Composition and abundance of zooplankton under the spring sea-ice of McMurdo Sound,Antarctica

Summary Zooplankton was sampled through holes in the sea-ice of McMurdo Sound from 8 November to 10 December, 1985. Replicated vertical hauls were made to 100 and 300 m off Pram Point in the inner Sound, near the edge of the permanent McMurdo Ice Shelf. The zooplankton was sparse, averaging 2.5 mg/m³ wet weight. The numbers of individual species varied between catches, depths, and occasions. Generally, small copepods, particularly Oithona similis, Ctenocalanus citer and Oncaea curvata, numerically dominated the catches, and higher densities of these were present in the shallower 100 m layer. Deeper hauls contained higher numbers of larger crustaceans, particularly copepods Metridia gerlachei, Calanoides acutus and Euchaeta spp., ostracod Conchoecia belgicae and euphausiid Euphausia crystallorophias. Pteropods Limacina helicina and Clione limacina were also consistently caught, but in equal densities in 100 m and 300 m hauls. Numerous other plankters were caught in low numbers, including amphipods, chaetognaths, medusae, radiolarians, and larval nemerteans, barnacles, shrimps, polychaetes and echinoderms. Comparative samples from 40 km further north, off Cape Royds and near the sea edge of the fast sea-ice in Wohlschlag Bay, and to 100 m deep, contained a similar species diversity to those near the McMurdo Ice Shelf, but always with higher densities of L. helicina. On the last sampling occasions, when microalgae were conspicuous under the ice off Cape Royds, there were increased densities of microcopepods and Paralabidocera antarcticus, indicating different ecosystem processes from the inner Sound location.