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. Lawrence (1992–1994, Canada) indicate that hydrographic conditions were in close agreement with the physical oceanographic climatology of the Gulf, and that winds during the sampling period were relatively weaker than their corresponding monthly means. Using temperature and salinity profiles with meteorological conditions averaged over 4 days prior to each sampling date, several physical parameters were computed to characterize the hydrodynamic conditions in the euphotic zone. Phytoplankton chlorophyll a biomass (Chla) averaged over the euphotic zone was examined, both total and size-fractionated (>5 and <5 μm), in combination with nitrate data. The seasonal pattern in total phytoplankton reflected changes in the large size fraction, the concentration of phytoplankton <5 μm being low throughout the year. A modified buoyancy length scale (BL*) was derived from wind-induced turbulence and upper-layer stratification; BL* co-varied with the depth of the surface mixed layer. When considering the proportion of the small size fraction ([Chla]small/[Chla]total), maximum Chla biomass corresponded to intermediate values of BL* (∼0.1 m), with phytoplankton concentrations decreasing in the two ends of the buoyancy length spectrum.