High-resolution observations of chlorophyll-a biomass from an instrumented ferry: Influence of the Fraser River plume from 2003 to 2006

An instrumented ferry made eight transects per day across the Fraser River plume over the years 2003–2006 as part of the STRATOGEM program studying biophysical coupling in the Strait of Georgia, British Columbia, Canada. Seawater properties inside and outside the Fraser plume were measured with high spatial and temporal resolution for an extended period. Here the salinity and chlorophyll-a fluorescence records are used to determine how the Fraser River plume affects phytoplankton biomass in the Strait of Georgia over a range of time scales. The fluorescence record is corrected for non-photochemical quenching by comparison of daytime and nighttime samples, and then calibrated in a manner traceable to extracted chlorophyll measurements. Analysis shows that the instantaneous along-track structure of near-surface chlorophyll-a is often sensitive to the presence of the Fraser River plume. However, when averaged over the 4-year time series, the plume has little impact. Depth-integrated chlorophyll-a biomass is then estimated by applying scaling factors derived from vertical profiles. Unlike the near-surface measurements, depth-integrated biomass in the plume is on average only 74% of the amount found in neighboring waters. Potential reasons for this discrepancy are discussed in terms of light attenuation, the vertical distribution of chlorophyll-a, plume fresh water fraction, and flushing time.

• We use an instrumented ferry to study the impact of a river plume on chlorophyll-a.
• In situ fluorescence was corrected for non-photochemical quenching by a novel method.
• Near-surface chlorophyll-a can be sensitive to the presence of the Fraser River plume.
• When seasonally averaged, the plume has little impact on near-surface phytoplankton.
• On average, however, depth-integrated phytoplankton biomass is 74% lower in the plume.