Cold comfort for change: Stream mats as biological indicators of ecosystem processes in the McMurdo Dry Valleys,...

Glacier-fed streams (GFSs) make ideal systems for studying climate-related changes. Some of the best-studied GFSs are found in the McMurdo Dry Valleys (MDVs) of Antarctica, one of the Earth's coldest and driest deserts. Despite their harsh and isolated nature, MDV GFSs represent an oasis of life in a landscape visually devoid of it, with biomass dominated by photosynthetic microorganisms (including chlorophytes, cyanobacteria, and diatoms) and manifesting as benthic "mats." Mats form the basis of MDV GFS ecosystems, drive biogeochemical cycles, and harbor high proportions of the regional biodiversity. Furthermore, the biomass and composition of these mats respond to environmental fluctuations, making them ideal bioindicators for ecological monitoring. In this review, we have (1) distinguished the three major photosynthetic mat types by their taxonomic structure, habitat use, and elemental composition; (2) demonstrated how mat type distribution, coverage, and biomass are dictated by a combination of geomorphology, suspended sediment loads, and hydrology, among other factors; (3) introduced MDV diatoms as model organisms for investigating mat community assembly; and (4) speculated on how the biomass, community structure, and functional process rates of different mat types will change in a warmer and more connected world. Synthesizing this information, we suggest future opportunities for research, with the most promising avenues centering upon questions, methodologies, and scales that would have been inconceivable for the Heroic Age explorers that discovered them, ranging from studies of gene expression to cataloging changes in mat abundance by satellite.