Mesozooplankton size-selectivity and grazing impact on the phytoplankton community of the Prince Edward Archipelago (Southern Ocean)

Abstract

Dense phytoplankton blooms are often found during austral summer and autumn in the Prince Edward Archipelago, dominated by the large diatoms Chaetoceros radicans and Rhizosolenia curvata and by the silicoflagellate Dictyocha speculum. Nano-phytoplankton is preferentially selected by all taxa. Only two euphausiid species appear to have a significant positive selection for the smaller netplankton (20-37 μm). When phytoplankton biomass is low, the zooplankton community ingests 33-48% of the euphotic zone chlorophyll a per day, or 76-81% of daily primary production, but during bloom conditions the grazing impact is reduced to only 5-10% of chlorophyll a concentration and 9-17% of primary production per day. Preferential selection for the smaller cells may have an energetic significance as nanoplankton seems to represent a food source of higher nutritional value than bloom-generating diatoms. The bulk of the phytoplankton blooms presumably sinks out of the photic zone and provides an important transfer of biomass from the pelagic and the benthic subsystems. -from Author