Natural populations of phytoplankton from the western tropical Pacific Ocean were analyzed by flow cytometry from a transect along 165°E between 20°S and 7°N. The abnormal hydrological situation corresponded to a weak El Niño event, with no equatorial upwelling and a marked nutrient ridge centered on 10°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 4.4 x 105 cells/ml, were found in oligotrophic waters (< 0.1 µM NO3). In contrast, the highest concentrations of orange cyanobacteria and redfluorescing picoeukaryotes were observed when nitrate was present in the photic layer, i.e. around 10°S (up to 6.4 x 104 cells/ml and 1.3 x 10**4 cells/ml), and, to a lesser extent in the vicinity of the deep nitracline north of 8°S. Along the transect we encountered two hydrological situations, characterized by different community structures. The first one, found from 15°S to 7°N, except at 10°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 (~4%) and phytoplankton biomass ( 30% of total fluorescence and ~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.
Supplement to: Blanchot, Jean; Rodier, Martine (1996): Picophytoplankton abundance and biomass in the western tropical Pacific Ocean during the 1992 El Niño year: results from flow cytometry. Deep Sea Research Part I: Oceanographic Research Papers, 43(6), 877-895
