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UBC Theses and Dissertations

Multi-timescale analysis of the salinity and algal biomass of the Fraser River plume from repeated ferry transects Halverson, Mark J.


An instrumented ferry made eight transects per day across the Fraser River plume over the years 2003 - 2006 as part of the STRATOGEM program to study biophysical coupling in the Strait of Georgia. Water temperature, salinity, chlorophyll-a fluorescence, nitrate concentration, and dissolved oxygen were measured. This thesis utilizes salinity and chlorophyll-a fluorescence to study mixing in the plume, and the impact of the plume on algal biomass. First, the effects of river discharge and tides on plume salinity and surface area are quantified. Tidal fluctuations are caused by advection of the estuarine salt field, while fortnightly variations are caused by modulation of mixing in the estuary. Tidal and fortnightly variations are strongest at high river discharge and weakest at low discharge. Plume salinity decreases quasi-linearly with river discharge. Plume surface area increases with river discharge, from about 300 km² at low river flow to about 1,200 km² at high river flow, and can be predicted by scaling the river mouth deformation radius. Second, the plume fresh water flushing time is estimated and a salinity budget is constructed. Fresh water flushing time is 2.2 days, independent of river discharge. The quasi-steady budget predicts a vertical entrainment flux which varies with river discharge. The discharge-dependent vertical entrainment velocities in the estuary and plume implied by the entrainment flux are consistent with other methods. Flow speeds at the edge of the plume estimated from this method are too weak to maintain a plume front, suggesting fronts are transient and created on tidal time-scales. Third, a time series of surface and depth-integrated chlorophyll-a biomass is constructed. Chlorophyll-a fluorometer data are corrected for fluorescence quenching with a parameterization specific to the region, and then calibrated with extracted samples. Instantaneous along-track differences in surface chlorophyll-a can be large, however, averaged over the whole time series, the distribution is nearly uniform. In contrast, depth-integrated values are about 35% lower on average in the plume compared to surrounding waters. Interannual variability in biomass is partly due to the magnitude and duration of the spring bloom, which is itself influenced by wind mixing and grazing.

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