Our paper describes the ecological controls on algal-mat diatom communities in the dynamic stream ecosystems of the McMurdo Dry Valleys in Antarctica. Dry Valley diatom communities are relatively diverse, and nearly ½ of the taxa found in these mats are considered endemic. Diatom community composition was examined in 5 streams in Taylor Valley during a 15-y cooling period that included a discrete flood event. Two hydrologic variables, total annual discharge and historical variation in discharge, gave the most parsimonious model of among-stream and interannual variation in diatom communities. Algal-mat biomass and chlorophyll a concentrations decreased after the flood, which occurred during the 2001/2002 summer season. Most algal-mat diatom communities recovered quickly after the flood. However, Green Creek, a relatively high-flow stream with low historical variation in discharge, appears to have experienced a persistent diatom community shift toward increased relative abundance of small, generalist species. Diatom relative biovolume, a proxy for the size of diatoms within a sample, was negatively correlated with stream flow, such that higher-discharge streams contained greater relative abundances of smaller diatoms than lower-flow streams. Therefore, diatom size may play a role in determining the distribution of a species in these streams and may be useful for monitoring environmental changes. Our study demonstrates the importance of understanding factors affecting ecosystem resilience, especially in polar regions, which are experiencing rapid climate changes.