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 tomcod Microgadus tomcod and rainbow smelt Osmerus 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 gm-net plankton and that of autotrophs indicated the impact of zooplankton grazing on autotrophic biomass, which was largely composed of diatoms. Within the 63 mum-net plankton, nauplii, copepodites and the adults of Eurytemora affinis appeared 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 (<20%) of total microplankton in the freshwater samples, but dominated the total community biomass at higher salinities. This shift towards heterotrophic dominance implies a spatial coupling between upstream autotrophic production and downstream consumer processes. Analysis of stomach contents showed that the calanoid copepod E. affinis was the primary food source for larval fishes and mysids, although the combined ingestion rates of the 2 fish species are unlikely to have any impact on copepod standing stocks. By far the most important predators of the zooplankton are Neomysis americana and Mysis stenolepis. Estuarine circulation and associated entrapment processes ultimately control the trophic relationships and gradients in community structure within the ETM.