A complex interplay among agricultural land uses, urbanization, and landscape attributes shapes the concentration-discharge relationships in Ontario, Canada

The shape of a concentration-discharge (C-Q) relationship and its evolution in space and time can be used to elucidate the predominant hydrological pathways and residence time of a chemical of interest, to determine the rate and timing of biogeochemical production and uptake, and to evaluate how catchment characteristics (topography, land cover/soil vegetation, anthropogenic activities) modulate export dynamics. Being a powerful tool to address ecohydrology’s fundamental questions, the investigation of C–Q relationships for multiple elements at multiple spatial and temporal scales has been instrumental in the context of water-quality management. In this study, we use a statistical (Segmented, Classification and Regression Trees) modelling framework to establish linkages among watershed morphological attributes, weather variability, land-use patterns, and biogeochemical “hot spots” across Ontario, Canada. Our analysis signifies the presence of enriching behavior, either across the entire streamflow domain or at high flow regimes, for several water quality constituents, such as total phosphorus, suspended solids, nitrate, calcium, and dissolved organic carbon. Alkalinity, calcium, and dissolved inorganic carbon consistently displayed negative slopes at higher flows, indicative of source limitation for these constituents. Similarly, although many of our study watersheds are influenced by anthropogenic activities, the predominant C-Q relationship for chloride was suggestive of either a monotonically diluting pattern or a diluting one followed by a chemostatic response to increasing streamflows. Our results offer novel insights into the interplay between anthropogenic stressors and biogeochemical processes that shapes the severity of nonpoint-source pollution, as hydrological and nutrient cycles will be increasingly subjected to major disturbances by food-production systems and urbanization.