Dynamics of the Dissociation of Dissolved Organic Acids across Swedish Streams over 35 Years

Dissolved organic matter (DOM) plays a key role in weakly-buffered surface water ecosystems by influencing pH, metal speciation, and the bioavailability of trace elements through complexation reactions. Despite a substantial decrease in acid deposition over recent decades, many Swedish streams and lakes remain relatively acidic, largely due to the natural acidity associated with DOM.  A robust understanding of the acid–base properties of DOM is therefore essential for accurately assessing natural water acidity and determining the continued need for liming as a mitigation measure. In this study, we analyzed national monitoring data comprising more than 42,000 samples collected between 1990 and 2024 from 136 streams across Sweden to quantify long-term changes in DOM acidity. Organic matter charge density at pH 5.6 (OMCD_5.6) was used as an indicator of the dissociation behaviour of dissolved organic acids. While spatial variation in median OMCD_5.6 among sites was relatively small, compared to variation within individual stations, approximately two-thirds of the sites exhibited significant temporal trends, predominantly reflecting a decline in organic matter charge density over time. Lower charge density implies reduced dissociation of organic acids and, at constant DOM concentrations, a diminished influence of DOM on stream water pH. We further examine how catchment characteristics, such as atmospheric deposition, land-use, and water chemistry, and their long-term changes, relate to observed trends in OMCD_5.6. Our findings challenge the common assumption that changes in DOM dissociation properties can be neglected in surface water chemistry assessments and highlight the need to explicitly consider shifts in DOM acid–base properties.