Climate Change and Urbanization Decouple Dissolved Organic Carbon Quantity and Composition in Streams

Dissolved organic carbon (DOC) is a ubiquitous component of freshwater ecosystems that is sensitive to global change. In turn, DOC controls fundamental biogeochemical processes and functions. These controls depend on both the amount and composition of organic molecules comprising the dissolved organic matter (DOM) pool, which reflects the relative contributions of catchment‐derived terrestrial inputs and in situ production. Stream DOM fluctuates with land use, soil mobility, and hydrology; however, few studies have monitored long‐term changes in DOM composition to investigate links with climate. Here, we characterized 17‐year trends in DOC and DOM in 48 streams across a land use gradient and modeled patterns therein with climatic and hydrological conditions. Across streams, Mann‐Kendall trend analyses showed that DOC decreased through time, while DOM became fresher, more aromatic, and contained an increased proportion of urban‐derived DOM from the terrestrial catchment. Using generalized additive models, we observed significant linear, unimodal, and multimodal patterns in DOM composition with precipitation and soil temperature. Generally, precipitation increased terrestrial DOM, whereas soil temperature increased urban‐derived DOM, particularly in catchments characterized by increasing levels of urbanization. Our study highlights the importance of long‐term monitoring in understanding dynamic interactions between terrestrial—fluvial carbon transfer and biogeochemical effects of global climate change and urbanization. Altogether, our results show that interactions between climate change and urbanization will shape future DOM dynamics in streams.