Pergamon Deep-Sea Research II, Vol. 42, No. 2-3. pp. 715~-733, 1995 0967~45(95)00033-X Copyright0 1995 Elsevier Science Ltd Prmted m Great Bntain. All rights reserved 0967X&45/95 $9 50 + 0 00 zyxwvutsrqpon Latitudinal gradients in zooplankton biomass in the tropical Pacific at 14O”W during the JGOFS EqPac study: Effects of El Niiio JACQUES R. WHITE,*§ XINSHENG ZHANG,‘( LEIGH A. WELLING,+ MICHAEL R. ROMAN* and HANS G. DAM? (Received 14 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA August 1994; in revised form 28 February 1995; accepted I March 199.5) Abstract-h 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 14O”W from 12”N to Ii%. The first cruise (Survey I) occurred during El NiAo 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 1. 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 (O-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 >lOOO pm 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 Nirio interrupt this balance. INTRODUCTION The equatorial region of the Pacific Ocean is characterized by high biological production relative to adjacent regions to the north and south in the basin. Upwelled nutrients in the euphotic zone are supplied by a wind-driven geostrophic divergence and support high biological production centered near the equator, decreasing to the north and south (Vinogradov, 1981). Nitrate supplied to equatorial surface waters from below the thermocline in the Pacific may support a significant fraction of the total new production of organic matter in the world oceans (Chavez and Barber, 1987)) i.e. production in excess of *Horn Point Environmental Laboratory, University of Maryland, Cambridge, MD 21613, U.S.A. tDepartment of Marine Science, University of Connecticut, Avery Point, Groton, CT 06340, U.S.A. $College of Oceanic and Atmospheric Sciences, Oregon State University, Ocean Administration Bldg. 104, Corvallis, OR 97331-5503, U.S.A. PPresent address: College of Marine Studies, University of Delaware, Lewes, DE 19958-1298, U.S.A. 715