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Abstract Wetlands have been used to treat anthropogenic effluents for decades due to their intense biogeochemical processes that transform and uptake nutrients, organic matter, and toxins. Despite these known functions, we lack generalizable knowledge of effluent-derived dissolved organic matter (DOM) cycling in wetlands. Here, we quantify the cycling of DOM in one of Canada’s more economically important wetland complexes (Frank Lake, Alberta), restored to hydrologic permanence in the 1980s using urban and agro-industrial effluents. Optical analyses and PARAFAC (parallel factor analysis) modelling showed a clear compositional change from more bioavailable and protein-like DOM at effluent input sites to more aromatic and humic-like at the wetland outflow, likely due to DOM processing and inputs from marsh plants and wetland soils. Microbial incubations showed that effluent DOM was rapidly consumed, with the half-life of DOM increasing from as low as 35 days for effluent, to 462 days at the outflow, as a function of compositional shifts toward aromatic, humic-like material. Long-term averaged dissolved organic carbon (DOC) export was low compared to many wetlands (10.3 ± 2.0 g C m⁻² yr⁻¹). Consistent with predictions based on water residence time, our mass balance showed Frank Lake was a net source of DOM across all measured years, but shifted from a source to sink among wet and drought years that respectively shortened or lengthened the water residence and DOM processing times. Overall, Frank Lake processes and transforms effluent DOM, despite being a longer-term net source of DOM to downstream environments. 

Abstract River organic matter transformations impact the cycling of energy, carbon, and nutrients. The delivery of distinct dissolved organic matter (DOM) sources can alter aquatic DOM cycling and associated biogeochemical processes. Yet DOM source and reactivity are not well‐defined for many river systems, including in western Canada. Here, we explore DOM cycling in the mainstem of the Oldman River (stream order 6–7), a heavily regulated river network in southern Alberta (Canada). We compared seasonal river DOM content, composition, and bioavailability with nine endmember leachates from the river valley using optical properties and incubations to estimate biodegradable dissolved organic carbon (BDOC). River DOM composition was most similar to terrestrial soil leachates, followed by autochthonous DOM leachates. River DOM bioavailability was low (BDOC = 0%–16.6%, mean of 7.1%). Endmember leachate bioavailability increased from soils (BDOC = 23.9%–53.7%), to autochthonous sources (fish excretion, macrophytes, biofilm; BDOC = 49.9%–80.0%), to terrestrial vegetation (leaves, shrubs, grass; BDOC >  80%), scaling positively with protein‐like DOM content and amount of leachable dissolved organic carbon (DOC), and negatively with aromaticity. Seasonally, DOC concentrations changed little despite >15‐fold increases in discharge during spring. River DOM composition shifted modestly toward soil‐like endmembers in spring and more bioavailable autochthonous end members in autumn and winter. Low DOM bioavailability in the river mainstem and low DOC yields shown in previous work point to limited internal processing of DOM and limited bioavailable DOM delivery to downstream habitats, possibly due to upstream flow regulation. Our observations provide important insights into the functioning of western Canadian aquatic networks.