Mesoscale physical—biological variability during a summer phytoplankton bloom in the lower St Lawrence Estuary

Wide estuaries can produce and sustain eddies that can then have a substantial impact on the fluxes of nutrients and on the production and export of carbon. As part of a multidisciplinary programme to evaluate the biological impact of cold-core eddies in the lower St Lawrence Estuary, six cruises were conducted between mid-June and mid-July 1989, during the period of the summer phytoplankton bloom. A description of the temperature (T), salinity (S), phytoplankton biomass (Chl), nutrient (NO3−), and primary production data for the surface layer (top 30 m) gathered during these cruises is presented along with a correlation analysis and a spatial classification of T-S and Chl-NO3− vertical profiles. The results show considerable variability and complexity in the physical-biological structure over this 1-month period. One-dimensional measures of physical structure, e.g. vertical stratification, could not account for the biological variability. Two types of coherent mesoscale physical-biological structures, unrelated to the semi-diurnal tide, were found: one with features oriented mostly along-shore and another displaying a cross-shore thermohaline front. The initiation of the summer phytoplankton bloom was associated with the development of the cross-shore front. The three-dimensional mesoscale physical-biological structure fluctuated every few days, faster than the biweekly spring-neap tidal cycle, and changes in biological production were not related to spring-neap transitions as expected. The physical processes responsible for these variations in distribution and production will be the subject of further study.