Photosynthesis rates of microbial mats were measured at a range of irradiances to determine the performance of mats at low irradiances at Lake Hoare and Fryxell. Underwater irradiance measurements were taken under the ice and through the water column. Photosynthesis was measured in situ using PAM fluorometry. Profiles along the depth gradient in Lake Hoare and Lake Fryxell were sampled with benthic microbial mats analysed for pigments (HPLC), species composition, O2 and H2S profiles and ash free dry mass. Water samples were analysed for DRP, NH4-N, NO3-N, TDN and TDP.