Trophic coupling across the St. Lawrence River estuarine transition zone

The objective of this study was to analyze the coupling between trophic levels of the frontal area of the St. Lawrence estuary transition zone, which is the site of an estuarine turbidity maximum (ETM) and is an important nursery area for the juveniles of Atlantic tomcodMicrogadus tomcodand rainbow smeltOsmerus mordax. A detailed series of measurements and sampling were conducted over 6 tidal cycles within the frontal zone. An inverse relationship between the abundance of the 63 μm-net plankton and that of autotrophs indicated the impact of zooplankton grazing on autotrophic biomass, which was largely composed of diatoms. Within the 63 μm-net plankton, nauplii, copepodites and the adults ofEurytemora affinisappeared to be the most important grazers of autotrophs. First-order calculations illustrated that the primary production observed in the ETM is capable of supporting the biomass of this copepod and that its grazing pressure is capable of reducing autotrophic biomass in the brackish waters of the transition zone. Heterotrophic species were a small component (