Many lake ecosystems that have been severely disturbed by eutrophication, have also experienced large human efforts to restore “natural” conditions. However, the trajectories and the extent of recovery of these lake ecosystems are still poorly understood. In many shallow lakes, recovery was often delayed and counter-clockwise hysteretic. Here, we study recovery and ecosystem trajectories in a large and deep lake using diatom remains in sediment cores and time series of phosphorus concentrations. We identified four periods of diatom community change: slow change during early eutrophication, thereafter a short period of rapid change after the 1950s, followed by community stability from the 1960s to the mid-1980s, and finally a recovery phase until 2010. Diatom community structure responded quickly and in a saturating way to increasing phosphorus concentrations, but also fast to phosphorus decline. Hence, diatom community dynamics did not show counter-clockwise hysteresis but was characterized by a high degree of recovery and clock-wise hysteresis (CWH). We suggest that CWH in response to eutrophication and recovery is a typical and previously overlooked feature of deep lakes, which results from a more rapid change of average nutrient concentrations and thus productivity in the epilimnion compared to average nutrient concentrations across the entire water column. Such nonlinear and hysteretic responses to changing nutrients need to be considered when analyzing the effects of other stressors such as climate warming on ecosystem dynamics to prevent erroneous attribution of ecosystem change to other stressors instead of nutrient change.

中文翻译：

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硅藻顺时针滞后响应富营养化和恢复过程中的营养动态

许多受到富营养化严重干扰的湖泊生态系统也经历了人类为恢复“自然”条件所做的巨大努力。然而，这些湖泊生态系统的轨迹和恢复程度仍然知之甚少。在许多浅水湖中，恢复通常被延迟并且逆时针滞后。在这里，我们使用沉积物核心中的硅藻残骸和磷浓度的时间序列来研究一个大而深的湖泊中的恢复和生态系统轨迹。我们确定了硅藻群落变化的四个时期：早期富营养化期间的缓慢变化，此后在 1950 年代之后的短期快速变化，随后是 1960 年代至 1980 年代中期的群落稳定，最后是直到 2010 年的恢复阶段。硅藻群落结构对磷浓度的增加响应迅速且饱和，但对磷的下降也很快。因此，硅藻群落动力学没有表现出逆时针滞后，而是以高度恢复和顺时针滞后（CWH）为特征。我们认为响应富营养化和恢复的 CWH 是深湖的一个典型且以前被忽视的特征，这是由于与整个水柱的平均养分浓度相比，平均养分浓度的变化更快，从而导致表层的生产力变化。