The pond ecosystems of the McMurdo Ice Shelf contain abundant growths of algae and cyanobacteria. The role of the anaerobic sediments underlying the cyanobacterial mats was investigated. Sediment samples were collected from several ponds to determine in situ rates of methanogenesis, methane oxidation and ammonium turnover. Sub-samples were used to determine sulfate reduction. All experiments were ... carried out in triplicates with initial samples preserved and used for analysis. In the laboratory, sediment from six ponds were subjected to experiments to compare rates of sulphate reduction with methanogenesis. Samples of sediment and mats were collected from the six study ponds and tested for the effects of freeze-thaw and salinity on anaerobic microbial processes. Studies were conducted over several field seasons from the 96-97 season to the 99-00 season.