Zooplankton dynamics in a mesoscale eddy-jet system off California

Zooplankton in the central jet of the California Current and an adjacent me soscale cyclonic eddy centered at 125.1 degrees W, 38.4 degrees N were stud ied in early July, 1993, using a SeaSoar-mounted 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 c opepod Calanus pacificus accounted for most of the zooplankton in the upper 50 m of the eddy, while the copepod Metridia pacifica dominated the abunda nce maximum of medium size zooplankton at 150 m. These species were also pr esent in the jet, but male:female ratios of the 2 copepod species differed greatly, suggesting that populations within the jet and the eddy were disti nct from one another. Earlier observations of the cyclonic eddy indicate th at it departed California coastal waters in April; resident zooplankton pop ulations may have gone through several generations before they reached the position at which we found it in July. Waters of the jet, by contrast, prob ably departed from the California coastal region in mid-June, so that its p opulations of zooplankton may have been essentially the same as those advec ted to our sampling location.