Influence of wind mixing and upper-layer stratification on phytoplankton biomass in the Gulf of St. Lawrence

Analyses of CTD profiles collected during 9 cruises in the Gulf of St. Lawr ence (1992-1994, Canada) indicate that hydrographic conditions were in clos e agreement with the physical oceanographic climatology of the Gulf, and th at winds during the sampling period were relatively weaker than their corre sponding monthly means. Using temperature and salinity profiles with meteor ological conditions averaged over 4 days prior to each sampling date, sever al physical parameters were computed to characterize the hydrodynamic condi tions in the euphotic zone. Phytoplankton chlorophyll a biomass (Ch1a) aver aged over the euphotic zone was examined, both total and size-fractionated( > 5 and < 5 mu m), in combination with nitrate data. The seasonal pattern in total phytoplankton reflected changes in the large size fraction, the co ncentration of phytoplankton < 5 Irm being low throughout the year. A modif ied buoyancy length scale (BL*) was derived from wind-induced turbulence an d upper-layer stratification; BL* co-varied with the depth of the surface m ixed layer. When considering the proportion of the small size fraction ([Ch 1a](small)/[Ch1a](total)), maximum Chla biomass corresponded to intermediat e values of BL* (similar to 0.1 m), with phytoplankton concentrations decre asing in the two ends of the buoyancy length spectrum. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.