SPATIAL AND TEMPORAL PATTERNS OF ZOOPLANKTON BIOMASS IN MONTEREY BAY,CALIFORNIA, DURING THE 1991-1993 EL-NINO, AND AN ASSESSMENT OF THE SAMPLING DESIGN

To estimate spatial and temporal zooplankton biomass, and the appropri ateness of the sampling design, eighteen surveys were conducted in Mon terey Bay, California, between November 1991 and August 1993. Vertical rows were taken to estimate zooplankton biomass in six regions of the bay on each survey day. In year 1, when 100-m vertical tows were made , zooplankton biomass peaked in January, March, and August; in year 2, when 50-m vertical tows were made, biomass peaked in April, August, a nd October. Mean zooplankton biomass differed significantly among seas ons for both years, but trends differed between years. In year 1, mean biomass measured in the Davidson and oceanic seasons was significantl y greater than in the upwelling season. In year 2, mean biomass measur ed in the upwelling and oceanic seasons was significantly greater than in the Davidson period. The seasonal trends in zooplankton biomass du ring this study were representative of similar trends for the phytopla nkton cycle in Monterey Bay, which had a spring and an autumn bloom an d decreased biomass in winter. Low zooplankton levels recorded in Mont erey Bay during February and April 1992 and January and March 1993 wer e probably related to an El Nino-Southern Oscillation warm-water event (ENSO) in 1991-93. The sampling regime adequately revealed large-scal e spatial (tens of km) and temporal (seasonal) differences in zooplank ton biomass, but probably does not adequately describe smaller spatial and shorter temporal processes.