Urbanization accelerates the weathering of natural and man-made materials resulting in increased salinity and alkalinity in aquatic ecosystems, potentially driving the freshwater salinization syndrome (FSS). This urbanization-induced FSS is caused by the urban karst phenomenon, which reflects the prevalence of anthropogenic materials (e.g., cement) in urban watersheds that exhibit accelerated weathering, contributing alkaline substances and base cations (e.g., Ca2+) into downstream waterbodies. The ability of Ca2+ and other major cations to interact with and bind organic matter (OM) is recognized across disciplines such as industrial engineering, wastewater management, and pharmaceutical manufacturing. While not unrecognized in freshwater ecology, the effects of FSS on aquatic ecosystems are understudied relative to more traditional or emerging contaminants. Further examination is needed to understand whether FSS affects OM cycling and ecosystem dynamics. As OM degrades, molecular size decreases while solubility and reactivity increases. More reactive DOM can evade complete decomposition to CO2 via sorption to mineral surfaces or supramolecular aggregation, flocculation, and precipitation. Increasing ionic strengths and pH in freshwater systems (as seen with FSS) further enhances the sorptive capacity of DOM via deprotonation of DOM-associated carboxylic and phenolic groups. To better understand how increasing alkalinity and salinity might affect the properties of DOM in real-world conditions, we manipulated site water from five streams spanning an urbanization gradient to alter both Ca2+ concentrations and pH. We selected Ca2+ as a representative cation due to its high cation exchange capacity and prevalence in building material (e.g., cement). Using the differential approach, we assessed changes in DOM reactivity following Ca2+ exposure. Additionally, we monitored respiration across treatment levels to assess how OM shifts might affect ecosystem functions. Through this research we aim to identify how changes in cation composition driven by FSS and urban karst interact with biogeochemical functioning of streams to alter carbon and energy dynamics.