LATITUDINAL GRADIENTS IN ZOOPLANKTON BIOMASS IN THE TROPICAL PACIFIC AT 140-DEGREES-W DURING THE JGOFS EQPAC STUDY - EFFECTS OF EL-NINO

In 1992, as part of the equatorial Pacific study (EqPac) of the U.S. J GOFS Program, we collected zooplankton samples during a set of cruises that crossed the equator in the Pacific Ocean roughly along 140 degre es W from 12 degrees N to 12 degrees S. The first cruise (Survey I) oc curred during El Nino conditions in February-March when anomolously wa rm surface waters (>28 degrees C) were present over most of the transe ct. The second cruise (Survey II) was during August-September when sur face temperatures had returned to near or below the climatological mea n over much of the region, and inorganic nitrogen and chlorophyll conc entrations had increased compared with Survey I. Zooplankton biomass w as higher in the equatorial region during Survey II compared with Surv ey I. For both cruises, chlorophyll and zooplankton biomass generally increased in the epipelagic zone (0-200 m) towards the equator. Howeve r, whereas primary production and chlorophyll were highest on or near the equator, zooplankton biomass was usually highest several degrees t o the north and south of the equator. Zooplankton biomass was reduced below 100 m in newly upwelled water near the equator during both cruis es, while biomass was distributed more evenly with depth in the presen ce of a weak thermocline at higher latitudes. The size structure of th e zooplankton community changed between cruises, with more biomass in the >1000 mu m size fraction during Survey 2 in waters near (but not o n) the equator. As noted by previous investigators, zooplankton biomas s in this region appears to be in a dynamic balance between utilizatio n of a relatively rich food supply and advection out of the area. Phys ical changes caused by El Nino interrupt this balance.