| Summary: | Vertical profiles of temperature, salinity, NO3 and fluorescence were taken along
the Fraser River estuary and during time series at a station near the river mouth to
investigate entrainment of N0₃. In late spring and early summer, the N0₃-poorer
estuarine plume in the Strait of Georgia invaded the river with the advancing salt wedge on the flood tide in the middle layer between the river water and the N0₃-rich deep seawater, forming a three-layered system. Thus, upward entrainment of seawater into the riverine plume does not necessarily result in an upward entrainment of NO₃. More NO₃was entrained during the spring tide than during the neap tide; more during a higher river discharge than during a lower river discharge; and more under windy conditions than weak winds. Under all the conditions investigated, the contribution of the entrained N0₃to the surface layer was (2-11 times) more than that of the river-borne N0₃. The spring bloom was underway in early April, 1991 in the Strait of Georgia
estuary when it was interrupted by a wind event. Five days after the wind event,
phytoplankton biomass and production were even lower. During the next four days, they
gradually increased, and N0₃concentrations in the water column decreased slowly, which
indicated a slow recovery of the spring bloom. Higher zooplankton abundance were
responsible for the slow recovery.
The interaction between tidal ranges, river discharge and winds in the Strait during
spring controls the stability of the water column and hence, the development of the spring bloom. In 1988, nutrients, phytoplankton biomass, production and species composition during early June indicated the delayed spring bloom, due to a later initiation of the annual freshet and strong winds during March-mid-April. Whereas in 1992 (an El Nino year), an earlier initiation of the freshet and calm weather in March appeared to result in an earlier
onset of the spring bloom. The observations in all those years strongly suggest that
massive recruitment of the copepod Neocalanus plumchrus to the surface, due to its ontogenic migration from deep waters, regulates the development of the spring bloom. Thus, the interannual variability in the timing of the spring bloom determines the matching (phasing) between phytoplankton and zooplankton.
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