Increased cultural eutrophication since the 20th century, caused by phosphorus (P) enrichment, has become a major problem worldwide. In deep, stratified lakes, eutrophication-induced hypolimnetic anoxia often stimulates the release of labile P from the sediment into the water column. This positive feedback, termed internal P loading, maintains or even accelerates eutrophication. However, most studies on internal P loading have focused on recent times. Little is known about whether such positive feedbacks caused by labile P release from sediments also played a role under natural conditions with little or no human impact. We investigated a high-resolution 15,000-year sediment record of paleoproduction, anoxia, and five sedimentary P fractions from a small, deep lake, Soppensee, on the Swiss Central Plateau. We estimated long-term qualitative internal P loading by comparing the Holocene record of diatom-inferred epilimnetic total P (DI-TP) concentrations with labile P fraction (FeP) concentrations in sediments under changing trophic state, redox, and lake mixing regimes. Intensified P cycling from sediments into the water column (enhanced internal P loading) apparently occurred as a positive feedback to natural eutrophication with persistent bottom-water anoxia during the early to mid-Holocene (~9000–6000 cal BP). However, this positive feedback was not inferred for other eutrophic phases. Fe-rich layers formed during seasonal mixing of the lake in the late Holocene (~2000–200 cal BP) and magnetite-type minerals produced by magnetotactic bacteria (MTB) internal P loading during anoxic phases in the mid- to late Holocene (~6000–2000 cal BP) appeared to prevent internal P loading. MTB presence resulted in high concentrations of potentially labile FeP in sediments. Our study demonstrates the potential contribution of internal P loading during long-term natural eutrophication of deep stratified lakes and has wide implications for lake management and restoration. Our results highlight the importance of the coupled geochemical cycles of P and Fe in the long-term trophic state evolution of stratified, ferruginous, low-sulfate-water lakes, conditions that have been reported to serve as analogs for the Archaean Ocean.

自 20 世纪以来，由磷 (P) 富集引起的文化富营养化加剧已成为世界范围内的一个主要问题。在深层次的湖泊中，富营养化引起的水下缺氧通常会刺激不稳定的磷从沉积物中释放到水体中。这种正反馈，称为内部磷负荷，维持甚至加速富营养化。然而，大多数关于内部 P 负载的研究都集中在最近。很少有人知道沉积物中不稳定的磷释放引起的这种正反馈是否也在自然条件下发挥作用，几乎没有或没有人类影响。我们调查了来自瑞士中部高原 Soppensee 小深湖的古生产、缺氧和五种沉积 P 组分的高分辨率 15,000 年沉积物记录。P) 在不断变化的营养状态、氧化还原和湖泊混合状态下沉积物中的浓度。在全新世早期至中期（~9000-6000 cal BP）期间，从沉积物到水体的强化 P 循环（增强的内部 P 负载）显然是对自然富营养化的正反馈，持续性底水缺氧。然而，对于其他富营养化阶段并没有推断出这种正反馈。在全新世晚期（~2000-200 cal BP）湖泊季节性混合期间形成的富铁层和由趋磁细菌（MTB）在全新世中晚期缺氧阶段（~ 6000–2000 cal BP）似乎可以防止内部 P 加载。MTB 的存在导致高浓度的潜在不稳定 Fe沉积物中的 P。我们的研究证明了内部磷负荷在深层分层湖泊长期自然富营养化过程中的潜在贡献，并对湖泊管理和恢复具有广泛的意义。我们的研究结果强调了 P 和 Fe 耦合地球化学循环在分层、含铁、低硫酸盐水湖泊的长期营养状态演变中的重要性，据报道，这些条件可作为太古代海洋的类似物。