Abstract
Abstract. Heterotrophic prokaryotic production (BP) was studied in the western tropical South Pacific (WTSP) using the leucine technique, revealing spatial and temporal variability within the region. Integrated over the euphotic zone, BP ranged from 58 to 120 mg C m−2 d−1 within the Melanesian Archipelago, and from 31 to 50 mg C m−2 d−1 within the western subtropical gyre. The collapse of a bloom was followed during 6 days in the south of Vanuatu using a Lagrangian sampling strategy. During this period, rapid evolution was observed in the three main parameters influencing the metabolic state: BP, primary production (PP) and bacterial growth efficiency. With N2 fixation being one of the most important fluxes fueling new production, we explored relationships between BP, PP and N2 fixation rates over the WTSP. The contribution of N2 fixation rates to bacterial nitrogen demand ranged from 3 to 81 %. 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). This is consistent with the fact that nitrogen was often one of the main factors controlling BP on short timescales, as shown using enrichment experiments, followed by dissolved inorganic phosphate (DIP) near the surface and labile organic carbon deeper in the euphotic zone. However, BP was more significantly correlated with PP, but not with N2 fixation rates where DIP was more available (TDIP > 100 h), deeper in the Melanesian Archipelago, or within the entire euphotic zone in the subtropical gyre. The bacterial carbon demand to gross primary production ratio ranged from 0.75 to 3.1. These values are discussed in the framework of various assumptions and conversion factors used to estimate this ratio, including the methodological errors, the daily variability of BP, the bacterial growth efficiency and one bias so far not considered: the ability for Prochlorococcus to assimilate leucine in the dark.
Highlights
Heterotrophic prokaryotes can process, on average, 50 % of the carbon (C) fixed by photosynthesis in many aquatic systems (Cole, 1988)
The longitudinal transect started northwest of New Caledonia, crossed the Vanuatu and Fiji Arcs and finished inside the western part of the ultra-oligotrophic South Pacific Gyre. It covered a vast region of the western tropical South Pacific (WTSP) and the main gradient of biogeochemical and biological properties between the Melanesian Archipelago (MA) and the western part of the South Pacific Gyre (WGY) (SD13 to SD15 and LDC) separated by the Tonga Volcanic Arc (Fig. 1)
Our results provide a unique set of simultaneous measurements of bacterial production (BP), primary production (PP) and N2fix rates in the WTSP
Summary
Heterotrophic prokaryotes can process, on average, 50 % of the carbon (C) fixed by photosynthesis in many aquatic systems (Cole, 1988). Understanding the controls of heterotrophic bacterial production and respiration rates is fundamental for two major aspects of marine C cycling: (i) to explore the possible fate of primary production through the microbial food web, and (ii) to construct a metabolic balance based on C fluxes. To assess these two major features, bac-. The metabolic state of the ocean, and in particular the status of net heterotrophy within oligotrophic systems, has been largely debated in the last decade (see for example the review in Duarte et al, 2013; Ducklow and Doney, 2013; Williams et al, 2013)
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