IMPORTANCE OF WIND AND RIVER DISCHARGE IN INFLUENCING NUTRIENT DYNAMICS AND PHYTOPLANKTON PRODUCTION IN SUMMER IN THE CENTRAL STRAIT OF GEORGIA

A cruise was conducted during August 6-14, 1991 to investigate the dyn amics of nutrients and phytoplankton production in the central Strait of Georgia, British Columbia, Canada, during a period when strong stra tification resulted in nitrogen-limited primary productivity. High res olution vertical profiles of salinity, temperature, fluorescence and n utrients (nitrate and phosphate) were taken daily along a transect. A wind event occurred on August 7 and a rapid increase in the Fraser Riv er discharge took place from August 8 to 14. The wind event mixed the water column and nutrients increased at the same time. Phytoplankton r esponded to the increase in nutrients and a bloom occurred soon after the wind event. The rapid increase in river discharge caused the entra inment of nitrate in the estuarine plume and, as a result, a subsurfac e maximum of chi a was developed. Our results clearly demonstrated tha t summer phytoplankton productivity in the central Strait of Georgia i s fueled by a supply of nutrients from the nitracline through vertical mixing induced by the interaction of winds, river discharge and tidal cycles. Of these 3 factors, winds are the most variable and therefore a summer with frequent wind events could result in higher than normal productivity. The mechanism for this is that part of the nitracline w as maintained above the euphotic zone due to various physical processe s in spite of the strong stratification, and therefore, nutrients were frequently available for phytoplankton uptake caused by across-pycnoc line mixing due to wind, river discharge and tides.