Testing potential bias in marine plankton respiration rates by dark bottle incubations in the NW Iberian shelf: Incubation time and bottle volume

The accurate determination of the balance between plankton production and respiration in the ocean is important for C budgets and global change predictions. Disagreements on the measurement of such a balance at different scales (from microbiological to biogeochemical) have produced a controversy over the trophic status of the ocean. This is especially striking in the oligotrophic open ocean, where plankton community O consumption rates in 24h incubations have frequently produced a net heterotrophic balance, but similar difficulties emerge in coastal systems. These results have been criticised due to the possibility that the standard 24h in vitro incubations are biased because of the long incubation time needed and the so-called "bottle effect". To study the influence of the incubation time and bottle volume on the measurement of plankton net metabolism, we carried out several time series experiments in the NW Iberian coastal system. Here we present measurements of plankton community respiration rates concurrently obtained through (1) standard in vitro changes in dissolved oxygen concentration after different incubation times ranging from 2 to 48. h, and with bottle volumes of 50, 125 and 570. mL, and (2) the decrease in the oxygen concentration measured every 20. s with oxygen microsensors, during 48. h. Our results refute the contention that 24. h dark 125. mL bottle incubations are systematically biased, and highlight the validity of oxygen microsensors to study the dynamics of natural marine plankton respiration.