Predicting Organic Carbon Loss Across the Aquatic Continuum Using Water Retention Time

Abstract Predicting dissolved organic carbon (DOC) mineralization and removal rates across the aquatic continuum is vital for addressing questions relating to carbon cycling, ecosystem functioning, contaminant transport and drinking water safety. Previous research has shown a decline in DOC reactivity with increasing water retention time (WRT), implying reduced processing rates from headwaters to the coast. However, these findings were largely based on bioassays and lake data, which may not reflect real‐world conditions across the full aquatic continuum. Using an expanded field‐based data set and a statistical model comparison exercise, we found evidence for a more rapid decline in DOC reactivity with WRT than previously reported. Headwaters may therefore act as even stronger DOC processing hotspots than previously recognized. We present updated equations for predicting DOC removal as a function of WRT, which should replace existing formulations in modeling studies to avoid underestimating removal, particularly in headwaters. In a boreal case study, for example, updated equations predict nearly 40% higher DOC mineralization across the aquatic continuum than previous formulations. In addition, we recommend a steady‐state Vollenweider approach for simulating DOC transmission in open systems such as lakes, rather than the commonly used exponential decay model, which assumes closed‐system dynamics. Nonetheless, large residual variance highlights the limitations of these simple models. Future efforts should focus on developing more nuanced approaches that better capture the complexity of DOC dynamics across diverse aquatic environments.