The vertical distribution and abundance of copepod nauplii and other micro- and mesozooplankton in the seasonal ice zone of Lützow-Holm Bay during austral summer 2009

The Antarctic seasonal ice zone (SIZ) is an ecologically important region of the Southern Ocean. However, the abundance and vertical distribution of micro- and mesozooplankton in this zone, in particular the contribution of nauplii of smaller copepod taxa, are poorly understood. We investigated the vertical distribution, abundance, and species composition of micro- and mesozooplankton in the SIZ of Lützow-Holm Bay, East Antarctica, in January 2009. Zooplankton samples were collected using a 60-µm mesh closing net (appropriate to estimate the quantitative abundance of copepod nauplii) vertically hauled from three depth layers (0–100, 100–200, 200–500 m). Small calanoid copepods, Oithonidae, and Oncaeidae were dominant among the zooplankton communities in this region. Cluster analysis revealed that the communities were separated by depth into epipelagic (0–200 m) and mesopelagic (>200 m) groups. Distribution patterns in the 0- to 100-m layer changed at the sea ice edge, but showed no clear relationship with frontal structures. Nauplii of copepods including Oithona spp., Oncaea spp., and Calanoida were major contributors to total zooplankton abundance. The abundance of these nauplii in the surface layer was high in open water and the marginal ice zone but low in the pack ice zone where chlorophyll a concentrations were high. Low water temperature associated with sea ice, rather than food availability, may have caused the low copepod nauplii abundance observed in the surface layer within the pack ice zone. These findings provide valuable background information to monitor variations in zooplankton community structure in response to changing environmental conditions.