Composition of Stream Dissolved Organic Matter Across Canadian Forested Ecozones Varies in Three Dimensions Linked to Landscape and Climate

Dissolved organic matter (DOM) is a key variable influencing aquatic ecosystem processes. The concentration and composition of DOM in streams depend on both the delivery of DOM from terrestrial sources and on aquatic DOM production and degradation. However, there is limited understanding of the variability of stream DOM composition at continental scales and the influence of landscape characteristics and disturbances on DOM across different regions. We assessed DOM composition in 52 streams at seven research sites across six forested ecozones in Canada in 2019-2022 using 26 indices derived from five analytical approaches: absorbance and fluorescence spectroscopy, liquid chromatography-organic carbon detection, Fourier-transform ion cyclotron resonance mass spectrometry, and asymmetric flow field-flow fractionation. Combined analyses showed clear clustering and redundancy across analytical techniques, and indicated that compositional variations were primarily related to three axes of DOM composition: (a) aromaticity, which was greater in low-relief, wetland-dominated catchments, (b) oxygenation, which was greater in colder and drier ecozones, and (c) biopolymer content, which was greater in lake-influenced catchments. Variability in DOM composition among research sites was greater than variability of streams within a site and variability over time within a stream. Forest harvesting and wildfire disturbances had no common influence on DOM composition across research sites, emphasizing the need for regional studies. Our study provides a broad understanding of the variability of stream DOM composition and its associations with landscape and catchment characteristics at a subcontinental scale, and provides key insights for the choice and interpretation of DOM indices from various analytical approaches. Dissolved organic matter (DOM) in surface waters influences water quality, aquatic organisms, and carbon cycling, but variability in its composition across different regions has not been extensively studied. We collected water samples from 52 streams across 6 different forested regions in Canada spanning from coast to coast, and analyzed them using five analytical approaches varying in complexity to characterize the composition of DOM in order to assess the differences in stream DOM among the forested regions, the environmental controls on DOM composition, and which approaches were most useful in our characterization. We found that many regions had distinct DOM composition, and climatic factors like mean annual temperature, the presence of wetlands and lakes explained most of the variations, but we were unable to detect any common effects of land disturbance. For assessing differences in DOM across regions, simple analytical approaches were as useful as the more complex approaches. Our findings are important for understanding the function of aquatic ecosystems, potential impacts of climate change and land management, and implications for drinking water treatment. We analyzed dissolved organic matter (DOM) composition in samples from 52 streams across 6 forested regions in Canada using multiple analytical techniques DOM composition varied in three dimensions: aromaticity, oxygenation and biopolymer content, linked to climate, wetlands and lakes Our subcontinental-scale assessment provides insights for data interpretation, monitoring program design and land management