EFFECT OF A FRESH-WATER PULSE ON MESOSCALE CIRCULATION AND PHYTOPLANKTON DISTRIBUTION IN THE LOWER ST-LAWRENCE ESTUARY

As pad of a multidisciplinary program to study the physical-biological interactions regulating carbon flows in the lower St. Lawrence Estuar y (LSLE), three cruises were conducted in June-July 1990 during a neap -spring tidal cycle when biological production was expected to be maxi mal. Nutrient (nitrates and silicates), phytoplankton biomass (chlorop hyll), oxygen, temperature, salinity, and current fields were used to elucidate the effect of a freshwater pulse produced by the discharge o f the St. Lawrence and Saguenay rivers on the current fields and the b iological variability and productivity of the LSLE. A simple Rossby ad justment model is presented to explain the temporal (3-5 days) and spa tial (40-50 km) scales of motion in our study region (impact of the fr eshwater pulse on the circulation). Prior to the passage of the pulse during the neap tide, the circulation was dominated by a downstream ou tflow and phytoplankton blooms were limited to areas of weak baroclini c currents downstream and along the south shore. The arrival of the pu lse during the tidal transition led to the intensification of a transv erse current that most likely reduced flushing and allowed phytoplankt on biomass to develop further upstream and toward the north shore. Dur ing the spring tide, lower salinity waters and the bloom spread along the north shore, as the transverse current weakened. Based on these ob servations, a new conceptual model of mesoscale physical-biological in teractions in the LSLE is presented that emphasizes the importance of transverse motions in regulating mesoscale patterns in phytoplankton b looms.