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We evaluated 2 contrasting perspectives on the St Lawrence Estuary maxlmum turb~dlty zone (MTZ) the f~ r s t being that ~t is a zone of stress and mortal~ty for the plankton community, the second that it is a biologically productive area with a complex structured food web Two crulses were undertaken under different hydrolog~c reglmes one at the end of the spnng flood penod and the other durlng mean summer flow conditions The same pattern of change in conlmunlty structure of the phytoplankton and protozoa (2 to 200 pm) was observed across the saltwater transition in both cruises there was a shift in the size d~stnbutlon of tava towards dominance by larger cells and an increase In the number of taxa (including endemic taxa conf~ned to a specific salinity range) at the bottom of the transition zone There was no ev~dence of a decline in evenness nor of a shlft towards a heterotrophbased food web that would result flom the mortality of autotrophs advected Into the MTZ These changes ~ m p l y a prolonged residence tlme for the cells in the MTZ and a comb~natlon of imrnigratlon and emigration pathways that favours larger particles and high taxon dlverslty The iesults a re conslstent wlth a 'hydrodynamic entrapment model Our observat~ons do not support the earller vlews that the St Lawrence MTZ IS a reglon of unfavou~able biolog~cal conditions Conversely, the large average cell slze combined w ~ t h a longer cell-retention time in this region may contnbute towards ~ t s product lv~ty as a larval fish nursery K E Y WORDS: Estuary . Phytoplankton . Protozoa . B~ovolume . Turbidity . Community

Size-dependent variations in phytoplankton and protozoan community structure across the St. Lawrence River transition region