Department of Plankton and Microbial Ecology, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Stechlin, Germany.
Department of Ecohydrology and Biogeochemistry, Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany.
Glob Chang Biol. 2024 Nov;30(11):e17575. doi: 10.1111/gcb.17575.
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.
许多清澈的湖泊越来越多地出现富营养化症状，但根本原因在很大程度上尚不清楚。我们结合了长期水质数据、多年的沉积物捕获器测量、沉积物分析和简单的质量平衡模型，以阐明一个深层温带清澈湖泊富营养化的潜在原因，该湖泊总磷 (TP) 浓度在十年内翻了两番，同时伴缺氧区扩大。模型和经验确定的磷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。与此同时，TP 在季节性变化的幅度大大增加，无论是在地表水还是深水，与深水氧气条件部分脱钩。上覆水柱中每年 P 损失与缺氧层 P 积累之间的正相关可能暗示着一个短循环 P 周期，涉及从浅水区域侧向 TP 运输以及从深水沉积物中沉积和释放。这一假设也得到了稳定夏季分层期间上覆 20 米范围内 P 预算的支持，表明浅水区沉积物在 2018 年之前一直是 P 的净源。这些变化可能与冬季绿色的轮藻草甸（Nitellopsis obtusa）转变为一年生自由漂浮的金鱼藻（Ceratophyllum demersum）的沉水植物的变化有关，以及硫化物形成的增加，促进了沉积物中的铁固定。与硫结合的铁不能与 P 结合，导致浅水区 P 释放、初级生产力、大型植物群落结构和氧化还原依赖的沉积物生物地球化学之间的正反馈。总的来说，我们的结果表明，与在越来越缺氧的条件下通常推断的内部 P 释放增加相比，更复杂的关系可能会导致快速的湖泊富营养化。由于即使在深层分层湖泊中，滨岸区的比例通常也很大，因此滨岸过程可能比目前认识到的更频繁地导致世界各地观察到的快速湖泊富营养化趋势。
