Controls on Dissolved Organic Carbon Bioavailability Across an Urban Stream Network: Disturbance, Metabolism, and Seasonality

Urban impacts on aquatic ecosystems are heterogeneous, and the drivers of these impacts vary across space and time. One area that remains poorly understood is how urbanization impacts the bioavailability of dissolved organic carbon (BDOC), a measure of how readily heterotrophic microorganisms utilize organic carbon. BDOC depends on the size and molecular composition of the dissolved organic carbon pool, both of which are shaped by watershed characteristics and seasonal hydrology. We hypothesized that BDOC would peak in the dry season and decline through the wet season, and that urban factors would significantly influence BDOC. Seasonal surface water sampling was conducted across eight watersheds in the Portland, OR metropolitan area between July 2023 and May 2024. Samples were filtered, inoculated with mixed unfiltered water, and incubated in the dark at 20°C for 28 days. Temporal and spatial difference were evaluated using mixed-model ANOVA and Tukey HSD tests. A redundancy analysis (RDA) identified the primary relationships between BDOC, water quality, and urbanization, and explanatory variables from the RDA were subsequently modeled against BDOC using univariate linear regression for each season. Summer BDOC was significantly higher than in other seasons, while differences among wet-season sampling periods were not significant. The RDA indicated that urban factors and stream metabolism are significant drivers of BDOC, with distinct correlative patterns for BDOC concentrations versus %BDOC. Seasonal regression analysis further showed that the strength of these urban drivers is seasonally dependent, with the strongest relationships occurring in the summer and spring. These findings advance understanding the heterogeneous urban impacts interact with seasonality to shape freshwater carbon cycling.