The protistan microzooplankton community in the oligotrophic north-eastern Atlantic: Large- and mesoscale patterns

Abstract

We surveyed the oligotrophic waters of the north-eastern Atlantic subtropical gyre to investigate the biomass, abundance, composition and variability of the protistan microzooplankton community. Aloricate ciliates and gymnodinoid dinoflagellates dominated the community, which did not show broad seasonal variations. Large mixotrophic ciliates, with a high carbon content per cell, accounted for a substantial amount of the variability of the standing stock. We found average values of microzooplankton biomass and abundance of 1.1 mg C m-3 and 3.7 × 103 cells l-1, respectively, which are among the lowest measured in marine systems, 5- to 10-fold lower than in the neighbouring waters of the North Atlantic Drift. The mesoscale structures surveyed, a cyclonic eddy and the hydrographical structures over a submarine mount, significantly enhanced microzooplankton biomass and sustained the presence of cells with a carbon content higher than in neighbouring waters. Microzooplankton followed complex patterns of distribution that did not correspond to the variability of primary production or of chlorophyll a.