Seasonal variability of microplankton community respiration in the Celtic Sea, UK

The downward flux and ultimate storage of particulate and dissolved organic carbon in shelf seas is constrained by the respiration of bacteria, phytoplankton and zooplankton in the water column. In order to apportion respiration to the different plankton size classes in the water column and associated with particles, we undertook a seasonal study of plankton community respiration in the Celtic Sea (UK) as part of the NERC Shelf Sea Biogeochemistry programme. Fast, slow sinking and suspended particles were collected using Marine Snow Catchers. Community respiration rates (CR) were estimated from oxygen consumption measured with both traditional Winkler titrations and optodes and from in vivo reduction rates of a tetrazolium salt (INT) in two size-fractions (0.2-0.8 µm and >0.8 µm). The concentration of reduced INT was significantly correlated with the concentration of oxygen consumed (r = 0.641, p<0.001, n = 113) supporting previous suggestions that INT is a good proxy of respiration.

As expected, CR varied seasonally, with the highest depth-integrated rates (surface to 1% incident irradiance) in April 2015 (152 ± 4 mmol O₂ m^-2 d^-1) in response to the spring phytoplankton bloom and the lowest rates in November 2014 (20 ± 4 mmol O₂ m^-2 d^-1). CR usually decreased with depth except in November when it was similar above and below a weak thermocline. In general, 32 % of community respiration was attributable to cells in the 0.2 to 0.8 µm size class, however, during the April phytoplankton bloom this contribution decreased to 17 %. In the fast sinking particles the contribution of the 0.2 to 0.8 µm size class to CR increased with depth with an average of 25 % above the thermocline and an average of 33 % below it. These results will be interpreted alongside concentrations of particulate and dissolved organic material and plankton biomass in the context of this coordinated physical and biogeochemical annual study.