DIEL FLOW PULSES DRIVE TRANSPORT OF PARTICULATE ORGANIC MATTER AND DIATOMS FROM MICROBIAL MATS IN GLACIAL MELTWATER STREAMS INTO LAKES IN THE MCMURDO DRY VALLEYS

The permanently ice-covered lakes in the McMurdo Dry Valleys (MDV) of Antarctica are fed by glacial meltwater streams which contain abundant microbial mats. These mats are composed of cyanobacteria, diatoms and other bacteria and persist through the winter in a freeze-dried state. When the mats grow in the summer they experience a dynamic hydrologic regime as streamflow varies on a diel cycle and with weather conditions. During diel peaks in flow these streams transport particulate organic matter (POM) to the lakes, and lake sediments have been shown to contain stream diatoms. We investigated the spatial and temporal dynamics of transport of POM and associated diatoms derived from the scouring of microbial mats in Von Guerard Stream of the MDV. The results show clockwise hysteresis effects in POM concentration over diel flood pulses, which indicates that POM transport is supply limited. The hysteresis effect was modeled using an approach derived from models of sediment transport in streams. Spatial variations in POM transport indicate that patch-scale variations in flow conditions and benthic biomass influence transport which are integrated by downstream transport over several 100’s of meters. Large variations in the POM transport dynamics between different diel flood pulses were found to be related to the time since a resetting flood event and the regrowth of potentially mobile benthic biomass. Similarly the diatom community in the POM was found to vary, with different sources being dominant during periods of high flow. Taken together, these results provide further evidence of the importance of supply limitation in controlling the organic matter flux from streams to the lakes, and facilitate the interpretation of the paleorecord from the lake sediment diatom distributions in the MDV.