Size-fractionated primary production, bacterial production and net community production in subtropical and tropical domains of the oligotrophic NE Atlantic in autumn

The biomass and metabolism of the planktonic microbial community was assessed at 7 stations in the eastern subtropical and tropical domains of the North Atlantic subtropical gyre in autumn 2001. Picoplankton formed the bulk of phytoplankton (70 ± 3% SE of total chlorophyll a), with Prochlorococcus spp. as the dominant group and an increasing importance of picoeukaryotes and Synechococcus spp. in tropical waters. Autotrophic plankton biomass clearly exceeded that of heterotrophic bacteria in the upper 150 m, contradicting previous claims of marked inverted biomass pyramids in the oligotrophic ocean. Despite the dominance of picoplankton in biomass, primary production (range 153 to 337 mgC m^-2 d^-1) was mostly due to cells >2 μm (59 ± 3%). Primary production by microphytoplankton was significantly greater in the tropical domain. Bacterial heterotrophic production (range 6 to 22 mgC m^-2 d^-1) was always below 10% of primary production. This low value was probably related to the high proportion (56 ± 2%) of inactive bacteria (identified by their DNA content). Although bacterial activity covaried with primary productivity, bacterial carbon demand estimated by 2 models from the literature generally required organic matter input other than dissolved primary production. Given that community respiration (116 ± 32 mmol O2 m_-2 d^-1) also exceeded gross primary production (58 ± 32 mmol O₂ m^-2 d ^-1) at all but 1 station, our results confirm previous observations of the net heterotrophic balance in the oligotrophic NE Atlantic.