SIZE-DEPENDENT VARIATIONS IN PHYTOPLANKTON AND PROTOZOAN COMMUNITY STRUCTURE ACROSS THE ST-LAWRENCE-RIVER TRANSITION REGIONS

We evaluated 2 contrasting perspectives on the St. Lawrence Estuary maximum turbidity zone (MTZ): the first being that it is a zone of stress and mortality for the plankton community, the second that it is a biologically productive area with a complex, structured food web. Two cruises were undertaken under different hydrologic regimes, one at the end of the spring flood period and the other during mean summer flow conditions. The same pattern of change in community structure of the phytoplankton and protozoa (2 to 200 mu m) was observed across the saltwater transition in both cruises: there was a shift in the size distribution of taxa towards dominance by larger cells and an increase in the number of taxa (including 'endemic taxa' confined to a specific salinity range) at the bottom of the transition zone. There was no evidence of a decline in evenness, nor of a shift towards a heterotroph-based food web that would result from the mortality of autotrophs advected into the MTZ. These changes imply a prolonged residence time for the cells in the MTZ and a combination of immigration and emigration pathways that favours larger particles and high taxon diversity. The results are consistent with a 'hydrodynamic entrapment' model. Our observations do not support the earlier views that the St. Lawrence MTZ is a region of unfavourable biological conditions. Conversely, the large average cell size combined with a longer cell-retention time in this region may contribute towards its productivity as a larval fish nursery.