Dissolved Organic Matter Charge Dynamics over Three Decades

Dissolved organic matter (DOM) is a crucial component of aquatic ecosystems, influencing pH, metal speciation, and the bioavailability of trace elements. The influence of DOM on water quality depends not only on its concentration but also on its chemical characteristics. To quantify the site density (SD) of organic acid functional groups, a key characteristic of DOM, this study utilized data from over 31,000 water samples collected from 110 watercourses between 1995 and 2024 by Swedish national monitoring programs. We found systematic dynamic trends in SD across Sweden. The analysis revealed an increase in SD from 1995 to 2003, followed by a decline from 2003 to 2010. Since 2010, SD has stabilized, although modest increases remain evident at many sites. Earlier trends were potentially governed by changes in DOM mobility associated with recovery from acid deposition, whereas more recent trends are likely linked to DOM source characteristics.