Sullivan Samantha N, Bowen Jennifer C, Kaplan Louis A, Cory Rose M, Hatcher Patrick G
Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, VA, USA.
Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, USA.
Water Res. 2025 Aug 1;281:123668. doi: 10.1016/j.watres.2025.123668. Epub 2025 Apr 28.
Dissolved organic matter (DOM) contributes to the energy flow in tropical rivers that dominate the global budget of riverine runoff to the ocean. Yet, the processes that control downstream DOM concentration and composition in the tropics are much less understood compared to higher latitudes. Here, we investigate seasonal and downstream changes in DOM concentration and composition in headwater streams draining the seasonally dry evergreen forest of the Río Tempisquito watershed in Costa Rica. We used dissolved organic carbon (DOC) analysis as a measure of DOM concentrations and UV-Vis, fluorescence spectroscopy and FT-ICR-MS to measure DOM composition. Seasonal differences in litterfall and day-to-day changes in rainfall controlled the DOC concentration. Elevated DOC concentrations were associated with inputs of terrestrial, humic-like DOM into the stream, and light rain falling on fresh leaf litter in the dry season led to nearly equivalent DOC concentrations as heavier rain falling on degraded litter in the wet season. The average nominal oxidation state of carbon (NOSC) in DOM molecular formulae differed between wet and dry seasons (seasonally) and with distance downstream (longitudinally). A higher NOSC of stream DOM in the dry versus wet season may be due to a peak in fresh litterfall in the dry season followed by the leaching of degraded litter during precipitation in the wet season. The shift in DOM toward a higher NOSC downstream in both seasons may be due to the selective metabolism of a more reduced, labile pool within DOM during downstream transport. We propose that the downstream changes in DOM composition in the dry tropical headwater streams of this study are driven by the preferential microbial metabolism of labile, reduced DOM compounds. These findings extend the River Continuum Concept by showing that DOM oxidation state can be a sensitive indicator of microbial metabolism in the absence of measurable decreases in downstream DOM concentrations.
溶解有机物（DOM）对热带河流的能量流动有贡献，这些热带河流在全球河流径流注入海洋的预算中占主导地位。然而，与高纬度地区相比，控制热带地区下游DOM浓度和组成的过程却知之甚少。在这里，我们调查了哥斯达黎加里奥坦皮斯quito流域季节性干燥常绿森林源头溪流中DOM浓度和组成的季节性和下游变化。我们使用溶解有机碳（DOC）分析作为DOM浓度的度量，并使用紫外可见光谱、荧光光谱和傅里叶变换离子回旋共振质谱来测量DOM组成。凋落物的季节性差异和降雨的每日变化控制了DOC浓度。DOC浓度升高与陆地类腐殖质DOM输入到溪流中有关，旱季落在新鲜落叶上的小雨导致的DOC浓度与雨季落在降解落叶上的大雨导致的DOC浓度几乎相当。DOM分子式中碳的平均标称氧化态（NOSC）在湿季和干季（季节性）以及下游距离（纵向）上有所不同。旱季溪流DOM的NOSC高于湿季，这可能是由于旱季新鲜凋落物峰值之后，湿季降水期间降解凋落物的淋溶所致。两个季节中DOM向下游NOSC升高的转变可能是由于下游运输过程中DOM中更还原、更不稳定的部分被选择性代谢。我们提出，本研究中热带干燥源头溪流中DOM组成的下游变化是由不稳定、还原的DOM化合物的优先微生物代谢驱动的。这些发现扩展了河流连续体概念，表明在下游DOM浓度没有可测量下降的情况下，DOM氧化态可以是微生物代谢的敏感指标。
