Seasonal variation in abundance and size structure of phytoplankton in Baie des Chaleurs, southwestern Gulf of St. Lawrence, in relation to physical oceanographic conditions

The seasonal development of the phytoplankton community in the Baie des Chaleurs, eastern Canada, was investigated by collecting water samples at 6 different depths (3-24 m) during May-December 1989 at a north-shore site and recording physical conditions. Phytoplankton biomass generally decreased from spring to autumn except for a brief diatom bloom in late October. Picoplankton (less than 5 um) accounted for more than 70 per cent of chlorophyll-a in the water column and dominated particulate organic carbon and nitrogen throughout the year although only 40 per cent of total particulate organic matter was associated with phytoplankton. In summer a strong temperature-driven stratification was observed. A positive correlation between the Brunt-Vailsala water column stability index, particulate matter and phytoplankton abundance, small (less than 5 um) particle abundance and the carbon:nitrogen ratio of small particles indicated dominance by a microbial food web during summer with nutrients regenerated through microbial organisms. The carbon:nitrogen ratios were low and relatively uniform for small particles throughout the study period but more variable for large particles which were abundant only during May-June and late October and probably had a heterogeneous composition. There are 45 references.