Alternating Drying and Flowing Phases Control Stream Metabolism Through Short- and Long-Term Effects: Insights From a River Network

Stream metabolism is a key biogeochemical process in river networks, synthesizing the balance between gross primary production (GPP) and ecosystem respiration (ER). Globally, more rivers and streams are drying due to climate change and water abstraction for human uses and this can alter the organic carbon residence time leading to decoupled ER and terrestrial organic matter supply. Although the consequences of drying on CO2 emissions have been recently quantified, its effects on stream metabolism are still poorly studied. We addressed the long-term effects of drying and rewetting events on stream metabolism by monitoring oxygen dynamics at 20 reaches across a drying river network, including perennial (PR) and nonperennial reaches (NPR) for one year. We also calculated several climatic and land use variables and characterized local abiotic conditions and biofilm and sediment communities at five sampling dates. ER was significantly higher in NPR than in PR reaches demonstrating in situ the effects of drying on stream metabolism. When analyzing the long-term drivers of ER and GPP, we found a direct positive effect of drying on ER and a negative effect on GPP. Drying also altered microbial community composition with algal communities from NPRs being different from those in PRs. In the short-term, the total oxygen consumption (respiration) during rewetting events was positively related to the duration of precedent nonflow period. Our results show that drying had an important effect on stream metabolism both in the short- and long term, supporting the need for including NPRs in global estimates of stream metabolism.