A TESTATE RHIZOPOD ASSEMBLAGE IN AN EXTREME ENVIRONMENT: THE ANTARCTIC PERMANENT ICE-COVER LAKE HOARE (TAYLOR VALLEY)

The McMurdo Dry Valleys comprise the largest ice-free area in Antarctica and are characterized by low precipitation rates. Perennial ice cover on the dry valley lakes shelters the amount of solar radiation reaching the water column and reduces the interaction between the atmosphere and the water column. However the ice provides a form of shelter from the harsh Antarctic climate, that maintains the lake ecosystem. Two cores were collected in Lake Hoare from the Taylor Valley and the samples were examined for their thecamoebian (i.e. testate rhizopod) content. The testate rhizopods in these lakes have only chitinous organic tests with no agglutinated material, unlike most testate rhizopods, which use detrital material to build a test. The few species here are referred to the genus Arcella, which have only a chitinous organic wall, unlike most thecamoebians in temperate climates where sediment is plentiful. The testate rhizopod concentration is generally higher in the layers where bacterial/algal mat-fragments were more abundant which are net producers of oxygen. Additionally there were a limited number of ostracod carapaces and possibly a rotifer resting stage. This reflects the sedimentation pattern occurring here where there are intervals with mat growth alternating with sediment accumulation on the ice surface and subsequent fallout to the lake bottom. New data from cores collected at a water depth of over 30m by others in 2002-03 penetrated to 2m depth with the upper 6-7cm being <5000ybp and having mixed layers of organics (diatoms and bacterial and sand), similar to the cores here. However, below 7cm the sediment was partly unsorted, with gravel, sand and gravel mixed with sand, indicating glacial outwash into the lake; this middle unit was dated at 14.5kybp with radiocarbon dates of >20kybp from 135cm to the 2m level.