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 mum fraction of total Chl a dropped from 20-30% in mid-May to 15% in late June. Photosynthesis by the <2.0 mum fraction in May surface waters contributed 6-38% of the total primary production. Picoplankton dropped from 8.4 X 10(6) (freshwater zone, FZ) to 3.7 x 10(6)) cells.L-1 (TZ) in mid-May and from 17 x 10(6) (FZ) to 5.8 x 10(6) 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.