Picophytoplankton in the equatorial Pacific: vertical distributions in thewarm pool and in the high nutrient low chlorophyll conditions

Abundance distribution and cellular characteristics of picophytoplankton we re studied in two distinct regions of the equatorial Pacific: the western w arm pool (0 degrees, 167 degreesE), where oligotrophic conditions prevail, and the equatorial upwelling at 150 degreesW characterized by high-nutrient low-chlorophyll (HNLC) conditions. The study was done in September-October 1994 during abnormally warm conditions. Populations of Prochlorococcus, or ange fluorescing Synechococcus and picoeukaryotes were enumerated by flow c ytometry. Pigment concentrations were studied by spectrofluorometry. In the warm pool, Prochlorococcus were clearly the dominant organisms in terms of cell abundance, estimated carbon biomass and measured pigment concentratio n. Integrated concentrations of Prochlorococcus, Synechococcus and picoeuka ryotes were 1.5x10(13), 1.3x10(11) and 1.5x10(11)cells m(-2), respectively. Integrated estimated carbon biomass of picophytoplankton was 1 gm(-2), and the respective contributions of each group to the biomass were 69, 3 and 2 8%. In the HNLC waters, Prochlorococcus cells were slightly less numerous t han in the warm pool, whereas the other groups were several times more abun dant (from 3 to 5 times). Abundance of Prochlorococcus, Synechococcus and p icoeukaryotes were 1.2 x 10(13), 6.2 x 10(11) and 5.1 x 10(11) cells m(-2), respectively. The integrated biomass was 1.9 g C m(-2). Prochlorococcus wa s again the dominant group in terms of abundance and biomass (chlorophyll, carbon); the respective contributions of each group to the carbon biomass w ere 58, 7 and 35%. In the warm pool the total chlorophyll biomass was 28 mg m(-2), 57% of which was divinyl chlorophyll a. In the HNLC waters, the tot al chlorophyll biomass was 38 mg m(-2), 44% of which was divinyl chlorophyl l a. Estimates of Prochlorococcus, Synechococcus and picoeukaryotes cell si ze were made in both hydrological conditions. (C) 2000 Elsevier Science Ltd . All rights reserved.