Seasonal drivers and infrastructure impacts on dissolved oxygen dynamics in complex environmental systems.

Urban streams are increasingly regulated by the interaction of climate-driven thermal stress and episodic infrastructure disturbances, yet their combined influence on dissolved oxygen (DO) dynamics remains insufficiently resolved across temporal scales. This study quantified the combined effects of temperature, hydrology, and combined sewer overflow (CSO) activity on DO across four urban watersheds in Philadelphia, USA, using 12 years of high-frequency monitoring integrated with precipitation, streamflow, and modeled CSO data. A multi-scale framework combining event-stratified comparisons, distributed lag models (0-3 days), and multivariate regression was used to evaluate controls on DO variability. Across all watersheds, DO exhibited a consistent seasonal cycle that reflects background thermal constraints on oxygen solubility and metabolic demand. This seasonal pattern establishes a baseline physical regime, against which shorter-term variability becomes apparent. At event-scales, CSO and wet-weather conditions consistently reduced minimum daily DO relative to dry conditions, with strongest effects observed in more hydraulically connected systems. Distributed lag models further revealed watershed-dependent persistence of CSO impacts, ranging from rapid recovery to multi-day DO suppression in systems with greater retention and reduced flushing capacity. Multivariate models identified temperature as the dominant control on baseline DO conditions, while event-based and lagged analyses showed that CSO activity and hydrologic variability govern short-term deviations and persistence in DO dynamics. Overall, urban DO dynamics are structured primarily by event-scale and lagged CSO and hydrologic responses, with seasonal thermal conditions providing a baseline context for variability.