Tephra and ecological studies of Limnopolar Lake, Byers Peninsula, Livingston Island.

Limnopolar Lake, Livingston Island, situated close to the tip of the northern Antarctic Peninsula (62°40'S, 61°00'W), has experienced a long history of primary and secondary tephra inputs from nearby volcanic centres. Background levels of tephra in a sediment core collected from the lake were ~30,000-35,000 gdm-1, and were caused by the continual washing in of tephra deposited in the surrounding watershed. Tephra abundances above these levels were assigned to tephra horizons, of which 5 were identified to have occurred over the last 2000 years. Electron probe microanalysis of the tephra shards revealed that the predominant composition of the tephra was basaltic andesite, with few basalt and andesite pyroclasts. Limited numbers of shards with trachy-basalt and basaltic trachy-andesite compositions were also recorded. Deception Island was attributed as the most likely source of the tephra. Eight distinct biological communities were identified in the sediment core on the basis of abundance and distribution of organism remains. Most community shifts over the last ~2000 years appear to be due to direct or indirect effects of tephra deposition into the lake, with tephra abundance levels being an important factor. However, some community shifts can be attributed to climate variability and grazing pressures. The apparent lack of zooplankton species, with only a recent colonisation of the anostracan Branchinecta gainii and possibly the copepod Boeckella poppei, suggests that zooplankton species may not be adapted to surviving in lakes with continual tephral inputs or alternately have recently colonised the island.