SEASONAL-VARIATION IN ABUNDANCE AND SIZE STRUCTURE OF PHYTOPLANKTON IN BALE DES CHALEURS, SOUTHWESTERN GULF OF ST-LAWRENCE, IN RELATION TO PHYSICAL OCEANOGRAPHIC CONDITIONS

We investigated changes in the abundance and size structure of phytopl ankton and organic seston in relation to temperature, stratification a nd current patterns at Gascons on the north shore of Pale des Chaleurs , eastern Canada. Phytoplankton biomass showed a general decrease duri ng the study (May to November 1989), except for a brief diatom bloom i n late October. During most of the summer, a strong temperature driven stratification was present and <5 mu m cells dominated the phytoplank ton community. Particles measuring <5 mu m also dominated the particul ate organic matter (POC and PON) throughout the year. However, only 40 % of these particles could be associated with phytoplankton cells. For both particulate matter and phytoplankton, the abundance of the <5 mu m size fraction was positively correlated with the Brunt-Vailsala ind ex of stability of the water column. Inorganic nitrogen may have limit ed the phytoplankton growth, as generally reported for stratified envi ronments. Most of the biomass was probably supported by nitrogen regen erated through microbial organisms. A large bacterioplankton community was suggested by the abundance of small (<5 mu m) non-phytoplanktonic particles with a low and relatively uniform C/N ratio. Larger particl es were only abundant at the beginning of the study (May-June) and on one date in October. Their C/N ratios indicated they were of varied or igins.