Picophytoplankton abundance and biomass in the western tropical Pacific Ocean during the 1992 El Niٌo 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°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 × 105 cells ml−1, 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 × 104 cells ml−1 and 1.3 × 104 cells ml−1), 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 (< 1%) in comparison to the other cell types. The second structure observed in the southernmost part of the transect (20°S-16°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 ≈ 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.