Ephemeral streams of the McMurdo Dry Valleys, Antarctica (MDV), present a unique environment to examine ecological processes without complicating influences of allochthonous materials and human impacts. Numerous ecological and physical/chemical factors influence algal mat standing stock and community composition in streams. In MDV streams, top-down controls such as predation are not considered important due to the low abundance of grazers. As a result, other physical and ecological factors - light, stream flow, competition, and migration – may have a more dominant influence on shaping benthic diatom communities. Previous work demonstrated that historical stream flow conditions influence diatom community composition and diversity in MDV streams. However, the ecological factors influencing algal mat habitats on shorter (seasonal) time scales are not understood. Drift is an important influence on benthic diatom communities in that movement of diatoms in the water column drives mat successional processes through emigration and immigration. The goal of this study is to determine the role of drift in shaping benthic diatom communities and transporting organic material. A drift experiment was conducted in Von Guerard Stream (MDV), where a continuous stream flow and temperature record exists since 1990. A plankton net collected drift material at 4-hour intervals over a 28-hour period.

Results/Conclusions

Von Guerard stream experiences a diurnal flow pattern in which stream flow can fluctuate ten times over a 24-hour period. It was hypothesized that the daily flood pulse mobilizes algal mat communities along the stream margins and that these mats contribute most of the biomass and migratory organisms to the water column. Algal biomass and Chlorophyll-a increase by over 200 times and 70 times (respectively) during the daily flood pulse and therefore support this hypothesis. Water chemistry remained fairly constant throughout the sample period. A diatom metric is being developed to distinguish the relative contributions of diatoms and organic material from in-stream and marginal mat communities. Preliminary diatom composition data suggest that diatoms present in the water column differ from benthic diatom community composition, indicating that drift contributes to algal mat successional processes via species-specific differences in emigration rates.