Zooplankton-mediated carbon flux in the Southern Ocean: influence of community structure, metabolism and behaviour

The biological carbon pump (BCP) exerts an important control on climate, exporting organic carbon from the ocean surface to interior. Zooplankton are a key component of the BCP and may enhance it through diel vertical migration (DVM) and faecal pellet production at depth. However, variability in these processes mean the zooplankton term is insufficiently constrained in global climate models. I investigated the role of zooplankton in the BCP at four locations in the Scotia Sea, Southern Ocean (SO), combining observations, in situ experiments and modelling. I found that carbon flux is highly dependent on zooplankton structure, behaviour and community dynamics, with strong latitudinal variations. Zooplankton demonstrated a high degree of behavioural plasticity. Normal, reverse and non‐migration modes were common within species and at the community level, with implications for seasonal export flux. Carbon export (faecal pellet and respiration flux) from migrants was generally higher north of the Southern Antarctic Circumpolar Current Front (SACCF), corresponding to greater species biomass and diversity, but could be highly variable. Faecal pellet attenuation depth, defined as the depth from which faecal pellet flux decreased from its maximal value, also corresponded to zooplankton biomass, being deeper in the north, and shallowest nearer the ice. DVM may explain enhanced night‐time fluxes which comprised a shallower input of dense, fastsinking faecal pellets with high sinking velocities. Community‐scale reverse migrations during summer reduced flux from migrants suggesting that at high latitudes, mode of migration may be important in determining zooplankton community carbon flux. Variability in water column temperature during DVM also affected carbon flux from respiration from Euphausia triacantha, a widespread SO euphausiid and interzonal migrant. The first comprehensive measurements of this kind showed short‐term respiration rates to vary with a Q10 of ~4.7 between 0.2 and 4.7 °C. In model simulations, respiration flux accounted for two‐thirds of total annual carbon flux from E. triacantha, dominating during summer when upper water column temperature was most variable. Modelled carbon flux was also highly sensitive to feeding dynamics and migratory behaviour. Flux was enhanced during carnivorous feeding and asynchronous migration. However, when considering feeding dynamics and seasonal mixed layer depths, lower export was predicted from foray compared to ‘classical’ DVM. In summary, regions of high mesozooplankton biomass can generate large fluxes of carbon which penetrate the mid‐mesopelagic but are also more difficult to predict due to variability in zooplankton behaviour and ecology. This will be improved with a deeper understanding of species‐specific feeding and migration behaviours.

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