Size distribution patterns of phytoplankton in the western Pacific: towards a generalization for the tropical open ocean

The size distribution of chlorophyll a (Chl a) was comprehensively investigated during four cruises along 165°E from 20°S to 6°N, with cell counts by epifluorescence microscopy and nutrient analysis being performed at the same stations. Observations tool place in two contrasting periods, an El Niño Southern Oscillation event in 1987 and a non ENSO period in 1988 and 1989. One micrometre Nuclepore filters proved to separate efficiently cyanobacteria from eucaryotic microalgae, in nutrient-rich water masses as well as in poor ones, and whatever the depth or the cell abundance. The Chl a distribution in the <1 μm and >1 μm fractions resulted from the relative contribution of procaryotic and eucaryotic cells to the total Chl a. In a stratified system, the euphotic zone was found to be divided into two parts: (1) an upper nitrate-depleted layer in which cyanobacteria were always numerically predominant, closely linked with Chl a in the <1 μm fraction which accounted for 60% of total Chl a on average; (2) a lower nutrient-rich layer in which Chl a > 1 μm dominated, belonging mainly to eucaryotic microalgae, as confirmed by the correlation between Chl a > 1 μm and the number of eucaryotes. The rapid change of the Chl a size pattern repeatedly observed at the top of the nitracline, whatever the depth, clearly demonstrated the major effect of nutrient increase on the size structure of phytoplankton. In systems such as the equatorial upwelling, where there is no oligotrophic mixed layer, Chl a > 1 μm predominated from the top to the bottom of the euphotic layer, in spite of very numerous cyanobacteria in the surface waters. Below the deep Chl a maximum, relatively large amounts of Chl b in the <1 μm fraction can be attributed to minute cells such as prochlorophytes. These results were compared with Chl a fractionations previously performed in the tropical Atlantic Ocean. Analysis of 230 profiles of Chl a in the <0.6, <0.8, <1, <2, <3, <10 and <20 μm fractions did not reveal any significant difference between the two areas. Since the Chl a size structure properties with respect to nutrient are common to both oceanic systems, the relationship evidenced in the western Pacific between the Chl a size pattern and the distribution of procaryotic and eucaryotic algal cells is likely to be similar in the tropical Atlantic: phytoplankton over wide areas has certain well-defined size distribution properties, probably typical of the whole of the tropical open ocean.