The impacts of drought on the microbial states and processes in the hyporheic zone of temperate streams

Intermittent streams as well as extreme events are expected to become more common as the climate changes. Therefore, it is important to understand how drought affects the biofilms that are essential for nutrient and DOM processing within streams. Previous work has largely focused on Mediterranean streams. This project evaluates how drought affects the state and processes of the microbial biofilms in the hyporheic zone of temperate intermittent streams. The experiment was conducted with outdoor experimental hyporheic flumes (5 m long, 0.6 m wide, 1.2 m deep) that were allowed to fall dry for periods ranging from 4 to 100 days. Sediment was sampled before drying, during the drought, and at several time points after rewetting. Samples were analyzed for extracellular enzymatic activity, respiration, bacterial growth, live to dead cell ratios, bacterial abundances, and extracellular polymeric substances.

Extracellular enzymatic activities remained unaffected by the drought in the hyporheic zone but showed an increase on the surface during the dry phase. Upon rewetting, the enzymatic activities generally fell to pre-drought levels on the surface. Extracellular polymeric substances also remained unaffected by drought in the hyporheic zone. However, surface values for extracellular polymeric substances showed a similar pattern to enzymatic activities during the longer dry periods (70+ days) and subsequent rewetting. These results indicate that the hyporheic zone retained enough moisture in the sediment to continue functioning, while the surface sediment was impacted by the loss of flowing water.