ABUNDANCES AND DISTRIBUTIONS OF PICOPLANKTON POPULATIONS IN THE CENTRAL EQUATORIAL PACIFIC FROM 12-DEGREES-N TO 12-DEGREES-S, 140-DEGREES-W

Abundances and distributions of picoplankton were studied on two cross -equatorial transect cruises (12 degrees N, 140 degrees W-12 degrees S , 135 degrees W) during February-March (TT007) and August-September 19 92 (TT011). Samples were collected in the upper 200 m from early-morni ng and late-afternoon hydrocasts at 15 stations on each cruise (60 dep th profiles, 820 samples). Populations of heterotrophic bacteria, Proc hlorococcus, Synechococcus and small autotrophic eukaryotes were enume rated by dual-beam how cytometry. At the northern end of the transect (7-12 degrees N), abundances and vertical distributions were similar t o those reported for the oligotrophic North Pacific gyre, with Prochlo rococcus and heterotrophic bacteria dominating in the upper euphotic z one, and Synechococccus and eukaryotes exhibiting peaks in cell abunda nce at depth. All populations were abundant throughout the euphotic zo ne close to the equator and at the southern end of the transect. Heter otrophic bacteria and Synechococcus were generally more abundant in la te-afternoon hydrocasts. The diel enhancement followed the temporal pa ttern in beam c and suspended particulates, and was particularly stron g in the equatorial upwelling region where it averaged 13.6% of the mo rning population for heterotrophic bacteria and 22.3% for Synechococcu s. Conservative estimates of daily growth rates from these data are 0. 25 and 0.40 day(-1), respectively, for the two populations. Near-surfa ce maxima in heterotrophic bacteria were symmetrical around the equato r, centered around 5 degrees S and 5 degrees N. Prochlorococcus was mo st abundant during local summer conditions at the respective ends of t he transect. A minimum occurred in association with a dense aggregatio n of buoyant diatoms at the convergent front of a tropical instability wave (2 degrees N, TT011). The ratio of Prochlorococcus to total bact eria 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 degrees N front. At higher latitudes, Synechococcus was more numerous during El Nino conditions (TT007) on both sides of the equator and at southern stations on both cruises. Autotrophic eukaryot es were more abundant during local winters at the ends of the transect and during the ''cold tongue'' conditions (TT011) at the equator. Pic oplankton account for most of the chlorophyll biomass and primary prod uction in the central equatorial Pacific. Nonetheless, their abundance s and distributions are relatively stable and conservative while other populations, such as diatoms, respond more dramatically to environmen tal forcing. Copyright (C) 1996 Elsevier Science Ltd.