Climate change is expected to alter precipitation and surface water flow, increasing drought and decreasing stream permanence. This will expand non-perennial streams globally, impacting resident microbial communities and their ecosystem functions, such as primary production, decomposition, and nutrient cycling. To assess microbial structure and carbon cycling responses to hydrologic dynamics on different scales, we conducted an incubation experiment using sediments from perennial and non-perennial stream reaches in the US Rocky Mountains (RM), Great Plains (GP), and Southeast Forests (SE). Sediment samples were collected from pool and riffle habitats and subjected to three treatments: continuously wet (saturated), continuously dry (unsaturated), and fluctuating between the two (flux). Microbial respiration and methane production were tracked via carbon dioxide (CO2) and methane (CH4) concentrations using a Picarro spectrometer.
We predicted that under fluctuating saturation, microbial activity would be lower in sediments from perennial reaches and regions with more stable flow, due to microbial adaptation to stable hydrology. Aerobic respiration responses varied among region (P<0.001), and were weakest from the least perennial location (GP), where there were no reach or habitat influences on lab treatment effect. CO2 production was lowest in unsaturated sediments, and highest under saturated conditions in all GP and MW sediments (P<0.001), but not in non-perennial SE sediments (P=0.003). Habitat (pool or riffle) influences were inconsistent and may be more related to local sediment texture and organic matter content. CH4 efflux was greatest in saturated non-perennial sediments (P<0.010). Overall, aerobic respiration was less sensitive to water fluctuation in sediments from regions with lower flow permanence, but local dynamics were also important, and non-perennial sediments showed unexpected respiration and CH₄ efflux patterns. These findings improve our understanding of microbial responses to changing hydrology and contributions to greenhouse gas balance in lotic ecosystems under conditions of reduced flow permanence.