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 (SR), absorbance at 254 nm (a254), and DOC-specific absorbance (SUVA; a254: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 a254 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 they are more responsive to climatic variations than DOC concentration. SUVA in particular tended to increase with greater moisture and decrease with drier conditions. These results demonstrate that DOC quantity and quality can exhibit different complex long-term trends and responses to climate components, with important implications for aquatic ecosystems.

溶解态有机质（Dissolved organic matter, DOM）是驱动诸多湖泊生态过程的核心要素。近数十年来，诸多湖泊的DOM总量——以溶解态有机碳（dissolved organic carbon, DOC）浓度表征——已出现显著提升。尽管DOC浓度升高这一现象已得到学界广泛共识，但针对DOM特性（即DOM质量）的长期演变规律的研究仍相对稀缺。DOM质量可通过多种光学特性指标进行表征，包括光谱斜率（S275-295）、光谱比值（SR）、254nm处吸光度（a254）以及溶解性有机碳特异性吸光度（SUVA; a254:DOC）等。然而，针对DOM质量的长期监测数据远少于DOC浓度的长期监测序列。本研究依托美国威斯康星州北部北温带湖泊长期生态研究（North Temperate Lakes Long Term Ecological Research）站点内7个湖泊的24年DOC与吸光度监测数据，探究了DOC浓度与DOM质量的时间变化趋势及同步性特征。研究提出如下假设：与DOC浓度升高及外源有机质输入占比提升的规律相符，DOM质量将呈现光谱比值（SR）与光谱斜率（S275-295）降低、254nm处吸光度（a254）与溶解性有机碳特异性吸光度（SUVA）升高的趋势。监测结果显示，不同湖泊的DOC浓度分别呈现出显著的上升与下降趋势。与之形成鲜明对比的是，DOM质量的时间趋势表明其外源输入占比有所降低或降解程度有所提升：其中3个湖泊的光谱比值（SR）出现了显著的长期升高。DOM质量参数的变化模式与同步性特征显示，其对气候波动的响应敏感性远高于DOC浓度。其中，溶解性有机碳特异性吸光度（SUVA）往往随湿度升高而上升、随干旱条件加剧而下降。本研究结果表明，DOC的总量与质量可呈现出截然不同的复杂长期趋势，且对气候要素的响应模式存在显著差异，该发现对水生生态系统的研究与保护具有重要的启示意义。