The McMurdo Dry Valleys is a polar desert comprised of glaciers, areas of bare patterned ground, glacial meltwater streams that flow in summer and permanently ice-covered lakes in the valley floors. The microbial mats that are abundant in the streams persist through the winter and control the flux of nutrients to the lakes during the summer. In 1994, a monitoring program studying sixteen reaches in thirteen streams was established as part of the McMurdo Dry Valleys Long-term Ecological Research project. This long term dataset characterizes the microbial mat community abundance and location in the channel and overall channel geometry for a period including multiple years of low flows and three historic flood events. The location and channel geometry data were initially collected using total station surveys, with points of interest noted outside and inside the stream channel. In 2010 some reaches were surveyed using terrestrial LiDAR, which provides for much greater resolution than traditional surveying methods. We overlapped the total station data with a data cloud representing the entire stream reach, and a history of channel elevations was generated. We evaluated mat resilience in the context of relative change in streambed level.
Results/Conclusions
In general, five of the stream reaches did not change significantly in the 20 year record, but two reaches which received large sediment loads experienced large variations in streambed elevation. The microbial mats were consistently most abundant, as measured by ash free dry mass, in areas of near zero bed change. Four microbial mat types differed greatly in spatial position and in resilience to bed change. Green microbial mats, which are composed of filamentous chlorophytes, were found between or on the exposed underside of large immobile rocks near the thalweg, and were rarely found in locations with any significant bed change. Conversely, orange mats, which are composed of filamentous cyanobacteria, were found in dynamic areas of the stream bed near the thalweg, with some orange mats found in locations with bed change exceeding 50 cm. Black microbial mats, primarily composed of Nostoc sp., occurred on the stream margins or in seeps with little change in bed elevation. For all four mat types the greatest biomass occurred in locations with little bed change, and the position in the channel was remarkably stable. The two stream sites with large changes in bed elevation had the least coverage of the bed by microbial mats.