Mesozooplankton biomass, abundance and community composition in the Ross Sea and the Pacific sector of the Southern Ocean

Due in part to its remote location, the zooplankton of the Ross Sea and adjacent waters is poorly characterized. Very little depth-integrated information exists for this region, including measurements of mesozooplankton biomass, abundance and community structure throughout the water column. Furthermore, because large-mesh nets are often used, small copepods have been undersampled. Mesozooplankton research during the New Zealand International Polar Year-Census of Antarctic Marine Life voyage attempted to address these knowledge gaps. Depth-stratified net sampling was conducted from the surface to the seafloor at 11 stations in the Ross Sea and the adjacent Antarctic Circumpolar Current region of the Southern Ocean. Apart from high, localized contributions from pteropods and salps, mesozooplankton was numerically dominated by small calanoid and cyclopoid copepods, and densities were highest near the surface. Maximum mesozooplankton densities reached 640 ind m(-3), and those of Oncaea spp. and Oithona spp. reached 111 and 256 ind m(-3), respectively. On the Ross Sea shelf, Ctenocalanus sp. made significant contributions to total mesozooplankton numbers, while Oithona similis was highly abundant on and around Admiralty Seamount. On the Ross Sea slope, mixtures of Oithona spp. and Oncaea spp. dominated, and at the Scott Seamounts, mixtures of Oithona spp. and Ctenocalanus sp. occurred. Total water column, depth-integrated mesozooplankton biomass ranged between 0.6 and 7.1 g C m(-2). Mesozooplankton biomass in the Ross Sea was at the higher end of previously recorded levels in the region and may rival that of productive sub-Antarctic regions.