Picophytoplankton abundance and biomass in the western tropical Pacific Ocean during the 1992 El Nino year: Results from flow cytometry

Natural populations of phytoplankton from the western tropical Pacific Ocean were analyzed by flow cytometry from a transect along 165 degrees E between 20 degrees S and 7 degrees N. The abnormal hydrological situation corresponded to a weak Fl Nino event, with no equatorial upwelling and a marked nutrient ridge centered on 10 degrees S. Prochlorophytes dominated numerically everywhere along the vertical, whatever the depth, in the 0-160-m layer (96% of cell abundance). Paradoxically, the highest concentrations, up to 4.4x10(5) cells ml(-1), were found in oligotrophic waters (<0.1 mu M NO3). In contrast, the highest concentrations of orange cyanobacteria and red-fluorescing picoeukaryotes were observed when nitrate was present in the photic layer, i.e. around 10 degrees S (up to 6.4x10(4) cells ml(-1) and 1.3x10(4) cells ml(-1)), and, to a lesser extent in the vicinity of the deep nitracline north of 8 degrees S. Along the transect we encountered two hydrological situations, characterized by different community structures. The first one, found from 15 degrees S to 7 degrees N, except at 10 degrees S, was a two-layer structure (Typical Tropical Structure, TTS) defined by a strong pycnocline in the upper 180 m and a well-marked nitracline. In this region, Prochlorococcus and picoeukaryotes co-dominated the 180-m integrated fluorescence and carbon biomass, but Prochlorococcus were the major component in the upper nitrate-depleted layer, while picoeukaryotes dominated the underlying rich layer. Inversely, Synechococcus were a relatively minor contributor to fluorescence (similar to 4%) and phytoplankton biomass (<1%) in comparison to the other cell types. The second structure observed in the southernmost part of the transect (20 degrees S-16 degrees S) was defined by the absence of a density gradient, and therefore by deep vertical mixing. In this case, the concentration of Prochlorococcus in the upper nitrate-depleted layer was reduced, whereas Synechococcus percentage contribution in the upper 180 m was significantly higher than in the TTS (>30% of total fluorescence and similar to 4% of carbon biomass). According to our results, we discuss the expected role of each phytoplankton group in the regenerated and new production. Finally, we discuss the importance of cell size as a factor in the expected roles of the different phytoplankton groups in the carbon sink. Copyright (C) 1996 Elsevier Science Ltd.