Planktonic carbon budget in the eastern subtropical North Atlantic

We used data collected at >60 stations over a 10 yr period to build the carbon budget of the plankton community in the euphotic layer of the Eastern North Atlantic Subtropical Gyre (NASE). Autotrophic biomass exceeded microbial heterotrophic biomass by a factor of 1.7. Mean (±SE), integrated chlorophyll a concentration and net particulate primary production (PP) were 17 ± 1 mg m–2 and 271 ± 29 mgC m–2 d–1, respectively. Protist grazing on phytoplankton represented >90% of PP. Bacterial production (BP) was 17 ± 3 mgC m–2 d–1. In vitro O2-evolution experiments indicated that net community production was –65 ± 16 mmolO2 m–2 d–1, while community respiration (CR) averaged 124 ± 13 mmolO2 m–2 d–1, equivalent to 1324 ± 142 mgC m–2 d–1. However, the sum of the respiration rates by each microbial group, estimated from their biomass and metabolic rates, ranged from 402 to 848 mgC m–2 d–1. Therefore, CR could not be reconciled with the respiratory fluxes sustained by each microbial group. Comparison between estimated gross photosynthesis by phytoplankton (481 to 616 mgC m–2 d–1) and the sum of respiration by each group suggests that the microbial community in the NASE province is close to metabolic balance, which would agree with the observed O2 supersaturation in the euphotic layer. Taking into account the mean open-ocean values for PP, BP, CR and bacterial growth efficiency, we show that bacteria account for approximately 20% of CR. Our results suggest that the view that bacteria dominate carbon cycling in the unproductive ocean must be reconsidered, or else that in vitro incubations misrepresent the real metabolic rates of one or several microbial groups.