Daphnia are key organisms in pelagic food webs, acting as a food resource for fish and predatory zooplankton and regulating phytoplankton through grazing. Its population dynamic follows regular seasonal patterns, with spring peaks followed by summer population declines (midsummer declines, MSDs). Midsummer declines show high inter‐annual variation, which has been attributed to different causes. However, the mechanisms controlling the MSD remain poorly understood, especially in deep stratified lakes. We tried to disentangle the factors causing Daphnia MSDs in Lake Lugano and Lake Iseo (in Switzerland and Italy), two deep peri‐alpine lakes with similar trophic status and vertical mixing dynamics, characterised by phosphorus accumulation in the hypolimnion and variable mixing during late‐winter turnovers. Specifically, we assessed the effects of three different hypothetical pathways according to which: (1) winter air temperature controls MSDs by influencing mixing depth during turnovers and epilimnetic phosphorus replenishment; (2) vernal air temperature influences MSD by accelerating the timing of spring population peak; and (3) summer temperature influences MSDs by increasing fish predation. We assessed the relative strength of these pathways using structural equation modelling on long‐term datasets for the two lakes (29 years for Lake Lugano and 19 years for Lake Iseo). Between the hypothesised pathways, the one driven by winter air temperature (through P replenishment) influenced Daphnia abundance in spring in both lakes, but the effects propagated to summer Daphnia abundance only in Lake Lugano. Additionally, summer Daphnia abundance was influenced by the summer air temperature through a positive (although weak) effect. By comparison, vernal air temperature had no detectable effects on summer Daphnia abundance. The results revealed marked differences between the meromictic study lakes and the shallow hypertrophic water bodies that were the focus of previous research on Daphnia MSD, and also between the two study lakes. The influence of epilimnetic P replenishment on the summer Daphnia abundance in Lake Lugano, which was recovering from past eutrophication, may have reflected the greater susceptibility of deep, stratified lakes to P depletion after spring compared to shallow hypertrophic lakes or reservoirs. This effect might not have been detected in Lake Iseo because P was more consistently depleted during the study period (i.e. variance in the predictor was too low to detect an effect). This study highlighted the complexity of the effects of climate variability on Daphnia MSD in deep lakes, showing that the responses can differ even between two neighbouring lakes with similar vertical mixing dynamics and trophic status. At the same time, the results suggest that future increases in winter air temperature, caused by global warming, may cause critically low densities of Daphnia during spring and summer and compromise the ability of zooplankton to control phytoplankton biomass.

中文翻译：

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两个深海分流亚高山湖泊水蚤仲夏衰退的原因

水蚤是远洋食物网中的关键生物，是鱼类和捕食性浮游动物的食物资源，并通过放牧调节浮游植物。其人口动态遵循规律的季节性模式，春季高峰后夏季人口下降（仲夏下降，MSD）。仲夏的下降显示出较高的年际变化，这归因于不同的原因。然而，控制 MSD 的机制仍然知之甚少，尤其是在深层分层的湖泊中。我们试图解开导致卢加诺湖和伊塞奥湖（瑞士和意大利）水蚤 MSD 的因素，这两个深高山湖泊具有相似的营养状态和垂直混合动态，其特征是下层水层中磷的积累和后期的可变混合。冬季营业额。具体来说，我们评估了三种不同假设途径的影响，根据这些途径：（1）冬季气温通过影响周转和浮游磷补充期间的混合深度来控制 MSD；(2)春季气温通过加速春季人口高峰时间影响MSD；(3) 夏季温度通过增加鱼类捕食来影响 MSD。我们使用结构方程模型对两个湖泊的长期数据集（卢加诺湖为 29 年，伊塞奥湖为 19 年）评估了这些途径的相对强度。在假设的途径之间，由冬季气温（通过 P 补充）驱动的途径影响了两个湖泊春季水蚤丰度，但这种影响仅在卢加诺湖传播到夏季水蚤丰度。此外，夏季水蚤丰度受到夏季气温的积极（虽然微弱）影响。相比之下，春季气温对夏季水蚤丰度没有可检测到的影响。结果表明，分流研究湖泊和浅层肥厚水体之间存在显着差异，这是先前对水蚤 MSD 研究的重点，两个研究湖泊之间也存在显着差异。与浅层肥厚湖泊或水库相比，从过去的富营养化中恢复的流水磷补充对卢加诺湖夏季水蚤丰度的影响可能反映了深层次的湖泊在春季后对磷消耗的敏感性更大。在 Iseo 湖中可能没有检测到这种影响，因为在研究期间 P 更持续地耗尽（即 预测变量的方差太低而无法检测到效果）。这项研究强调了气候变异对深湖水蚤 MSD 影响的复杂性，表明即使两个相邻的湖泊之间具有相似的垂直混合动态和营养状态，响应也可能不同。同时，结果表明，未来由全球变暖引起的冬季气温升高可能会导致春季和夏季水蚤的密度极低，并损害浮游动物控制浮游植物生物量的能力。