Title: Glacial meltwater streams in the McMurdo Dry Valleys, Antarctica: ecological drivers of diatom biodiversity
The McMurdo Dry Valleys in East Antarctica is a harsh landscape of alpine and terminal glaciers, expanses of patterned ground, and ice-covered lakes in the valley floors, which are linked by glacial meltwater streams that flow during the austral summer. Perennial microbial mats composed primarily of cyanobacteria and diatoms are abundant in many streams. As part of the McMurdo Dry Valleys Long-Term Ecological Research (MCMLTER) project, about 17 first and second order streams have been monitored since 1994, with measurements of flow, water quality and the abundance and composition of microbial mats. Diatoms are the most diverse eukaryotic group present in the Dry Valleys and many of the over 50 diatom taxa are endemic to the Antarctic Region. In addition to the stream microbial mats, diatoms are present in cryoconite holes on the glaciers surfaces and in aeolian material transported throughout the valleys by the strong winds. The MCMLTER study has encompassed an extended cooling period in the 1990’s associated with the ozone hole followed by a warmer period during which "flood events" have occurred. Interannual variation in flow regime strongly influences the composition of the diatom community. For example, a few endemic taxa are abundant in wetland systems that only become active during flood events, and are commonly found in low abundance in other streams. During the recent floods, the microbial mats were scoured from the streambed and mat regrowth occurred slowly over several summers. Laboratory experiments with diatom taxa isolated from the streams have confirmed the slow growth rate and showed that thermal optima and tolerances varied among taxa. Overall, understanding the relationship between the diatoms in the mat communities and environmental change is useful for interpreting the record of the stream diatoms preserved in lake sediments and for considering future scenarios for the Dry Valleys.