SPRING BLOOM IN THE CENTRAL STRAIT OF GEORGIA - INTERACTIONS OF RIVERDISCHARGE, WINDS AND GRAZING

A 3 wk cruise was conducted to investigate how the dynamics of nutrien ts and plankton biomass and production are coupled with the Fraser Riv er discharge and a wind event in the Strait of Georgia estuary (B.C., Canada). The spring bloom was underway in late March and early April, 1991, in the Strait of Georgia estuary. The magnitude of the bloom was greater near the river mouth, indicating an earlier onset of the spri ng bloom there. A week-long wind event (wind speed >4 m s(-1)) occurre d during April 3-10. The spring bloom was interrupted, with phytoplank ton biomass and production being reduced and NO3 in the surface mixing layer increasing at the end of the wind event. Five days after the wi nd event (on April 15), NO3 concentrations were lower than they had be en at the end of the wind event, indicating a utilization of NO3 durin g April 10-14. However, the utilized NO3 did not show up in phytoplank ton biomass and production, which were lower than they had been at the end (April 9) of the wind event. During the next 4 d, April 15-18, ph ytoplankton biomass and production gradually increased, and NO3 concen trations in the water column decreased slowly, indicating a slow recov ery of the spring bloom. Zooplankton data indicated that grazing press ure had prevented rapid accumulation of phytoplankton biomass and rapi d utilization of NO3 after the wind event and during these 4 d. As a r esult, NH4 was generated at a rate faster than it was utilized by phyt oplankton and hence, its concentrations remained at higher levels duri ng April 15-18 than during the wind event. Also, total nitrogen in the water column decreased alter the wind event. This study presents the first set of data on daily scales to demonstrate how biological variab les are coupled with physical variables in vertical profiles in the St rait of Georgia estuary and to reveal how a wind event affected the sp ring bloom and consequently the phasing between phytoplankton and zoop lankton in the region.