Spatial and temporal variation in dissolved organic matter in urban streams in metropolitan Boston, Massachusetts (USA)

Urban riverine systems are heterogeneous, and the substantial variability in impervious cover, riparian cover, wetlands, and wastewater and stormwater infrastructure affect sources and transport of dissolved organic matter (DOM), of which dissolved organic carbon (DOC) is a significant component. . An understanding of the quantity, bioavailability, and timing of DOM inputs (a key energy source for food webs and component of nutrient cycling) to streams within cities can help to better evaluate drivers of DOM variability. We sampled 100 stream sites in the greater Boston (Massachusetts, USA) area spanning a range of land cover, riparian vegetation, stream size, housing and infrastructure age, and sociodemographic characteristics. Water samples collected during four seasonal synoptic events in 2021–2022 were analyzed for DOC concentration and DOM characteristics (using fluorescence excitation-emission matrices and absorbance spectra). Temporally, we observed more autochthonous DOM and lower DOC concentrations in the summer, possibly due to low precipitation and streamflow, disconnecting streams from humic wetland and soil carbon sources. Consistent with other studies, we observed that more urbanized streams had less humic, more autotrophic DOM. Higher wetland cover was associated with more humic, higher molecular weight DOM and was the strongest predictor of DOM characteristics. Interestingly, sites downstream of combined sewer outfalls (CSOs) showed distinctly higher concentrations of protein-like dissolved organic matter, suggesting the influence of sewage overflows; this pattern was absent during the very dry summer. Although sociodemographic variables were not strong predictors, we did observe a possible association between lower income areas with less canopy cover and more autochthonous DOM, and between areas with older housing, more canopy cover, and more humic DOM These results may guide management decisions regarding the development of wetlands and its impact on DOM and stream ecological functions and CO2 emissions, monitoring of wastewater contamination using protein-like DOM, and planning for the equitable repair of wastewater infrastructure and restoration of riparian vegetation.