Lagrangian study of microbial plankton respiration in the subtropical North Atlantic Ocean: bacterial contribution and short-term temporal variability

In oligotrophic regions, in situ diel variability of many biological variables has been shown to be often larger than the variability observed over scales of days. However, the short-term temporal variability (scale of hours to days) of microbial plankton respiration has been poorly assessed in oligotrophic waters, due to methodological limitations of the standard oxygen consumption methods (e.g. incubation time >= 24 h). In order to avoid such constraints, we estimated bacterial and microbial plankton respiration from size-fractionated (< 0.8 mu m and total) in vivo electron transport system (ETS) activity measurements (incubation time 3 to 5.5 h). Temporal variability in both bacterial and total microbial plankton respiration was assessed during 2 Lagrangian studies lasting 8 d and conducted at the center and the eastern margin of the North Atlantic Subtropical Gyre in autumn 2006. In these studies, size-fractionated in vivo ETS activity was measured in the same parcel of water on 12 occasions at different times of the day. The contribution of bacteria to total microbial community respiration averaged 29 +/- 20 (SD) % and was significantly higher in the upper mixed layer than at the base of the euphotic zone. Euphotic depth-integrated microbial community respiration showed high inter- and intra-day variability and was significantly higher in samples collected at night than during the day, despite constant biomass of bacteria, Prochlorococcus, eukaryotic pico-phytoplankton and total phytoplankton throughout both Lagrangian studies. Our results indicate that microbial community respiration estimates could be biased if short-term variability is not considered.