PHOTOSYNTHETIC AND BACTERIAL PRODUCTION GRADIENTS IN A LARVAL FISH NURSERY - THE ST-LAWRENCE-RIVER TRANSITION ZONE

Phytoplankton biomass, primary production and bacterial abundance and production were measured across the freshwater-saltwater transition zo ne of the St. Lawrence River (Canada) during the seasonal period of ma ximum concentrations of fish larvae and macrozooplankton. The estuarin e front was characterized by steep gradients in biological as well as physical properties. Maximum turbidity and high phytoplankton biomass occurred in the well-mixed, low salinity (0.2 to 4 psu) region immedia tely 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 exclu sively by allochthonous carbon and bacterial heterotrophy was not supp orted. Photosynthetic rates per unit chlorophyll a (chl a) remained hi gh across the freshwater-saltwater transition and the low light penetr ation was offset by a shallow mean depth of mixing. Bacterial concentr ations and activity remained relatively constant across the transition , while chi a declined sharply downstream of the front, consistent wit h grazing losses. Photosynthesis contributed 34 to 66% of the total pr oduction (bacteria + phytoplankton). Freshwater phytoplankton advected from upstream contributed another 20 to 30%. These first-order estima tes underscore the combined importance of photosynthesis plus bacteria l processes within the downstream food web of large river ecosystems.