Carbon and phosphate incorporation rates of microbial assemblages in contrasting environments in the Southeast Pacific

Size-fractionated chlorophyll a (chl a) was determined along with carbon (C) and phosphate (P) incorporation rates over a west-east transect in the Southeast Pacific (between 146.36° W and 72.49° W). A clear longitudinal gradient was observed for both chl a and C and P incorporation rates from the productive areas, near the Marquesas Islands and the Chilean upwelling, to the hyperoligo-trophic area associated with the central part of the Southeast Pacific Gyre (SPG). The 0.2-0.6 μm fraction represented 15 to 43 % (29 ± 7 % mean ± SD, n = 24) of the P incorporation rate along the transect, suggesting that heterotrophic bacteria were important factors in the P cycle as dissolved inorganic P consumers. Small-sized cells (<2 μm) formed the bulk of chl a biomass (69 ± 4%) and accounted for 49 ± 7 % of total primary production in the SPG. In contrast, large phytoplankton (>2 μm) accounted for a significantly larger fraction of the total primary production, despite having lower chl a biomass and P incorporation rates. Consequently, larger cells presented higher photosynthetic efficiency values than the smaller phytoplankton. The C:P incorporation rate ratios exhibited increasing deviation from the canonical stoichiometric ratios of 106 and 50 (for phytoplankton and heterotrophic bacteria, respectively) towards the centre of the SPG. We propose hypotheses to explain these deviations.