Phytoplankton abundances and community structure in the equatorial Pacific

In both the warm pool region and the high-nutrient low-chlorophyll (HNLC) region of the equatorial Pacific, the phytoplankton community is dominated by high concentrations of picophytoplankton regardless of whether El Niño, La Niña or normal conditions prevail. Flow cytometry measurements indicate that the picoplankton community consists of two species of prokaryotes, Prochlorococcus and Synechococcus, and a diverse mixture of picoeukaryotes that cannot be differentiated. Analyses of chlorophyll and carotenoid pigments, determined by HPLC, confirm the importance of Prochlorococcus and Synechococcus and indicate that the eukaryotes are predominantly haptophytes. There are at least three ecotypes of Prochlorococcus and, possibly, various ecotypes of Synechococcus. These ecotypes have quite different pigment ratios and, possibly, pigment to carbon ratios, which complicates comparisons between cell counts by flow cytometry and the contribution that prokaryotes make to total chlorophyll as calculated from pigment analysis. Increased productivity is associated with the transient influx of nutrients, El Niño conditions in the warm pool, or surface nutrients in HNLC waters. Under these conditions there seems to be a small increase in the relative abundances of Synechococcus, diatoms and chlorophytes. Grazing of picophytoplankton by macro-grazers may make an important contribution to the ‘biological pump’, which may not be dominated by large phytoplankton such as diatoms.