Le Borgne ¹, Robert, Richard Feely² and Denis Mackey ³

¹IRD, Centre de Nouméa, 98848 Nouméa, New Caledonia, Tel: (687) 26 07 86, Fax: (687) 26 43 26, E-mail: leborgne@noumea.ird.nc, ²Pacific Marine Environmental Laboratory, NOAA, 7600 Sand Point Way N.E., Seattle, Washington 98115-0070, USA and ³CSIRO Marine Research, PO Box 1538, Hobart 7001, Tasmania, Australia

Carbon fluxes in the equatorial Pacific: a synthesis of the JGOFS programme

The present synthesis, issued in Deep-Sea Research II (2002), uses published results on the carbon cycle of the equatorial Pacific which accounts for a major fraction of the net exchange of CO₂ between the atmosphere and the oceans. Most of the CO₂ evasion takes place in the upwelling influenced region in the eastern Pacific, while atmospheric and sea surface partial pressures of CO₂ are near equilibrium in the warm pool, located to the west. Large changes in the surface area of the upwelling region, which occur as a result of the ENSO (El Niño – Southern Oscillation) phenomenon, account for up to 70% of the interannual variability in the net air-sea flux of CO₂. On average, the export of biological production, which is approximately the same as new primary production, is similar to that of CO₂ evasion (0.8 – 1.0 Pg C y^-1) but there is less temporal variability due to the very slow uptake of new macronutrients in the equatorial ecosystem. As in other tropical ecosystems, both the warm pool and upwelling areas are characterized by; (i) the dominance of picophytoplankton, and; (ii) the steady state, achieved by the balance between predation and growth. In addition to the basic tropical population of nano- and picoplankton, larger phytoplankton are more abundant in the nutrient-replete waters of the upwelling region with the result that biomass, mean organism size and export fluxes are greater than in the nutrient-depleted waters of the warm pool. However, the difference in export flux of carbon between the two regions is rather modest (2 – 4 fold per unit area) because of the limitation of primary production in the upwelling zone by iron and, possibly, other nutrients. The latter is a typical HNLC (High Nutrient - Low Chlorophyll) zone with very low rates of uptake of macronutrients and an essentially constant export flux of carbon due to the ‘biological pump’. This general pattern is temporarily disturbed by the passage of equatorial Kelvin waves and Tropical Instability Waves (TIW) in the upwelling region through horizontal advection and possible inputs of micronutrients from the deeper layers. Finally, low-frequency variations on decadal timescales could influence the values of the CO₂ evasion and the ‘biological pump’. Studying their impact will require long term monitoring.