Water availability controls on community structure of an ephemeral meltwater stream ecosystem in the McMurdo Dry Valleys

The McMurdo Dry Valleys of Antarctica contain many glacial meltwater streams that flow during the summer into lakes on the valley floors.  Many streams have thriving cyanobacterial mats that are freeze-dried in winter and begin photosynthesis when flow arrives.  We studied the community structure in a formerly abandoned channel, which was reactivated by a flow diversion in 1994. Cyanobacterial mats immediately became abundant in the reactivated channel and have remained evident even through cold, low flow summers.  We recently compared the abundance and species distribution of invertebrates and diatoms in the cyanobacterial mats and hyporheic zone during cold (low flow) and warm (high flow) summers. During the warm summer, there were stream sites where the invertebrate abundance was greater in the mats than in the underlying hyporheic sediments. In contrast, during the cold summer the invertebrate biomass was lower in the mats than in the hyporheic sediments. These findings suggest that the optimal micro-habitat for invertebrates in these mats and sediments is partially driven by ephemeral stream hydrology. This limitation on potential invertebrate grazers (which are important nutrient transformers) may account for the accumulation of algal biomass and subsequent nutrient immobilization in the mats over many summers.