Climate and Landscape Controls on Dissolved Organic Matter in Wetland‐Dominated Catchments of the Boreal Plains, Canada

ABSTRACT Stream dissolved organic matter (DOM) provides a link between terrestrial and aquatic ecosystems and controls many aquatic functions. Spatial and temporal variability of stream DOM differs amongst regions due to interactions between climate and physiogeography, and few studies have investigated stream DOM characteristics in the Boreal Plains ecozone in western Canada. The Boreal Plains has a subhumid climate, flat terrain, widespread wetlands, sedimentary bedrock and thick heterogeneous glacial deposits which cause complex surface water–groundwater interactions. To describe the spatial and temporal variability and main controls on stream DOM in this region, we monitored dissolved organic carbon (DOC) concentrations and DOM composition (assessed through UV–vis absorbance and fluorescence spectroscopy) in 17 streams with wetland‐dominated catchments over 3 years (2018–2020). In general, our study streams had higher DOC concentrations, lower DOM aromaticity, and lower DOC export when compared to streams draining wetland‐dominated catchments in other boreal regions. Spatial differences in DOC concentration and DOM composition were primarily related to the presence of lakes and whether fine‐ or coarse‐textured glacial deposits dominated. Catchments with lakes and coarse‐textured glacial deposits had lower DOC concentrations and lower DOM aromaticity. The seasonal variation in DOC concentration and DOM aromaticity was primarily controlled by temperature, reflecting increased DOC production in wetlands during warmer periods. Runoff was a secondary influence on the temporal variability, with dilution in wetland‐dominated catchments during annual high flows in summer. However, wetlands became hydrologically disconnected during seasonal dry periods, and streams either dried up or became dominated by groundwater sources with very low concentration and aromaticity of DOM. Our results show that DOM in streams in the Boreal Plains has both similarities and differences with other boreal regions. The observed temperature sensitivity suggests that DOC concentration and DOM aromaticity in Boreal Plains streams may increase as summer temperatures continue to rise, although hydrological thresholds may cause a shift toward low DOC concentration and aromaticity during droughts.