Physical and biological processes over a submarine canyon during an upwelling event

Short, shelf-break canyons are shown to have a substantial influence on local water properties and zooplankton distribution. Barkley Canyon (6 km long) off the west coast of Vancouver Island was extensively sampled in July 1997 and found to have water property and current patterns similar to those observed over Astoria Canyon (22 km long) off the coast of Washington State. Results from Barkley Canyon reveal that the canyon influence can occur very close to the surface (at the thermocline depth of 10 m) and that, near the canyon rim, the stretching vorticity generated over the canyon is strong enough to produce a closed cyclonic eddy of sufficient strength to trap deep passively drifting tracers. Most zooplankton species are advected by the currents; those near the ocean surface pass over the canyon, while those at depth are advected toward the coast. Euphausiids (Euphausia pacifica and Thysanoessa spinifera), the strongest swimming zooplankton collected in the 1997 study, were most prevalent in the closed eddy region near the head of the canyon. The observed aggregation of these animals appears to be linked to their ability to remain at specific depths combined with advection by horizontally convergent flows in the eddy.