Degradation and recovery of microbial ecosystem functions in rivers exposed to multiple stressors

Microorganisms in sediments are key players for the functioning of rivers, yet, we do not understand how they respond to and recover from multiple anthropogenic stressors, which are increasingly threatening river ecosystems worldwide. We therefore investigated the effects of two globally relevant stressors, temperature increase and salinization, on two microbe-driven ecosystem processes, which are degradation of dissolved organic carbon (DOC) and degradation of bacterial dead biomass (necromass). Using controlled lab experiments with isolated sediment microbial communities, we found that temperature increase had clear positive and salinization clear negative effects on the degradation of DOC and necromass, even at rather low stressor levels. Using outdoor mesocosm experiments where sediment microbial communities are integrated into the riverine food web, we found that the same low stressor levels had no effect on DOC degradation but on necromass degradation, whereas stressor release increased DOC degradation but not necromass degradation. The results indicate that the effects of multiple stressor increase and release on sediment microbial processes depend on the concentration and the bioavailability of the carbon source, and that changes in energy availability may mask multiple stressor effects.