Climate change and microbial mats in Antarctic meltwater streams: Modeling the scouring of mats by flood pulses

Background/Question/Methods

The McMurdo Dry Valleys of Antarctica have experienced three warm summers over the past 20 years that have caused high flows in the glacial meltwater streams and may presage conditions that will occur under an overall warming climate. These flood events have scoured the perennial cyanobacterial mats in these streams. Mat regrowth progresses over several summers following the flood events. We investigated the spatial and temporal dynamics of the transport of particulate organic matter (POM) derived from the scouring of benthic algal mats in a glacial meltwater stream, Von Guerard Stream in Taylor Valley, which is studied by the MCM-LTER project. Following the approach used in sediment transport modeling, we developed a model that accounts for the interannual variation in POM transport rate by considering variations in the available pool of potentially mobile biomass due to differences in the preceding flow regime. We also used the diatom communities in the POM as tracers for the source of the microbial mat material mobilized under differing flow conditions.

Results/Conclusions

The sampling results from individual peak flows show hysteresis effects in POM concentration resulting from unsteady flow conditions, which indicates that the source of POM is limited. The POM transport model developed from these data quantifies the general result that benthic algae growing in dry valley streams are well adapted to the natural variability of flow and that removal due to normal daily peak flows is limited. Spatial variations in the POM transport rate indicate that in-channel variations in flow conditions and potential POM sources influence transport. The differences in POM sources was confirmed by differences in the diatom communities in POM following flood events. Inter-annual variations in the POM transport dynamics are great and further indicate the importance of the source of mobile mat material in controlling the POM flux. Model-based estimates of the total annual POM flux for Von Guerard Stream and other streams show that the organic matter load derived from the removal of algal mats during flood events is less than half of the total standing stock of benthic biomass and is low in comparison to annual primary production in the receiving closed basin lakes of the McMurdo Dry Valleys.