Hydrologic controls on stream microbial mat abundance over two decades in the McMurdo Dry Valleys, Antarctica
In the McMurdo Dry Valleys of Antarctica, glacial melt produces streamflow for several weeks to several months of the Austral summer. The magnitude of flow varies by year, and a period of low discharges were observed during the 1990’s, but have since increased with several summers of abnormally high flows. Living within these streams are three different perennial microbial mat types which persist in a desiccated state during winter, and are revived annually with hydration. Different mat types are found in different parts of the streambed, and because of geomorphology, some streams have high benthic coverage of mats, while some are very sparse. Using 20 years of monitored biomass data collected by the McMurdo Long-term Ecological Research program, we investigate the following questions: 1) How have different mat types responded to flow variability over the last two decades, and 2) do these patterns differ in streams with different streambed morphologies? We evaluate the hypotheses that mat biomass is controlled by high and low flows, geomorphology, and the habitat use by creating smoothed trends for mat biomass and hydrologic variables, and comparing resulting curves with Pearson correlation coefficients.
Biomass of all mat types collectively decreased during low flows until interrupted by a “flood” summer in 2001-2002, and has increased over the last decade corresponding with higher discharges. When mat biomasses from all transects were analyzed together over the study period, green mats living within the stream channel were strongly correlated with hydrologic variables and increased with greater discharges, while benthic orange mats and marginal black mats showed weaker correlations. Relationships between hydrologic variables and orange and green mat biomass improved substantially when separated by high and low coverage transects, though black mat correlations remained low. Additionally, orange mats from high coverage streams were more prone to scour under high flows, while orange mats in low coverages streams increased in biomass. These results show that hydrologic regime is strongly related to microbial biomass in Dry Valley streams, though mat type, habitat use and geomorphology must be taken into consideration. We suggest that our findings may be useful in predicting the ecological response to a changing hydrology in polar and arid desert streams in the future, and may have implications for the ecology of the early Earth.