Plankton community respiration during a coccolithophore bloom

Plankton dark community respiration (DCR), gross production (GP), bacterial production, protozoan herbivory, and phytoplankton, microzooplankton and heterotrophic bacterial abundance were measured during a bloom of the coccolithophore Emiliania huxleyi. The study, which was conducted in the northern North Sea during June 1999, included a spatial survey and a 6-day Lagrangian time series informed by a sulphur hexafluoride (SF6) tracer-release experiment. E. huxleyi abundance in surface waters ranged from 380 to 3000 cells ml−1, while DCR varied between 2 and 4 mmol O2 m−3 d−1 and GP between 2 and 5 mmol O2 m−3 d−1. Euphotic zone integrated DCR and GP were in approximate balance, with a mean (±SD) P:R ratio of 0.9±0.4 (n=9). However, highest GP occurred at the surface alongside maxima of E. huxleyi, whereas highest rates of DCR occurred at depths of 25–30 m associated with maxima in chlorophyll a and bacterial biomass. DCR was positively correlated with bacterial biomass, microzooplankton biomass, attenuance, particulate organic carbon, and chlorophyll a concentration; and negatively correlated with apparent oxygen utilisation. DCR was not correlated with in situ temperature, dissolved organic carbon concentration or E. huxleyi abundance. A100 h incubation of 0.8 μm filtered seawater enabled the estimation of a bacterial respiratory quotient (RQ) and growth efficiency (BGE) from the slopes of the linear regressions of the decrease in dissolved oxygen and increase in dissolved inorganic carbon (DIC) and bacterial carbon with time. During this experiment the bacterial RQ was 0.69 and the growth efficiency was 18%. This measured BGE was used in comparison with literature values to apportion DCR to that associated with bacterial (13–71%), microzooplankton (10–50%), and algal (11–28%) activity. This accounting exercise compared well with measured DCR (to within ±50%), the exact calculation being highly dependent on the BGE used.