1. Laminated microbial mats are important ecosystem components of perennially ice-coveredAntarctic dry valley lakes. In order to understand better their response to changing environment, wemade observations and carried out a manipulation experiment to determine their response tovariations in irradiance in Lake Hoare (77°380S, 162°530E). 2. Ice transparency was the most variable parameter that affected benthic light dose, both spatiallyand between years. Patterns of lamina accrual corresponded to irradiance history, with laminae thatwere initiated in high transmission years thicker than those from low transmission years. 3. A shading experiment confirmed that accrual of lamina thickness, calcite precipitation and ash-free dry mass were determined by irradiance, but photosynthetic biomass and phototrophic speciescomposition were less affected. 4. Buried laminae decomposed only slowly over time, with potentially viable phototrophs manylaminae down into the microbial mat. Decay rate increased only slightly with shading. 5. We conclude that the microbial mats in Lake Hoare are characterised by remarkable stability, withslow accumulation rates and turnover of biomass over time. Photosynthetic biomass and speciescomposition appeared to be stable across long time periods, with interannual variation in laminationpattern due to differential accumulation of extracellular polysaccharide and representing the visibleexpression of annual growth conditions.