Three decades of change in the character and charge density of dissolved organic matter in Swedish surface waters

Sammanfattning

Over the recent three decades, dissolved organic matter (DOM) have increased in Swedish surface waters, contributing to brownification. Previous research has evaluated the quantity of DOM, although its character is less explored and evaluated. This study investigates the character of DOM through evaluations of long-term trends, through both quantity and quality of DOM on over 130 watercourses in Sweden using chemistry data from the MVM data portal. The character of DOM was evaluated using four different variables, or proxies: total organic carbon (TOC), coloured dissolved organic matter (CDOM), specific visible absorbance (sVISa) and organic matter charge density at pH 5.6 (OMCD5.6). Generalized additive mixed models (GAMMs) were applied to evaluate temporal trends, while a non-parametric Kruskal-Wallis test followed with a Dunn’s test was used to examine differences between three determined time periods: 1990-1997, 1998-2012, 2012-2024. Results showed that although DOM concentrations have increased, its acid-base properties have changed. In specifics, the OMCD5.6 values have seen a reduction in negativity, indicating a reduction in dissociation of acidic functional groups. This suggests that DOM reflects weaker acidity and reduced overall charge, which affects DOM’s solubility and binding affinity with metals at pH 5.6, and presumably at other pHs in proximity to 5.6. By integrating OMCD5.6 as a proxy of DOM character, this study provides new insight on how DOM chemistry in Swedish surface waters is evolving.