Many clearwater lakes increasingly show symptoms of eutrophication, but the underlying causes are largely unknown. We combined long‐term water chemistry data, multi‐year sediment trap measurements, sediment analyses and simple mass balance models to elucidate potential causes of eutrophication of a deep temperate clearwater lake, where total phosphorus (TP) concentrations quadrupled within a decade, accompanied by expanding hypolimnetic anoxia. Discrepancies between modeled and empirically determined P inputs suggest that the observed sharp rise in TP was driven by internal processes. The magnitude of seasonal variation in TP greatly increased at the same time, both in surface and deep water, partly decoupled from deep water oxygen conditions. A positive correlation between annual P loss from the upper water column and hypolimnetic P accumulation could hint at a short‐circuited P cycle involving lateral TP transport from shallow‐water zones and deposition and release from sediments in deep water. This hypothesis is also supported by P budgets for the upper 20 m during stable summer stratification, suggesting that sediments in shallow lake areas acted as a P net source until 2018. These changes are potentially related to shifts in submerged macrophytes from wintergreen charophyte meadows (Nitellopsis obtusa) to annual free‐floating hornwort (Ceratophyllum demersum) and to increased sulfide formation, promoting iron fixation in the sediments. Iron bound to sulfur is unavailable for binding P, resulting in a positive feedback between P release in shallow lake areas, primary productivity, macrophyte community structure and redox‐dependent sediment biogeochemistry. Overall, our results suggest that relationships more complex than the commonly invoked increase in internal P release under increasingly anoxic conditions can drive rapid lake eutrophication. Since the proportion of littoral areas is typically large even in deep stratified lakes, littoral processes may contribute more frequently to the rapid lake eutrophication trends observed around the world than is currently recognized.

中文翻译：

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克利尔沃特湖的快速富营养化：从磷质量平衡分析推断的趋势和潜在原因


许多克利尔沃特湖越来越多地表现出富营养化的症状，但根本原因在很大程度上是未知的。我们结合了长期水化学数据、多年沉积物陷阱测量、沉积物分析和简单的质量平衡模型，以阐明深温带清水湖富营养化的潜在原因，其中总磷 （TP） 浓度在十年内翻了两番，并伴有不断扩大的低涧酸缺氧。建模和经验确定的 P 输入之间的差异表明，观察到的 TP 急剧上升是由内部过程驱动的。TP 的季节性变化幅度同时大大增加，无论是在表层水中还是在深水中，部分与深水氧气条件脱钩。上层水柱的年磷损失与低石灰质磷积累之间的正相关可能暗示一个短路的 P 循环，涉及浅水区的横向 TP 运输和深水沉积物的沉积和释放。在稳定的夏季分层期间，上部 20 m 的 P 预算也支持这一假设，这表明直到 2018 年，浅湖地区的沉积物一直是 P 净来源。这些变化可能与淹没的大型植物从冬绿草甸 （Nitellopsis obtusa） 转变为一年生自由漂浮的金缕梅 （Ceratophyllum demersum） 以及硫化物形成增加有关，从而促进了沉积物中的铁固定。与硫结合的铁不能结合 P，导致浅湖区 P 释放、初级生产力、大型植物群落结构和氧化还原依赖性沉积物生物地球化学之间存在正反馈。 总体而言，我们的结果表明，在日益缺氧的条件下，比通常调用的内部 P 释放增加更复杂的关系可以驱动湖泊的快速富营养化。由于即使在深层湖泊中，沿海地区的比例通常也很大，因此沿海过程对世界各地观察到的快速湖泊富营养化趋势的贡献可能比目前认识到的要高。