Spring bloom in the central Strait of Georgia:interactions of river discharge, winds and grazing

Abstract

A 3 wk cruise was conducted to investigate how the dynamics of nutrients and plankton biomass and production are coupled with the Fraser River discharge and a wind event in the Strait of Georgia estuary (B.C., Canada). The spring bloom was underway in late March and early Apnl, 1991. in the Strait of Georgia estuary. The magnitude of the bloom was greater near the river mouth, indicating an earher onset of the spring bloom there. A week-long wind event (wind speed > 4 m S-') occurred during April 3-10 The spring bloom was interrupted, with phytoplankton biomass and production being reduced and N o 3 in the surface mixing layer increasing at the end of the wind event. Five days after the lvind event (on April 15), NO3 concentrations were lower than they had been at the end of the wind event, Indicating a utilization of NO3 during April 10-14. However, the utilized NO3 did not show up in phytoplankton blomass 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, phytoplankton biomass and production gradually increased, and N o 3 concentrations in the water column decreased slowly, indicating a slow recovery of the spring bloom Zooplankton data indicated that grazing pressure had prevented rapid accumulation of phytoplankton biomass and rapid utilization of NO3 after the wind event and during these 4 d. As a result, NH, was generated at a rate faster than it was utilized by phytoplankton and hence, its concentrations remained at higher levels during April 15-18 than during the wind event. Also, total nitrogen in the water column decreased after the wind event. This study presents the first set of data on daily scales to demonstrate how biological variables are coupled with physical variables in vertical profiles in the Strait of Georgia estuary and to reveal how a wind event affected the spring bloom and consequently the phasing between phytoplankton and zooplankton in the region.