Physical factors and dissolved reactive silica affect phytoplankton community structure and dynamics in a lowland eutrophic river (Po river, Italy)

We tested the hypothesis that species composition and persistence of phytoplankton communities in nutrient rich lowland rivers depends mainly on physical factors. The study aimed to analyse the effects of water discharge, temperature and chemistry on phytoplankton dynamic and species composition in the lowland reach of the eutrophic Po river (Italy). Both taxonomical and morpho-functional methods were used. True planktonic and tychoplanktic (i.e. detached taxa of benthic origin that remain in suspension) species were found, among which only a few taxa and functional groups prevailed. Diatoms were the most abundant, with a clear dominance of species either sensitive to the onset of water stratification or well adapted to turbid waters. Phytoplankton abundance, biomass and chlorophyll-a followed similar trends, attaining the highest values in summer, at low discharge rates. Correlation and multivariate analysis revealed that the development of a stable phytoplankton community was mainly controlled by water discharge rates. Namely, changes in water flow rates induced major variations in the community structure. The seasonal succession of phytoplankton assemblages was also related to water temperature and dissolved reactive silica availability to some extent overlapping flow effects.