Influence of the Columbia River plume on cross‐shelf transport of zooplankton

Abstract

A high‐resolution sampling strategy along a cross‐shelf transect near the mouth of the Columbia River was used to determine the distribution, abundance, and spatial relationship between plankton and a tidally modulated river plume. During a strong ebb tide, zooplankton‐sized particles were concentrated on the seaward side of the plume front and transported 15 km across the shelf at a rate of 38 cm s−1, roughly fivefold faster than typical wind‐driven transport in the region. Physical processes associated with the developing plume vertically depressed relatively dense layers of phytoplankton and zooplankton an average of 7 m deeper into the water column beneath the plume and up to 10 km beyond the extent of the plume front. The enhanced cross‐shelf transport by the Columbia River plume is unique for this region of the Northern California Current. Its impact on the vertical and horizontal distribution of zooplankton shifts prey availability for higher trophic levels, such as larval fish, that primarily feed in the upper water column.