The distribution of zooplankton in McMurdo Sound - interannual variation, spatial variation and depth stratification

Building on previous studies of zooplankton distribution and abundance, a programme was planned to i) augment the interannual variability off Scott Base, ii) assess variability in surface waters on both sides of McMurdo Sound and iii) acquire extra information on depth distribution in relation to fish feeding studies. Two types of sampling was conducted at 4 sites, integrated vertical net samples off Scott Base, Marble Point and Cape Royds and depth restricted pump samples off Danger Slopes. The aim was to investigate in a quantitative manner, the distribution of net caught zooplankton within McMurdo Sound. To determine interannual variation, 20 surface samples were taken at sites off Scott Base where sampling was conducted in the 1985 and 1987 seasons. Sample variation was compared across the years. No difference could be detected between years. To determine spatial variation, 10 surface samples were collected from Cape Royds (approximately the same place collected in 1985) and at Marble Point (first season of sampling). Comparison of these sample sets, with each other and with the shallow samples from off Scott Base, revealed no quantitative differences between those taken off Scott Base and Marble Point. However, those from Cape Royds had 3 species of copepods in significantly greater densities and one in lower densities. This difference at Cape Royds supports the difference found between this site and Scott Base in 1985. To determine depth stratification, a pump was used to draw sea water from discrete depths through a 20 m water column off Danger Slopes (just north of Arrival Heights). The water was filtered and 3 replicate samples were obtained for each of 3 depths (3.2m, 7.8m and 14.6m) and 5 samples from just above the sea bed (19.0 m). No consistent difference could be found. Observations of the water column were made with the US observation chamber, from which large zooplankters could be readily observed throughout the water column. There appeared to be no consistent variation in their distribution with respect to depth, or any vertical transport.