Post-depositional degradation of organic matter (OM) in freshwater sediments is crucial for driving the biogeochemical dynamics and influencing the carbon burial. This process also often causes diagenetic alteration on paleoenvironmental proxies. Yet, mechanisms behind degradation of sedimentary OM and depth-related variations in stable isotope ratios can so far only be explained in part. Degradation of sedimentary OM in two drinking water reservoirs with contrasting eutrophic and mesotrophic states and different catchment land use (agriculture versus forestry) was studied. A 4-step procedure was used to chemically separate sedimentary OM in terms of biochemical composition. Here we presented depth profiles of biochemical composition of sedimentary OM that helped to quantify preferential degradation of aquatic proteins and carbohydrates and the following removal of aquatic lipids. Sediment in the eutrophic reservoir, which reflected a larger contribution of algal-derived OM than the mesotrophic reservoir with a forest dominated catchment, was therefore subject to more intensive degradation of sedimentary OM along with δ¹³C and δ¹⁵N alterations. In addition, changes in the relative proportions of biochemical components in sedimentary OM had more pronounced impact on δ¹⁵N values relative to δ¹³C. Our findings suggest that the lability of algae-derived OM leads to uncertainties for the estimation of carbon burial in water bodies and obscures paleo-limnological information derived from isotopic proxies. Post-depositional modifications are more pronounced in eutrophic freshwaters that accumulated more readily degradable OM of algal origin in their sediments. Recognition of these modifications will help constrain carbon burial rates of productive lakes and reservoirs and assess the role of reservoirs in carbon cycling.

淡水沉积物中有机物（OM）的沉积后降解对于驱动生物地球化学动力学和影响碳埋藏至关重要。这个过程也经常导致古环境代理的成岩作用改变。然而，沉积物 OM 退化背后的机制和稳定同位素比率与深度相关的变化到目前为止只能部分解释。研究了两个具有对比的富营养化和中营养化状态以及不同集水区土地利用（农业与林业）的饮用水水库中沉积物 OM 的退化。使用 4 步程序在生化成分方面对沉积 OM 进行化学分离。在这里，我们展示了沉积 OM 的生化组成的深度剖面，这有助于量化水生蛋白质和碳水化合物的优先降解以及随后的水生脂质去除。富营养化水库中的沉积物反映了藻类有机质的贡献大于具有以森林为主的集水区的中营养性水库，因此沉积物有机质与 δ 一起受到更强烈的降解¹³ C和δ ¹⁵ N变化。此外，相对于 δ ^{13 C，沉积 OM 中生化成分相对比例的变化对 δ 15} N 值的影响更为显着。我们的研究结果表明，藻类衍生的 OM 的不稳定性导致估算碳埋藏的不确定性。水体并掩盖了从同位素代理中获得的古湖沼学信息。沉积后修饰在富营养化淡水中更为明显，这些淡水在其沉积物中积累了更容易降解的藻类 OM。认识到这些修改将有助于限制生产性湖泊和水库的碳埋藏率，并评估水库在碳循环中的作用。