ECOGEOCHEMICAL CONNECTIONS: WEATHERING OF SILICATE MINERALS CONTROLS DIATOM BIODIVERSITY IN MICROBIAL MATS IN GLACIAL MELTWATER STREAMS OF THE MCMURDO DRY VALLEYS, ANTARCTICA

The McMurdo Dry Valleys of Antarctica is comprised of alpine and terminal glaciers, large 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. As part of the McMurdo Dry Valleys Long-Term Ecological research project, we have studied dominant biogeochemical processes in glacial meltwater as these ecosystems respond to a sustained 18 year cool period with low flows. Recently this cooling trend has been interrupted by three "flood events" during sunny, warm summers. Water quality monitoring and reactive transport modeling have shown that throughout this period rapid weathering of silicate minerals in the hyporheic zone accounts for the downstream increases in Si concentration which are observed in the longer streams. Many of these streams contain thriving microbial mats comprised of cyanobacteria and endemic diatoms, the most diverse group of eukaryotic organisms in the valleys. Of the 45 diatom taxa, some common taxa are heavily silicified, while others are only lightly silicified. By comparing diatom communities in streams which flow every summer with those in streams that only flow during flood events, we found that hydrologic flow regime acts as a strong environmental filter on diatom community composition. Following the first flood event in 2001/02, mat biomass was two-fold lower due to scouring and recovered over several years, with lesser declines following the subsequent floods. In the longer streams, the diatom community composition remained stable through the flood events, whereas in two of the shorter streams, Green and Bowles Creeks, the diatom community shifted after the first flood event to a greater abundance of lightly silicified taxa. One mechanism driving this shift in diatom community composition in shorter streams could be the greater dilution of the Si supply from hyporheic weathering under high flows in shorter streams. Given that the stream diatom community is well preserved in the 40,000-year sediment record from the receiving lake, this insight into biogeochemical controls on diatom community composition provides insight into both the hydrologic and biogeochemical history of the region.