The palaeobiogeography of the east Antarctic freshwater fauna

The mechanisms and timing of the colonisation of Antarctica by lacustrine freshwater fauna are unclear. Three scenarios are possible: anthropogenic introduction, dispersal to the continent following late Pleistocene or Holocene deglaciation, and survival of preglacial species in lacustrine refugia. Current distributions and dispersal modes can be used to infer the colonisation mode of a species. They cannot, however, be used to determine the minimum age of association between a species and a region, palaeodistributions, or faunal response to environmental change. This knowledge is important for understanding the adaptability and antiquity of the Antarctic fauna. Analysis of faunal microfossils preserved in lacustrine sediment cores can provide direct evidence of the timing and occurrence of these processes. Additionally, informed selection of study lakes allows the minimum age of association to be determined. This study applied this approach to three lakes that differ in glacial history, distance to coast, age and present-day fauna, yet represent a geographically coherent area. Lake Reid, Larsemann Hills (76°23'E; 69°23'8) has a continuous 130 000 year sedimentary record. Microfossils of the cladoceran Daphniopsis studeri and the rotifer Notholca sp. were recovered from every core depth, indicating their continued presence throughout the lake's existence. In contrast, an unidentified copepod became extinct during the Last Glacial Maximum (LGM). This record provides the first direct evidence of a glacial lacustrine refugium for invertebrates in Antarctica and indicates the presence of a relict freshwater fauna on the continent. Holocene colonisation, therefore, need not be invoked. Waterfall Lake, Vestfold Hills (68°32'S, 78°20'E; ca. 100 km from Lake Reid) has contained a diverse faunal community throughout its 6000-year history. Colonisation occurred immediately after development, suggesting a local faunal source. All the species present in Lake Reid (excluding the copepod) were present in Waterfall Lake, suggesting dispersal between the oases or the presence of a glacial refugium in the Vestfold Hills. Lake Terrasovoje (70°33'S, 68°01 'E) is located in the Amery Oasis, parts of which have remained ice free for ca. 2 Myr. The lake's palaeofauna differed almost entirely from those of the coastal lakes. The copepod Boeckella poppei was recorded in nearly all core sections, indicating that its disjunct Antarctic distribution is not the result of anthropogenic introduction, but rather the survival of an ancient relictual population. Complex interactions within the lake were also identified, with local extinction of three species, most likely due to environmental factors. This study indicates that components of the Antarctic freshwater fauna survived throughout the LGM, and provides the first direct evidence for an ancient component to this fauna. In light of these results previous assertions of Holocene dispersal must be questioned. Clear geographic distinctions in faunal distributions and community response to environmental pressures have additionally been identified.