ABSTRACT. Phytoplankton biomass, primary production and bacterial abundance and production were measured across the freshwater-saltwater transition zone of the St. Lawrence River [Canada) during the seasonal period of maximum concentrations of fish larvae and macrozooplankton. The estuarine front was characterized by steep gradients in b~olog~cal as well as physical properties. Maximum turbidity and high phytoplankton biomass occurred in the well-mixed, low salinity (0.2 to 4 psu) region immediately upstream of the salt wedge. Peak zooplankton and ichthyoplankton biomass occurred within and slightly downstream of this frontal region The hypothesis that lower food chain processes were controlled exclusively by allochthonous carbon and bacterial heterotrophy was not supported. Photosynthetic rates per unit chlorophyll a [chl a) remained high across the freshwater-saltwater transition and the low light penetration was offset by a shallow mean depth of mixing. Bacterial concentrations and activity remained relatively constant across the transition, while chl a declined sharply downstream of the front, consistent with grazing losses. Photosynthesis contributed 34 to 66% of the total production (bacteria + phytoplankton). Freshwater phytoplankton advected from upstream contributed another 20 to 30%. These first-order estimates underscore the combined importance of photosynthesis plus bacterial processes within the downstream food web of large river ecosystems.

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 w e b 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 e v~d e n c e 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 are 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 productlv~ty as a larval fish nursery