DOWNSTREAM VARIATIONS OF PHYTOPLANKTON IN THE ST-LAWRENCE-RIVER (QUEBEC, CANADA)

Longitudinal variations of phytoplankton biomass and composition were assessed in a 250 km-long section of the St.Lawrence River, which alte rnately runs through narrow (< 2 km) river cross sections and wide (up to 10 km) fluvial lakes. In the main river stem, concentrations of su spended matter and total phosphorus increased with distance downstream , whereas light penetration decreased. Seasonal changes in plankton co mposition and biomass were more important than those resulting from di fferences in water mass (tributary) of origin. Sampling at three cross river sections and in two fluvial lakes showed a progressive downstre am decrease in phytoplankton biomass and changes in size structure and taxonomic composition. River plankton was primarily composed of small (< 10 mu m equivalent spherical diameter), truly planktonic cells bel onging to Cryptophyceae and diatoms, with Chlorophyceae in summer. Pla nkton sampled in summer among rooted macrophytes in fluvial lakes exhi bited a higher biomass of resuspended periphytic algae than in the mai n river stem, which contributed slightly to downstream phytoplankton b iomass. Successive river cross sections always shared about 50% of the ir taxa, indicating a rapid downstream transport of algae within the m ain water mass. However, the proportion of species common to all cross sections was highest during the spring freshet, and lowest during sum mer low discharge, likely resulting from the development of a distinct flora in fluvial lakes during summer. Conversely, about 30% of the id entified taxa were exclusive to a cross section and were replaced by o thers occurring downstream. Overall, phytoplankton composition along t he St.Lawrence River is primarily controlled by advective forces, whic h result in a homogeneous flora in the main river stem, with a local c ontribution of resuspended periphyton from fluvial lakes.