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 sub stantial impact on the fluxes of nutrients and on the production and e xport of carbon. As part of a multidisciplinary programme to evaluate the biological impact of cold-core eddies in the lower St Lawrence Est uary, six cruises were conducted between mid-June and mid-July 1989, d uring the period of the summer phytoplankton bloom. A description of t he temperature (T), salinity (S), phytoplankton biomass (Chl), nutrien t (NO3-), and primary production data for the surface layer (top 30 m) gathered during these cruises is presented along with a correlation a nalysis and a spatial classification of T-S and Chi-NO3- vertical prof iles. The results show considerable variability and complexity in the physical-biological structure over this 1-month period. One-dimensiona l measures of physical structure, e.g. vertical stratification, could not account for the biological variability. Two types of coherent meso scale physical-biological structures, unrelated to the semi-diurnal ti de, were found: one with features oriented mostly along-shore and anot her 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-biologica l structure fluctuated every few days, faster than the biweekly spring -neap tidal cycle, and changes in biological production were not relat ed to spring-neap transitions as expected. The physical processes resp onsible for these variations in distribution and production will be th e subject of further study. (C) 1995 Academic Press Limited