The waters of Lake Vanda, Wright Valley have risen at a rate of 1m per year. The rise was largely attributed to increased flows in its only significant inflow, the Onyx River. The effects of this increase in flow on the physical, chemical and biological aspects of the river ecosystem were determined. River flows were monitored at the Lower Wright gauging station and a discharge record was ... obtained. Point quadrat analysis was used to determine the percentage cover of rock, sand and gravel, boulders, channels, flooded boulders and extent of microbial mat. The distribution of nutrient sources and sinks in the river system was investigated by nutrient profiling of the water as it flowed over the wide meandering section of the river (temperature, nutrient concentration and total and organic suspended solids collected daily over the period from first flow to when the river began to refreeze) and examining the diel variation of nutrient concentration in the river (using a longitudinal transect and sampling sites) and quantifying the distribution of major mat communities, their growth rates and the effects of the microbial mat communities on the nutrient concentration (biomass, pigment analysis and species composition). Observations on communities from the Onyx River including one longitudinal transect of nutrient concentration, the deployment of artificial substrates for long term colonisation experiments and the determination of light-photosynthesis relationships (rates of change of oxygen concentrations in sealed vials which controlled light and temperature conditions) in mat communities was investigated. To assess the role of the microbial mat communities as sources and sinks for nutrients experiments investigating nutrient release following diel (freeze-thaw) freezing, re-wetting nutrient release, nutrient uptake by growing mats and nitrogen fixation (rates of nitrogenase activity and phosphorus limitation on nitrogenase activity) were conducted.