MESOSCALE DISTRIBUTION OF ZOOPLANKTON IN THE CALIFORNIA CURRENT IN LATE SPRING, OBSERVED BY OPTICAL PLANKTON COUNTER

A survey of zooplankton in the upper 300 m of the central California C urrent, from the coastal shelf to 128W and between 36.5N and 39.5N, wa s conducted in early June 1993 using an Optical Plankton Counter (OPC) as part of an interdisciplinary study of mesoscale ocean circulation and biological dynamics. The OPC was part of a multi-instrument packag e towed on an undulating vehicle (SeaSoar). Estimates of normalized si ze spectra and absolute abundance from OPC data compared favorably wit h measurements based on Bongo net catches. After processing, the stand ardized OPC data set provides a resolution of approximate to 7 km in t he horizontal and 10 m in the vertical for 60 size categories of zoopl ankton ranging in estimated body weight from 3 mu g C to 3000 mu g C. Results reveal rich mesoscale variability in zooplankton distributions at scales of 30-60 km, with regions of enhanced biomass and abundance coinciding with the location of mesoscale eddies, both cyclonic and a nticyclonic. The central jet of the California Current departed from t he coastline near Cape Mendocino (39.5N), separating a region of gener ally greater zooplankton biomass near the coast from one of lower biom ass to the west. The jet usually contained lower biomass than adjacent eddies. Total zooplankton biomass ranged from <2g C m(-2) to >20 g C m(-2). The two mesoscale features containing the highest biomass, one 200 nm offshore of Cape Mendocino and another 100 nm west of Monterey Bay, were dominated by euphausiids.