The space and time variability of chlorophyll concentration detected by the satellite-borne sea color sensor SeaWiFS over years 1998 to 2001 in the south western Pacific Ocean (150°E - 140°W, 0° - 40°S) is first described using Fourier's analysis. The region usually has low chlorophyll concentration, except near New Zealand. Seasonal variations are small. The main pattern of seasonal variation is a maximum in austral winter between 20°S and 40°S when cooling of surface waters reduces the vertical stability of the water column and favors the supply of nutrients from below, and subsequent phytoplankton growth. Some areas have high interannual variability, such as the surface waters near the equator that were affected by the 1997 - 98 El Nino, and had abnormally high pigments concentration when conditions returned to normal, and the region for Vanuatu to Fiji Islands where sporadic blooms of Cyanobacteria Trichodesmium are known to occur. Field data collected quarterly along the track of a merchant ship are used to characterize the phytoplankton populations in this region. Indeed, it is recognized that different phytoplankton populations may have different impact on the geochemistry of the ocean, such as the Coccolithophorids that carry out both biocalcification and photosynthesis, thus reducing the alkalinity of seawater, or Trichodesmium that use atmospheric N2 to fufill its nitrogen requirements and thus is not dependent on inorganic nitrogen sources. Cells counts and pigments determinations show that the phytoplankton populations are dominated by the picoplankton (Prochlorococcus, Synechococcus and picoeucaryotes). High numbers of Synechococcus have sometimes been found Near New Zealand.