Interaction of currents and vertical migration in maintaining Calanus marshallae in the Oregon upwelling zone—a simulation

Abstract

The zonal distribution of five life stage categories (egg, nauplius, early copepodites CI–III, late copepodites CIV–V, and adult) of the neritic copepod Calanus marshallae observed by Peterson, Miller and Hutchinson ( Deep-Sea Research, 26, 467–494, 1979) during coastal upwelling off Oregon in August 1973 is simulated. The model uses linear formulations to express copepod development and mortality and a time-dependent velocity field predicted by a numerical, upwelling circulation model to carry the copepods. Simulations demonstrate how diel vertical migration by adults can interact with upwelling currents to increase the residence time of egg-laying copepods in the nearshore zone. Diel vertical migration by late stage copepodites may enhance rather than retard their offshore transport if they enter waters sinking below the permanent pycnocline 10 to 20 km from the coast. Predicted longshore trajectories of vertically migrating copepods trapped in the region of intense upwelling show a spiral path over the continental shelf. The extent of longshore transport for copepods seaward of this zone is strongly dependent on the structure in the flow field and on the behavior of the copepod; however, vertical migration always tends to retard longshore transport. using the theoretical flow field, the model hindcasts the shape and location of transects needed to sample migrating and non-migrating copepods as the population moved down the coast.