EFFECTS OF A FLUCTUATION IN FRASER-RIVER DISCHARGE ON PRIMARY PRODUCTION IN THE CENTRAL STRAIT OF GEORGIA, BRITISH-COLUMBIA, CANADA

High-resolution vertical profiles of salinity, temperature, fluorescen ce, and nutrients NO3 and SiO4) were taken along a transect in the cen tral Strait of Georgia, British Columbia. The Fraser River discharge i ncreased rapidly over 4 days and then decreased over the following 3 d ays (June 16-19, 1991). The thickness and extent of the estuarine plum e increased as a response to the increased river discharge. As the est uarine plume flowed seaward, the nutricline (NO3) became shallower and broader, resulting in an increase in NO3 in the euphotic zone. Entrai nment of NO3 may explain the increase in NO3 in the surface layer, and the amount of NO3 entrained was estimated to be 5-10 times higher tha n river-borne NO3. The utilization of entrained nutrients increased Ch i a concentrations and primary production to levels comparable with sp ring bloom values. Our results clearly demonstrated for the first time that entrainment of nutrients and phytoplankton production in the cen tral Strait of Georgia are closely coupled to fluctuations in the Fras er River discharge as the estuarine plume moves seaward. The timing an d magnitude of the May-June freshet could control the entrainment of n utrients and thus maintain high primary productivity in late spring - early summer.