Biomass, size structure and depth distributions of the microbial community in the eastern equatorial Pacific

We investigated the biomass, size structure and composition of microbial communities over a broad area of the eastern equatorial Pacific (4°N-4°S, 110-140°W) during cruises in December 2004 (EB04) and September 2005 (EB05). Vertical-profile samples were collected at 30 stations at depths extending from the surface to the 0.1% light level, and each sample was analyzed quantitatively by flow cytometry and epifluorescence microscopy. Autotrophic biomass averaged 14.8±4.2 (1 s.d.) μg C L−1 for the euphotic zone, with dinoflagellates comprising 39%, Prochlorococcus 28%, other flagellates 18%, Synechococcus 7.5%, and diatoms 6.3%. Nanoplankton accounted for 46% of autotroph biomass, while pico- and microphytoplankton comprised 39 and 16%, respectively. C:Chl averaged 64±14 for the euphotic zone, with a mean mixed-layer value of 78±20 and a minimum of 36±15 at the 1% light level. Heterotrophic biomass averaged 7.0±1.2 μg C L−1 for prokaryotes, 1.6±0.9 μg C L−1 for dinoflagellates, 1.5±1.1 μg C L−1 for other flagellates, and 2.1±0.4 μg C L−1 for ciliates. Euphotic zone integrated biomass varied 2-fold, 1.2 to 2.5 g C m−2, among stations, decreasing west to east with the gradient in euphotic zone concentrations of dissolved iron. Overall, community biomass and the contributions of functional groups displayed remarkable constancy over our study area, but some patterns were evident, such as the enhancement of picophytoplankton in the leading (upwelling) edges of tropical instability waves and larger diatoms in the trailing (downwelling) edges. Prochlorococcus, in particular, exhibited more variability than expected, given its generally assumed role as a stable background species in the tropical oceans, and was positively associated with the areas of enhanced autotrophic carbon and Chl a. With corrections for different methodological assumptions taken into account, our EB05 estimates of mixed-layer community biomass are 27-35% higher than values for JGOFS studies in 1992.