Characterization of the pelagic ecosystem in surface waters of the northern Gulf of St Lawrence in early summer: The larval redfish Calanus microplankton interaction

In June 1989, the water column along a transect in the north-central Gulf of St Lawrence was thermally stratified (10-14 degrees C at the surface; 0-1 degrees C at 30 m). In the surface layer, nitrate and chlorophyll concentrations were very low; the mean concentration of chlorophyll a > 5 mu m in the subsurface maximum was 0.26 mu g l(-1). Autotrophic and (presumably) heterotrophic flagellates and dinoflagellates were the most abundant microplankton. In this system, redfish (Sebastes spp.) larvae and the planktonic copepod Calanus finmarchicus overwhelmingly dominated the ichthyoplankton and zooplankton, respectively. Redfish larvae, Calanus females and Calanus eggs were most abundant in the surface layer (0-25 m) day and night. Daily specific egg production rates of Calanus, calculated from shipboard incubations of females, approached the predicted maximal level for this species at the ambient temperature of the surface layer, indicating no or little food limitation. The redfish larvae were feeding almost exclusively on the Calanus eggs' and were found in greatest abundance along the transect where Calanus egg production rates (eggs m(-2) day(-1)), calculated from the product of the specific egg production rate and female concentration, were highest. The monospecificity of the larval redfish diet and the co-dominance with Calanus Finmarchicus are consistent with the findings of previous research in regions of the North Atlantic, including the Irminger Sea and Flemish Cap. Contrary to predictions of the traditional view of the relationship between C. finmarchicus and phytoplankton blooms, however, Calanus was not dependent on high phytoplankton concentrations for egg production in this region, at least at this time of year. These observations lay the foundation for characterization of the surface layer of the north-central Gulf in late lune as a summer, post-bloom environment in which production of autotrophic and heterotrophic microplankton supports the Calanus-larval redfish interaction.