Nutrient and plankton dynamics in the Fraser River Plume Strait of Georgia British Columbia Canada

High discharge rates from the Fraser River create a riverine plume front that moves daily and fortnightly with the tides in the Strait of Georgia. During a spring-neap tidal cycle in July 1987, a study of nutrient and plankton dynamics including vertical profiles of temperature, salinity, fluorescence, nutrients (NO3, NH4, PO4, SiO4 and urea), zooplankton, phytoplankton and bacterial biomass and primary and heterotrophic productivities was conducted at 3 stations, one situated in the riverine plume and two at the inner and outer estuarine plume. Primary productivity was highest in the outer part of the estuarine plume and lowest in the riverine plume. Bottom to surface daytime vertical zooplankton hauls revealed no differences in species composition among the 3 stations, but euphausiids were significantly more abundant in the estuarine plume, where phytoplankton abundance was also the highest. During post-neap tides, nitrate was undetectable, a subsurface chlorophyll maximum was present and productivity was high in the estuarine plume. During the post-spring tidal period, nutrient concentrations were elevated, maximum chlorophyll concentrations occurred near the surface and primary productivity increased approaching neap tides. Utilizable nitrogen sources (NO3 + NH4 + urea) and phosphate concentrations in the river were similar to concentrations in the riverine plume, while silicate was significantly higher in the river. Therefore, in late July, nutrient enrichment of the surface waters of the plume, resulting in high primary productivity at the plume boundaries and beyond, appears to be mainly due to entrainment as the freshwater moves over the seawater.