Ross Sea deep-ocean and epipelagic microzooplankton during the summer-autumn transition period

Microzooplankton populations are key participants in the transfer, recycling and export of carbon in epipelagic waters, but little is known about their role, abundance and diversity at depth, especially in the Antarctic region. We surveyed microzooplankton populations and their potential prey in the Ross Sea area from 66 degrees S to 77 degrees S during the New Zealand IPY-CAML survey, 12 February to 11 March 2008. Samples were taken throughout the available water column at depths between similar to 5 and 3400 m, allowing us to compare deeper, largely unknown, mesopelagic and bathypelagic waters with the traditionally studied epipelagic zone. Microzooplankton diversity and abundance were highest in the high chlorophyll a, diatom-dominated waters of the epipelagic zone, but occasionally, peaks did occur in the upper mesopelagic before populations declined with depth. Ciliates and dinoflagellates declined more rapidly than heterotrophic nanoflagellates, which led to the latter dominating at depth. Ciliate populations in epipelagic waters were correlated with bacteria, picophytoplankton and chlorophyll a, but these potential prey did not correlate well with heterotrophic nanoflagellates in deeper or low-biomass waters. Despite their rapid decline with depth, due to the large volume of deeper waters, the total integrated microzooplankton biomass exceeded that found in the epipelagic zone. This indicates that the deeper waters of the Ross Sea, at this time, contained a significant pool of microzooplankton biomass. These deeper populations are likely to aid in the recycling and remineralisation of sinking phytoplankton biomass traditionally thought to be exported to the ocean floor.