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 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 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 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 chi 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.