Structure and Dynamics of Photosynthetic Picoplankton across the Saltwater Transition Zone of the St. Lawrence River

Changes in the biomass, community structure, and activity of photosynthetic picoplankton (PP) were measured across the turbid, freshwater–saltwater transition zone (TZ) of the St. Lawrence River. The <2 μm fraction of total Chl a dropped from 20–30% in mid-May to 15% in late June. Photosynthesis by the <2.0 μm fraction in May surface waters contributed 6–38% of the total primary production. Picoplankton dropped from 8.4 × 106 (freshwater zone, FZ) to 3.7 × 106 cells∙L−1 (TZ) in mid-May and from 17 × 106 (FZ) to 5.8 × 106 cells∙L−1 (TZ) in late June. Results implied that the picoplankton were subject to increased losses (e.g., by protozoan grazing) in the TZ. Phycoerythrin-rich cyanobacteria (PEC) dominated the PP counts in mid-May, with phycocyanin-rich forms (PCC) contributing 4–13% of the cell concentrations. Six weeks later the PP community had shifted completely to PCC dominance (60–69% of total counts). In late summer, PP contributed 6–56% of the planktonic Chl a in the St. Lawrence River system. There were large variations in the PEC/PCC ratio and the proportion of eukaryotic PP along certain reaches of the river. These observations underscore the spatially and temporally dynamic character of PP communities in the river–estuary environment.