Le Borgne, Robert

IRD, Centre de Nouméa, 98848 Nouméa, New Caledonia, Tel: (687) 26 07 86, Fax: (687) 26 43 26, E-mail: leborgne@noumea.ird.nc

Equatorial upwellings in the Atlantic and Pacific oceans: a comparative study

Atlantic and Pacific equatorial upwellings –characterized by occurrence of deep waters at the surface- display quite different features. Due to the presence of Africa, which limits the current system to the North and influences the wind regime, the Atlantic equatorial upwelling is centered south of the equator and only occurs between July and September. In the Pacific, it is centered on the equator and quasi permanent. In both upwellings, surface nutrients are observed permanently, indicating that nutrient uptake is less than their inputs. However, the steady state seems weaker in the Atlantic considering the higher biomass latitudinal variability and frequently observed zooplanktonic blooms. A possible reason is the time required for the steady-state to be established when the ecosystem shifts from a typical tropical situation (two-layered stucture) to a 3 month-long upwelling system. Another difference is that chlorophyll « a » and mesozooplankton biomasses integrated values are higher in the Atlantic, with a greater (twofold) increase for the mesozooplankton. Such a contrast between the two regions may be interpreted as the result of an observed higher contribution of large sizes in the phytoplankton and a closer trophic link with the mesozooplankton, in the Atlantic. More large sizes in this region may be ascribed to less iron and silicon limitation than in the Pacific.