The importance of benthic microbial production in the lakes of the McMurdo Dry Valleys and how the benthic cyanobacterial mats are adapted to survive and grow in deep water (extreme low light but no freezing) was investigated. Quantity and quality of light penetrating the ice and light attenuation properties of the water column were determined. Samples of benthic mat cores were collected for ... measurements of the response of photosynthesis and respiration to varying light intensities, analysis for chlorophyll a and biomass to enable expression of results in a quantitative manner and the determination of areal concentrations of chlorophyll a, phycobilin pigments and ash free dry mass. Additional cores were preserved for subsequent analysis of pigments, nutrients and carbon content. The distinct morphology of the mats, height and density of pinnacle formations were measured. Samples were collected from a wide range of depth, up to 40 m. The location of photosynthetic activity within the mats and the attenuation of light through microbial mats were determined from samples from all depths and the effects of light limitation on the rate of photosynthetic electron transport by the mats and the mechanism whereby plants cope with excess light was investigated. A series of experiments were also undertaken to determine the fine scale structure of the photosynthetic component of the mats and to relate this to within-mat light attenuation. At each depth, five replicate mat cores were taken for invertebrate counting to investigate their potential role in carbon cycling and to investigate the variable responsible for regulating the distribution of benthic meiofauna. All invertebrates were identified to the lowest level possible.