PHYTOPLANKTON PRODUCTIVITY AND THE CARBON-CYCLE IN THE WESTERN EQUATORIAL PACIFIC UNDER EL-NINO AND NON-EL-NINO CONDITIONS

The Australian contribution to the equatorial JGOFS program consisted of three cruises that focused on a transect from 10 degrees S to 10 de grees N along 155 degrees E. In this paper we report on the last two c ruises, in June/July 1992 and November 1993, which coincided with the middle and the end of a prolonged El Nino event, and compare the resul ts with those obtained in October 1990, i.e. after the 1988/1989 La Ni na but before the 1991/1992 El Nino. Compared with 1990, the depth of the thermocline was shallower in 1992 and 1993, with the result that t he barrier layer was thinner and the deep chlorophyll maximum was alwa ys found below the top of the thermocline. Maximum chlorophyll concent rations and depth-integrated concentrations were similar in 1990 and 1 992 (0.4-0.45 mu g 1(-1) and 20-30 mg m(-2)) but were higher in 1993 ( 0.75 mu g 1(-1) and 30-40 mg m(-2)). Modelled productivities varied fr om 20 to 50 mmol C m(-2) d(-1) and were strongly dependent on the dept h of the chlorophyll mixed layer and the nutrient supply. In 1992, the surface waters were undersaturated in CO2 by about 30 mu atm, while i n 1993 the waters were closer to equilibrium with atmospheric CO2, wit h pCO(2) values largely determined by sea surface temperature. During 1993, large changes in sea surface temperature (1.5 degrees C), salini ty (0.4), pCO(2) (40 mu atm) and near surface currents were observed n ear the equator over a period of about a week. These changes in the ci rculation, the physical and chemical structure, and phytoplankton dept h distribution and production, appeared to respond to both local meteo rological forcing and changes in the large scale equatorial circulatio n (ENSO). In contrast to sites further east, we always found a low sal inity surface layer at 155 degrees E which isolated the surface ocean from changes in carbon pools and fluxes associated with the productive deep chlorophyll maximum. (C) 1998 Elsevier Science Ltd. All rights r eserved.