Dynamics of phytoplankton and heterotrophic bacterioplankton in the western tropical South Pacific Ocean along a gradient of diversity and activity of diazotrophs

Abstract. Heterotrophic prokaryotic production (BP) was studied in the Western Tropical South Pacific using the leucine technique. Integrated over the euphotic zone, BP ranged from 58–120 mg C m−2 d−1 within the Melanesian Archipelago, and from 31–50 mg C m−2 within the subtropical gyre. Nitrogen was often one of the main factor controlling BP on short time scale as shown using enrichment experiments, followed by dissolved inorganic phosphate (DIP) near the surface and labile organic carbon deeper in the euphotic zone. With N2 fixation being one of the most important fluxes fueling new production, we explored relationships between BP, primary production (PP) and N2 fixation rates. BP variability was better explained by the variability of N2 fixation rates than by that of PP in surface waters of the Melanesian Archipelago, which were characterized by N depleted layers, and low DIP turnover times (TDIP < 100 h). However, BP was more significantly correlated with PP but not with N2 fixation rates where DIP was more available (TDIP > 100 h), i.e. in a layer deeper than the euphotic zone -including the deep chlorophyll maximum depths- in the Melanesian Archipelago, or within the entire euphotic zone in the subtropical gyre. Bacterial growth efficiency (BGE) ranged from 6–10 %. Applying correcting factors to estimate gross primary production and correcting BP for Prochlorococcus assimilation of leucine, we showed a large variability in the contribution of gross primary production to bacterial C demand. Exploration of a bloom collapse at one site south of Vanuatu showed the importance of blooms, which can persist over extensive distance for long periods of time, and can maintaining net autotrophy where they occur. Using a Lagragian sampling strategy during 6 days, long duration sites allowed for the study of the rapid changes including BP, primary production and BGE, that occurred during the bloom collapse.