1997. Large diatoms (fucoxanthin) dominated the phytoplankton in the coastal area, while small phytoflagellates occurred outside the front, defined by the steep density gradient in the surface layer separating low-salinity coastal waters from the offshore waters. The extreme variability and values of phytoplankton biomass in the innermost plume area was not always reflected in primary production. Modeling of circulation patterns and water mass resilience in the area helped in predicting phytoplankton response and biomass distributions. In the frontal area, despite considerable variability in environmental conditions, our findings showed that the phytoplankton assemblages compensate for nutrient depression and hydrographic constraints, by means of size and taxonomic composition and, consequently, the variability in the photosynthetic capacity was much less pronounced than that observed for other parameters.