Seasonal and interannual variability of ocean color and composition of phytoplankton communities in the North Atlantic, Equatorial Pacific and South Pacific

International audienceMonthly averaged level-3 SeaWiFS chlorophyll concentration data from 1998 to 2001 are globally analyzed using Fourier's analysis to determine the main patterns of temporal variability in all parts of the world ocean. In most regions, seasonal variability dominates over interannual variability, and the timing of the yearly bloom can generally be explained by the local cycle of solar energy. The studied period was influenced by the late consequences of the very strong El Niño of 1997-98. After this major event, the recovery to normal conditions followed different patterns at different locations. Right at the equator, chlorophyll concentration was abnormally high in 1998, and then decreased, while aside from the equator, it was low in 1998, and increased later when equatorial upwelled waters spread poleward. This resulted in opposed linear trends with time in these two zones. Other noticeable examples of interannual variability in the open ocean are blooms of Trichodesmium that develop episodically in austral summer in the south-western tropical Pacific, or abnormally high chlorophyll concentration at 5°S in the Indian Ocean after a strong Madden-Julian oscillation. Field data collected quarterly from November 1999 to August 2001, owing to surface sampling from a ship of opportunity, are presented to document the succession of phytoplankton populations that underlie the seasonal cycles of chlorophyll abundance. Indeed, the composition of the phytoplankton conditions the efficiency of the biological carbon pump in the various oceanic provinces. We focus on the north Atlantic, Caribbean Sea, Gulf of Panama, equatorial Pacific, south Pacific subtropical gyre, and south-western tropical Pacific where these field data have been collected,. These data are quantitative inventories of pigments (measured by HPLC and spectrofluorometry), and picoplankton abundance (Prochlorococcus, Synechococcus, Picoeucaryotes and bacteria). There is a contrast between temperate waters where nanoplankton (as revealed by pigments indexes) dominate during all the year, and tropical waters where picoplankton dominate. The larger microplankton, that make most of the world ocean export production to depth, rarely exceed 20% of the pigment biomass in the offshore waters sampled by these cruises. Most of the time, there are large differences in the phytoplankton composition between cruises made at the same season on two different years