COPEPOD INDICATORS OF SURFACE WATER MOVEMENTS OFF THE OREGON COAST1

Abstract

Two small copepods, Acartia danae and Centropages mcmurrichi, were used as reciprocal indicators of seasonal surface current changes off the Oregon coast in 1961 and 1962. A. danae was found inshore and offshore in winter, indicating the northward flow (with an onshore component) of the Davidson Current. C. mcmurrichi was found inshore in early summer and farther offshore by late summer, indicating a southerly flow (with an offshore component). In the spring and fall, these species occurred together inshore, indicating a transitional state.Temperature‐salinity‐plankton (TSP) diagrams were used to examine the relationships between distributions of the copepods and local processes affecting near‐surface water properties (upwelling, rainfall and runoff, and the Columbia River plume). These diagrams emphasized that the major effect on copepod distribution was the seasonal shift of surface currents, with rainfall and runoff possibly affecting the inshore distribution of A. danae in winter. Upwelling had no apparent effect on the distribution of either species. Columbia River discharge, which flows southerly in summer, probably overwhelmed the general offshore (westerly) movement of surface currents, and hence the offshore movement of C. mcmurrichi, near the mouth of the river. The effect of the Columbia River plume farther out to sea was not shown conclusively to have an effect on the distribution of either species in summer.