Zooplankton dynamics in a mesoscale eddy-jet system off California

Zooplankton in the central jet of the California Current and an adjacent mesoscale cyclonic eddy centered at 125.1°W, 38.4°N were studied in early July, 1993, using a SeaSoarmounted Optical Plankton Counter. Within 3 d after the 2 d survey of these mesoscale features we completed a MOCNESS transect across the study area. Zooplankton in the rapidly moving (> 40 cm s^-1 near surface) jet were negatively correlated with the vertical distribution of phytoplankton biomass, which displayed strong fluorescence maxima in the upper 200 m. Zooplankton in the recirculating eddy, however, were positively correlated with fluorescence maxima at the pycnocline (ca 50 m) and at 150 m. Euphausiids, dominated by Euphausia pacifica, and the copepod Calanus pacificus accounted for most of the zooplankton in the upper 50 m of the eddy, while the copepod Metridia pacifica dominated the abundance maximum of medium size zooplankton at 150 m. These species were also present in the jet, but male:female ratios of the 2 copepod species differed greatly, suggesting that populations within the jet and the eddy were distinct from one another. Earlier observations of the cyclonic eddy indicate that it departed California coastal waters in April; resident zooplankton populations may have gone through several generations before they reached the position at which we found it in July. Waters of the jet, by contrast, probably departed from the California coastal region in mid-June, so that its populations of zooplankton may have been essentially the same as those advected to our sampling location.