Tidal Mixing and Summer Plankton Distributions in Hecate Strait, British Columbia

Abstract

Summer plankton distributions in Hecate Strait, northern British Columbia, were examined during 1979 and 1980 and compared with tidal mixing characteristics. Calculations of the Simpson–Hunter tidal stratification parameter (h∙U−3) indicate that most of Hecate Strait is potentially stratified in summer (in the absence of excessive storms), with a well-mixed region over the shallow shelf adjacent to the Queen Charlotte Islands. Vertical temperature profiles collected from a coastal oil tanker ship of opportunity confirmed the presence of mixed and stratified water masses as predicted. Near-surface concentrations of chlorophyll a, nutrients, diatoms, and copepods were lower in the mixed region (< 1 μg Chl. a∙L−1) than the statified region, but highest (5 μg Chl. a∙L−1) at the front between these mixing regimes. We believe that low phytoplankton biomass in western mixed waters is due to nutrient rather than light limitation, whereas possible advection of nutrients along the edge of the western shelf may support the higher biomass of the frontal region. Such a frontal zone is likely important to groundfish production in this region.