Long-term trends and synchrony in dissolved organic matter characteristics in Wisconsin, USA, lakes: Quality, not quantity, is highly sensitive to climate

Dissolved organic matter (DOM) is a fundamental driver of many lake processes. In the past several decades, many lakes have exhibited a substantial increase in DOM quantity, measured as dissolved organic carbon (DOC) concentration. While increasing DOC is now widely recognized, fewer studies have sought to understand how characteristics of DOM (DOM quality) change over time. Quality can be measured in several ways, including the optical characteristics spectral slope (S275-295), spectral ratio (S-R), absorbance at 254nm (A(254)), and DOC-specific absorbance (SUVA; A(254): DOC). However, long-term measurements of quality are not nearly as common as long-term measurements of DOC concentration. We used 24 years of DOC and absorbance data for seven lakes in the North Temperate Lakes Long-Term Ecological Research site in northern Wisconsin, USA, to examine temporal trends and synchrony in both DOC concentration and quality. We predicted lower SR and S275-295 and higher A(254) and SUVA trends, consistent with increasing DOC and greater allochthony. DOC concentration exhibited both significant positive and negative trends among lakes. In contrast, DOC quality exhibited trends suggesting reduced allochthony or increased degradation, with significant long-term increases in SR in three lakes. Patterns and synchrony of DOM quality parameters suggest that they are more responsive to climatic variations than DOC concentration. SUVA was particularly responsive to the degree of soil moisture. These results demonstrate that DOC quantity and quality can exhibit different complex long-term trends and responses to climatic drivers, with implications for carbon cycling and microbial communities in aquatic ecosystems.