Picoplankton consumption supports the ascidian Cnemidocarpa verrucosa in McMurdo Sound, Antarctica

Abstract

Polar marine ecosystems commonly have a seasonal pulse of primary productivity with large diatoms or prymnesiophytes dominating. Along with benthic production (i.e. micro - phyto benthos), the annual phytoplankton bloom provides an essential source of food for several trophic levels, including many invertebrate communities. The oceanographic and productivity patterns in McMurdo Sound, Antarctica, result in benthic communities that include high-density assemblages of active and passive suspension feeders. For many years, it has been assumed that these benthic suspension-feeding communities went into a period of quiescence during the austral winter and spring in response to the low food and chronically low temperatures as a strategy to conserve energy. There is increasing evidence, however, that suspension feeders can feed throughout most of the year, with many using picoplankton (0.2 to 2.0 μm) as a food source. It is now recognized that picoplankton, especially heterotrophic prokaryotes, are a diverse and important component of the Southern Ocean bacterio plankton community and a dominant component of the plankton during austral winter and early austral spring in McMurdo Sound. Here, we show that the common ascidian Cnemidocarpa verrucosa consumes picoplankton prior to the annual spring bloom. Differences in food availability at different sites (Cape Armitage versus Cape Evans) and differences in filtration efficiencies on different fractions of the plankton community result in this ascidian acquiring significantly more carbon and energy at Cape Evans, where higher densities of C. verrucosa reside. This study emphasizes the importance of picoplankton as a food resource for the Antarctic benthic suspension-feeding community.