Excessive inputs of sediment and acceleration of primary production have been observed worldwide in a large number of water bodies. Human-environment interactions were recognized as one of the main drivers of this evolution during the 20th century with the occurrence of major landscape changes and a greater use of agricultural inputs. In this study, we used paleo-production proxies such as chlorophyll-a, organic matter properties (TOC and TN concentrations, δ13C and δ15N) measured in sediment cores dated with fallout 210Pbex and 137Cs activities for reconstructing changes in accumulation rates and sources of organic matter during the recent period of agricultural intensification (1920–2020). In order to record these changes at the regional scale, sediment cores were collected at the outlet of several headwater catchments (n = 9), covering a wide range of land covers / land uses across the Loire River basin (117,000 km²), France. The rates of sedimentary organic matter deposition in the studied water bodies accelerated from 1950 onwards (+48 %). Between 1950 and 1970, the signature of sedimentary organic matter indicates a dominant contribution of soil-derived inputs. This period corresponds to major landscape modifications across the basin (land consolidation, stream re-design, implementation of tile drains) driving a general acceleration of erosion rates. Then, from 1960 onwards, chlorophyll-a and C/N proxies indicate an increase in primary production coupled with a decrease of terrigenous supply in agricultural catchments. These proxies were strongly correlated to the agricultural inputs during the 1955–1990 period (e.g., r = 0.9 between chlorophyll-a content and N inputs), suggesting a progressive eutrophication of these reservoirs driven by increasing fertilizer use. During these 35 years, sedimentary organic matter deposition rates increased on average by 30 %. During the 1990s, despite a slight reduction in fertilizer use (−13 %), the paleo-production proxies (e.g. chlorophyll-a) still indicate a positive trend suggesting the contribution of another driving factor such as climate warming or again nutrient release from soils and sediments. In the absence of long term geochemical and sediment input monitoring, paleolimnological reconstructions provide a powerful tool to reconstruct past agricultural pressures in rural environments. This study illustrates the impact of intensive farming on water body siltation driven by varying sources of organic material during the 20th century. In addition, these results suggest that eutrophication processes of these reservoirs with contrasting land uses started during the 1960–1970 period and are still ongoing nowadays.

中文翻译：

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基于多代理古湖泊学重建的 20 世纪卢瓦尔河流域富营养化的区域趋势

在世界范围内的大量水体中都观察到沉积物的过度投入和初级生产的加速。人类与环境的相互作用被认为是 20 世纪这一演变的主要驱动力之一，发生了重大的景观变化和农业投入物的更多使用。在这项研究中，我们使用了古生产指标，例如叶绿素 a、有机质特性（TOC 和 TN 浓度、δ13C 和 δ15N）在沉积物岩心中测量，这些岩心在 210Pbex 和 137Cs 活动中测得，以重建积累率和有机物来源的变化。最近的农业集约化时期（1920-2020 年）。为了在区域尺度上记录这些变化，在几个源头集水区（n = 9）的出口处收集了沉积物核心，涵盖法国卢瓦尔河流域（117,000 平方公里）的各种土地覆盖/土地用途。从 1950 年开始，所研究水体中沉积有机物沉积的速度加快 (+48 %)。1950 年至 1970 年间，沉积有机质的特征表明土壤来源的投入占主导地位。这一时期对应于整个流域的重大景观改造（土地整理、河流重新设计、瓷砖排水沟的实施），推动了侵蚀率的普遍加速。然后，从 1960 年起，叶绿素 a 和 C/N 代理表明初级生产增加，同时农业集水区陆源供应减少。这些代理变量与 1955-1990 年期间的农业投入密切相关（例如，r = 0。9 在叶绿素 a 含量和 N 输入之间），表明这些水库因肥料使用增加而逐渐富营养化。在这 35 年中，沉积有机质沉积率平均增加了 30%。在 1990 年代，尽管化肥使用量略有减少 (-13 %)，但古生产指标（例如叶绿素-a）仍显示出积极的趋势，表明另一个驱动因素的贡献，如气候变暖或土壤养分再次释放和沉积物。在缺乏长期地球化学和沉积物输入监测的情况下，古湖泊学重建提供了一种强大的工具来重建农村环境中过去的农业压力。这项研究说明了 20 世纪集约化农业对由不同有机材料来源驱动的水体淤积的影响。此外，这些结果表明，这些具有对比土地用途的水库的富营养化过程始于 1960-1970 年期间，并且至今仍在进行中。