Zooplankton variability on the equator at 140°W during the JGOFS EqPac study

The vertical distributions of zooplankton biomass and community composition were measured on the equator at 140°W during March/April (19 days) and October 1992 (21 days). El Niño conditions prevailed during the March/April time series. The average integrated (200 m) zooplankton (>64 μm) biomass was 32 mM C m−2 in March/April and 41 mM C m−2 in October. The overall cruise means were not significantly different; however during October zooplankton were more aggregated in the upper euphotic zone, there were more copepods >200 μm, and there were more day/night differences in the vertical distribution of zooplankton biomass as a result of vertical migration behavior. Cyclopoid copepods were more abundant during the El Niño conditions of March compared to October. There were no apparent trends in integrated zooplankton biomass during the March/April time series. However, the species composition of the zooplankton community changed towards the end of the time series when meridional currents flowed to the south. Zooplankton biomass during the October cruise increased nearly five-fold with the passage of a tropical instability wave. A lag in the increase of zooplankton compared to nitrate and chlorophyll as well as changes in the zooplankton community composition over the time series reflect the response times of zooplankton to upwelling events.