Latitudinal gradients in zooplankton biomass in the tropical Pacific at 140°W during the JGOFS EqPac study: Effects of El Niٌo

In 1992, as part of the equatorial Pacific study (EqPac) of the U.S. JGOFS Program, we collected zooplankton samples during a set of cruises that crossed the equator in the Pacific Ocean roughly along 140°W from 12°N to 12°S. The first cruise (Survey 1) occurred during El Niño conditions in February-March when anomolously warm surface waters (>28°C) were present over most of the transect. The second cruise (Survey II) was during August-September when surface temperatures had returned to near or below the climatological mean over much of the region, and inorganic nitrogen and chlorophyll concentrations had increased compared with Survey I. Zooplankton biomass was higher in the equatorial region during Survey II compared with Survey I. For both cruises, chlorophyll and zooplankton biomass generally increased in the epipelagic zone (0–200 m) towards the equator. However, whereas primary production and chlorophyll were highest on or near the equator, zooplankton biomass was usually highest several degrees to the north and south of the equator. Zooplankton biomass was reduced below 100 m in newly upwelled water near the equator during both cruises, while biomass was distributed more evenly with depth in the presence of a weak thermocline at higher latitudes. The size structure of the zooplankton community changed between cruises, with more biomass in the >1000 μm size fraction during Survey 2 in waters near (but not on) the equator. As noted by previous investigators, zooplankton biomass in this region appears to be in a dynamic balance between utilization of a relatively rich food supply and advection out of the area. Physical changes caused by El Nifio interrupt this balance.