The impact of urban stream burial on DOM cycling: new insights from a mesocosm experiment

Abstract Urban landscapes can drastically alter light regimes through stream burial, and also modify water temperature patterns, both of which have profound implications for the degradation of dissolved organic matter (DOM) through photodegradation and biodegradation, respectively. Despite their likely significance, the dynamics of short-term biodegradation and photodegradation in urban environments remain poorly understood, with limited knowledge regarding the potential interplay between warming and stream burial effects. This study used a replicated flume experiment to investigate the effects of shading, warming (+ 4.5–6.6°C), and their interaction on DOM processing. We used optical techniques to characterize DOM quantity and composition, allowing us to assess photodegradation and biodegradation rates in urban stream analogues. Linear mixed effects models revealed that the degradation of the fluorescent DOM pool decreased under shaded conditions, accompanied by an increase in humic-like compounds. Additionally, shaded flumes exhibited a shift towards higher molecular weight organic matter, indicating the importance of photodegradation in DOM processing within urban rivers. Temperature effects on DOM processing rates were found to be relatively minor compared to shading, with no interaction with shading observed. Principal Component Analysis demonstrated clear distinctions between shaded and unshaded treatments. In contrast, no significant differences were observed between warmed and ambient temperature treatments. Our findings suggest that stream burial impedes DOM processing and alters DOM composition in urban headwaters by inhibiting the photodegradation of humic material. The temperature treatments examined had limited impacts on biodegradation over the relatively short timescales of this study. This study provides experimental support for daylighting interventions as a strategy to enhance DOM processing in urban streams and mitigate the flux of labile material to downstream ecosystems.