Lake Abraxas (68" 29' S., 78' 17' E.) has been cut off from the sea for c. 6000 yr. In December, 1985, it had an ice cover 1.38 m thick, a thermocline at 5.2 m, and zero oxygen at 17.7 m and below. The salinity of the oxylimnion was in the range 14-19 g l-'. The calanoid, Paralabidocera antarctica (I. C. Thompson), had a maximum density just above a thermocline at 5.0 m, at which depth adult males outnumbered females almost 3 : 1. Immediately beneath the ice adult females outnumbered males 4 : 1. Immature copepodite stages were more abundant relative to adults near the oxycline than at higher levels. The planktonic harpacticoid, Amphiascoides sp., had a maximum density close to the oxycline at 17.2 m.

No abstract

Colonisation of Lake Fletcher, a hypersaline, meromictic lake in the Vestfold Hills, Antarctica, by the calanoid copepod Drepanopus bispinosus, the cyclopoid copepod Oncea curvata and an undescribed cydippid ctenophore is discussed. In 1978, salinity directly under the ice was 66x, and repeated net hauls found no zooplankton. In 1983, adults of D. bispinosus were found, and in 1984, a reproductively active population of this species. Surface water salinity in 1984 was 56x,. During winter 1986, surface salinity was 54& and three zooplankton species (0. bispinosus, 0. cur&a and an undescribed cydippid ctenophore) had established populations in the lake. In 1986/87, high tides caused nearby Taynaya Bay to flood into the lake, and three further species (the calanoid, Paralabidocera antarctica, and two harpacticoids, Harpacticus furcatus and Idomene sp.) were found in the lake. It appears that periodic flooding after 1978 caused a salinity decrease in the lake from 66 to 54x,, and this enabled some invertebrate species to maintain year-round populations, whereas others require marine incursions to re-establish summer only populations.