Aggregation of the Copepod Neocalanus Plumchrus at the Margin of the Fraser River Plume in the Strait of Georgia
The springtime spatial distribution of the copepod Neocalanus plumchrus in the Strait of Georgia is extremely heterogeneous. Very high abundances (>10 animals·liter−1 have frequently been observed in the surface layer along the outer margin of the Fraser River plume. We use an electronic particle counter to examine these distributions in detail. Surface layer copcpod concentrations vary over three to four orders of magnitude. Lowest concentrations occur within the core of the plume, highest concentrations at its outer margin, and intermediate concentrations in the more saline water surrounding the plume. Although the salinity range characteristic of the plume margin had higher zooplankton density at all times within our sampling interval, by far the highest densities occurred when and where the plume margin was expanding outward from the river mouth. Physical factors associated with this expansion include high river discharge rates, southeast winds, and flooding tidal currents. Abundance changes in the patches are far too rapid (factors of 10 or more over time intervals of about a day or two) to be caused by growth and reproduction of the copepod population. We believe that the dominant process is behavioral interaction with velocity gradients at the plume margin; an advancing plume edge collects and retains animals from a large area.
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Document Type: Research Article
Publication date: 1988-11-01
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