Extreme streams: flow intermittency as a control on diatom communities in meltwater streams in the McMurdo Dry Valleys, Antarctica

In the McMurdo Dry Valleys of Antarctica, stream biota is limited by the brief availability of liquid water. The benthic microbial mats harbor diatoms that have adapted to hydrologic stresses, including numerous endemic species. We found a strong relationship between diatom community composition and flow intermittency in a data set including seven streams that spanned a gradient in flow intermittency. In particular, two genera represented by numerous endemic species in Dry Valley habitats, Hantzschia and Luticola, had high abundances in moderately and highly intermittent streams, respectively. The Shannon Index of diversity was greatest in streams with intermediate flow intermittency, with lower diversity in more stable streams resulting from lower evenness, and lower diversity in highly intermittent streams resulting from lower richness. These results indicate that multiple metrics of biodiversity may be useful in assessing the response of diatom communities to changing hydrologic regime. We propose that flow intermittency acts as a species filter that increases habitat heterogeneity in Dry Valley streams and may allow endemic species to persist. Future Antarctic warming may alter diatom community composition and habitats that act as refugia for desiccation-tolerant taxa.