Ultrastructure, morphology and flux of microzooplankton faecal pellets in an east Antarctic fjord

Abstract

Small copepods and protozoa are major contributors to heterotrophic biomass in Antarc- tic waters. They produce small (<300 µm) faecal pellets, the fates of which are largely unknown. We examined the distribution and abundance of microzooplankton and small faecal pellets in Ellis Fjord, east Antarctica. We determined statistical relationships between the abundance of microzooplankton and pellets, and examined pellet morphology and ultrastructure using light and scanning electron microscopy. Our results indicate species-specific differences in the morphology and fate of pellets produced by small copepods: Oithona similis and harpacticoid pellets were retained in upper waters, while Oncaea curvata and Paralabidocera antarctica pellets sank to depth. Protozoan pellets did not sink to depth irrespective of their source or morphology and despite the fact they can be larger than those produced by small copepods. The majority of microzooplankton pellets, composed of phyto- plankton that otherwise may have directly sedimented to depth, was retained in near surface waters and probably recycled and remineralised. Despite producing faecal aggregates, heterotrophic activ- ity of most microzooplankton do not contribute to vertical flux but instead support respiration of mat- ter in upper waters. This may reduce the vertical flux of particulate matter to depth, thereby reduc- ing the capacity of Antarctic waters to act as a carbon sink, with implications for global climate.