Copepod Assemblages in the Water Column and Drifting Sea-Ice Floes in the Ice-Edge Region in the Indian Ocean Sector of the Southern Ocean During the Austral Summer

The ice-associated copepods Stephos longipes, Paralabidocera antarctica, and Drescheriella glacialis are the dominant species inhabiting Antarctic sea ice. They influence the pelagic ecosystem after being released into the water column when the ice melts from spring to summer in the marginal ice zone (MIZ), although less is known about those in the off-shelf region. We investigated the occurrence of copepods in drifting sea-ice floes and the adjacent water column off Vincennes Bay in the Indian sector of the Southern Ocean in mid-summer (January) of 2014 and 2016–2018. Sea-ice samples from the ice-growth season (February–March) collected off Australia’s Mawson Station (2016, n = 1) and Cape Darnley (2018, n = 2) provide some comparison with the ice-melt season. In the ice floes off Vincennes Bay, S. longipes and harpacticoid nauplii were abundant, but also highly variable (2016: 2.2 ± 3.5 [× 104 ind. m−3], n = 8; 2017: 1.1 ± 5.4, n = 11; 2018: 0.94 ± 1.81, n = 10). Copepod nauplii were very abundant at 0–5 m depths (maximum of 8.1 × 102 ind. m−3), but absent at depths below 35 m; this implies that copepods disappear rapidly from the surface layer after being released, mainly due to predation/non-predation death or development from the nauplius to the copepodite stages. Overall, the results indicate that sea ice forming in shelf regions transports some of the ice-associated copepods further north, which supplies food to pelagic predators, although S. longipes is also a primary consumer in the MIZ.