Fluxes of biogenic carbon in the Southern Ocean: roles of large microphagous zooplankton

The Southern Ocean is an extreme environment, where waters are permanently cold, a seasonal ice cover extends over large areas, and the solar energy a vailable for photosynthesis is severely restricted, either by vertical mixi ng to considerable depths or, especially south of the Antarctic Circle, by prolonged seasonal periods of low or no irradiance. Such conditions would n ormally lead to low productivity and a water column dominated by recycling processes involving microbial components of pelagic communities but this do es not seem to be the case in the Southern Ocean, where there is efficient export to large apex predators and deep waters. This paper investigates the role of large microphagous zooplankton (salps, krill, and some large copep ods) in the partitioning of biogenic carbon among the pools of short- and l ong-lived organic carbon and sequestered biogenic carbon. Large microphagou s zooplankton are able to ingest microbial-sized particles and thus repacka ge small, non-sinking particles into both metazoan biomass and large, rapid ly sinking faeces. Given the wide spatio-temporal extent of microbial troph ic pathways in the Southern Ocean, large zooplankton that are omnivorous or able to ingest small food particles have a competitive advantage over herb ivorous zooplankton. Krill efficiently transfer carbon to a wide array of a pex predators and their faecal pellets are exported to depth during occasio nal brief sedimentation episodes in spring time. Salps may be a significant link towards some fish (directly) and other apex predators (indirectly) an d, at some locations (especially in offshore waters) and time, they may acc ount for most of the downward flux of biogenic carbon. Large copepods are a trophic link towards fish and at least one whale species, and their grazin g activity generally impedes the export of organic particles to depth. As a result, biogenic carbon is channelled mainly towards apex predators and ep isodically into the deep ocean. Without these original interactions, Antarc tic waters might well be dominated by microbial components and recycling pr ocesses instead of active export from the generally small primary producers towards large apex predators.