Phytoplankton productivity and the carbon cycle in the western Equatorial Pacific under El Nin˜o and non-El Nin˜o conditions

Abstract

The Australian contribution to the equatorial JGOFS program consisted of three cruises that focused on a transect from 10°S to 10°N along 155°E. In this paper we report on the last two cruises, in June/July 1992 and November 1993, which coincided with the middle and the end of a prolonged El Nin˜o event, and compare the results with those obtained in October 1990, i.e. after the 1988/1989 La Nin˜a but before the 1991/1992 El Nin˜o. Compared with 1990, the depth of the thermocline was shallower in 1992 and 1993, with the result that the barrier layer was thinner and the deep chlorophyll maximum was always found below the top of the thermocline. Maximum chlorophyll concentrations and depth-integrated concentrations were similar in 1990 and 1992 (0.4 – 0.45 μg 1 −1 and 20–30 mg m −2) but were higher in 1993 (0.75 μg 1 −1 and 30–40 mg m −2). Modelled productivities varied from 20 to 50 mmol Cm −2 d −1 and were strongly dependent on the depth of the chlorophyll mixed layer and the nutrient supply. In 1992, the surface waters were undersaturated in C0 2 by about 30 μatm, while in 1993 the waters were closer to equilibrium with atmospheric C0 2, with pCO 2 values largely determined by sea surface temperature. During 1993, large changes in sea surface temperature (1.5°C), salinity (0.4), pCO 2 (40 μatm) and near surface currents were observed near the equator over a period of about a week. These changes in the circulation, the physical and chemical structure, and phytoplankton depth distribution and production, appeared to respond to both local meteorological forcing and changes in the large scale equatorial circulation (ENSO). In contrast to sites further east, we always found a low salinity surface layer at 155°E which isolated the surface ocean from changes in carbon pools and fluxes associated with the productive deep chlorophyll maximum. All rights reserved