The McMurdo Dry Valleys of Antarctica contain many glacial meltwater streams that flow during the austral summer and link the glaciers to the lakes on the valley floors.  Many streams have thriving cyanobacterial mats that are freeze-dried through the winter and begin photosynthesis with the onset of flow.  We studied the community structure in a formerly abandoned stream channel, which was reactivated by a flow diversion in 1994. Cyanobacterial mats responded immediately to reactivation and have remained present and productive even through cold summers with very low flow.  We recently compared the abundance and species distribution of invertebrates and diatoms in the cyanobacterial mats and sediments during cold (low flow) and warm (high flow) summers in January 2008 and January 2009, respectively. We collected information on stream and sediment  characteristics that could potentially influence invertebrate assemblage, such as depth to permafrost, chlorophyll a content, and nutrient status.  We hypothesized that invertebrate diversity would correlate with areas along the stream that were continuously moist, high in available carbon and chlorophyll a content, and low in salinity.  We expected a positive relationship between invertebrate abundance and depth to permafrost, as thawing of this layer is expected to stimulate microbial resources for invertebrates. 
Results/Conclusions

Invertebrate abundances did not correlate strongly with chlorophyll a content, nutrient status, pH, salinity or depth to permafrost in the cold summer. Preliminary data from the warm summer show an increase in the depth to permafrost (P < 0.05), indicating a potential increase in the zone of biological activity. However, invertebrate abundances did not respond significantly to this change. Stream flow may considerably alter movement and distribution of invertebrates between aboveground and belowground habitats. During the cold summer invertebrate abundances were low in the mats than compared to the underlying sediments, while during the warm summer there were sites where invertebrate abundance was greater in the mats than in the underlying sediments. These findings suggest that the optimal habitat for invertebrates in mats and sediments is partially driven by stream hydrology. This limitation on potential grazers (which are important nutrient transformers) may account for the accumulation of algal biomass and subsequent nutrient immobilization in the mats over many summers. Given the influence of the biota on nutrient cycling in these areas, it is essential to understand the relationship between sediments and aboveground vegetation, especially in aquatic habitats of polar deserts that may increase in size and importance with climate change.