Abundances and distributions of picoplankton populations in the central equatorial Pacific from 12°N to 12°S, 140°W

Abstract

Abundances and distributions of picoplankton were studied on two cross-equatorial transect cruises (12°N, 140°W-12°S, 135°W) during February–March (TT007) and August–September 1992 (TT011). Samples were collected in the upper 200 m from early-morning and late-afternoon hydrocasts at 15 stations on each cruise (60 depth profiles, 820 samples). Populations of heterotrophic bacteria, Prochlorococcus, Synechococcus and small autotrophic eukaryotes were enumerated by dual-beam flow cytometry. At the northern end of the transect (7–12°N), abundances and vertical distributions were similar to those reported for the oligotrophic North Pacific gyre, with Prochlorococcus and heterotrophic bacteria dominating in the upper euphotic zone, and Synechococcus and eukaryotes exhibiting peaks in cell abundance at depth. All populations were abundant throughout the euphotic zone close to the equator and at the southern end of the transect. Heterotrophic bacteria and Synechococcus were generally more abundant in late-afternoon hydrocasts. The diel enhancement followed the temporal pattern in beam c and suspended particulates, and was particularly strong in the equatorial upwelling region where it averaged 13.6% of the morning population for heterotrophic bacteria and 22.3% for Synechococcus. Conservative estimates of daily growth rates from these data are 0.25 and 0.40 day −1, respectively, for the two populations. Near-surface maxima in heterotrophic bacteria were symmetrical around the equator, centered around 5°S and 5°N. Prochlorococcus was most abundant during local summer conditions at the respective ends of the transect. A minimum occurred in association with a dense aggregation of buoyant diatoms at the convergent front of a tropical instability wave (2°N, TT011). The ratio of Prochlorococcus to total bacteria was generally in the range of 0.15-0.2 for the upper water column, but varied during TT011 from > 0.3 for the most northern stations to < 0.1 at the 2°N front. At higher latitudes, Synechococcus was more numerous during El Niño conditions (TT007) on both sides of the equator and at southern stations on both cruises. Autographic eukaryotes were more abundant during local winters at the ends of the transect and during the “cold tongue” conditions (TT011) at the equator. Picoplankton account for most of the chlorophyll biomass and primary production in the central equatorial Pacific. Nonetheless, their abundances and distributions are relatively stable and conservative while other populations, such as diatoms, respond more dramatically to environmental forcing.