Summer plankton beneath the McMurdo Ice Shelf at white Island, McMurdo Sound, Antarctica

Abstract

The zooplankton of the under-shelf-ice eco-system at White Island (78°10′S, 167°30′E), McMurdo Sound, Antarctica was investigated during December 1976 and January 1977. The water column was sampled through a hole in the McMurdo Ice Shelf over a water depth of 67 m. Seawater temperatures under the ice shelf ranged from −1.91 to 1.96°C. Dissolved oxygen levels ranged from 5.0–6.05 ml l⁻¹ in early December to 4.65–4.8 ml l⁻¹ in late January. Current speeds of up to 0.13 m s⁻¹ were recorded at a depth of 50 m and a predominantly northward flow was detected. Light levels under the shelf ice were low with less than 1% of the incident light being transmitted to a depth of 3 m. No chlorophylla was detected within the water column throughout the investigation. Mean zooplankton biomass values in the water column ranged from 12 to 447 mg wet weight m⁻³ and were similar to values recorded elsewhere from Antarctic mshore waters, but were very much higher than those recorded from under seasonal sea ice in McMurdo Sound. Thirty-two zooplankton species were recorded including 1 ostracod, 21 copepods (10 calanoids, 3 cyclopoids and 8 harpacticoids), 4 amphipods, 2 euphausiids, a chaetognath and 3 pteropods. Larvae of polychaetes and fish were found on some occasions. The species composition in general was similar to that recorded from McMurdo Sound and other Antarctic inshore localities. Among the Copepoda, however, there were a number of species, especially among the Harpacticoidea, that have not been found previously in McMurdo Sound and the Ross Sea, but that are known to be associated with ice in other localities in Antarctica. Two recently described species are known only from White Island. They were present in the water column but were most abundant in the surface water of the tide crack where they were the most abundant zooplankters. The tide crack, which probably is an extension of the under-ice habitat, is apparently a significant nursery area for amphipods and copepod species.