To test the general assumption that global warming will induce body size reduction in aquatic organisms, we used a system of lakes continually heated for six decades by warm water discharge from power plants. Their temperature elevation of 3–4°C corresponds with climate change forecasts for the end of the 21st century. We compared body size and reproduction of Daphnia longispina complex communities inhabiting heated and non‐heated (control) lakes nearby. No difference in body size was found, but Daphnia communities from heated lakes had a wider thermal breadth for reproduction. The two lake groups varied in the taxonomic composition of Daphnia communities. Thus, to disentangle inter‐ and intraspecific sources of variation, and to examine evolution vs. phenotypic plasticity of investigated traits, we performed two life history experiments: (1) a between‐species experiment compared D. galeata inhabiting heated lakes with D. longispina typical of nearby control lakes, under three temperature regimes; (2) a within‐species experiment compared D. galeata from heated lakes with conspecifics from high latitude (cold control) and low latitude (warm control) lakes, under two temperature regimes. The experiments revealed countergradient variation: environmental constraints on body size in situ concealed evolution of larger potential body size in Daphnia from heated lakes. In turn, evolution of increased body size plasticity resulted in an efficient resource allocation trade‐off: more effective reproduction at high temperature, at the cost of size reduction. We suggest that large size is adaptive during active overwintering, while plastic size reduction is a coping strategy for high temperatures.

中文翻译：

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反梯度变化掩盖了加热湖泊中水蚤对温度升高的适应性响应

为了检验一般的假设，即全球变暖将导致水生生物的体型缩小，我们使用了由发电厂排出的热水连续加热六十年的湖泊系统。它们的温度升高3-4°C与21世纪末的气候变化预测相对应。我们比较了居住在附近加热和不加热（控制）湖中的水蚤复杂种群的体型和繁殖。没有发现体型的差异，但是来自加热的湖泊的水蚤社区具有更宽的繁殖繁殖力。这两个湖泊群的水蚤的生物分类组成各不相同社区。因此，为了弄清种间和种内的变异源，并研究所研究性状的进化与表型可塑性，我们进行了两个生活史实验：（1）物种间实验比较了居住在加热湖泊中的加里氏石D和长石pin。在三种温度条件下，典型的附近控制湖；（2）一项物种内实验比较了在两个温度范围内加热湖中的D. galeata与高纬度（冷控制）和低纬度（温暖控制）湖的同种异体。实验揭示了梯度梯度的变化：原位生物体的环境限制掩盖了水蚤较大体型的进化。从加热的湖泊。反过来，增加体型可塑性的演变导致了有效的资源分配折衷：在高温下更有效的繁殖，但以减小尺寸为代价。我们建议大尺寸在主动越冬过程中具有适应性，而减小塑料尺寸是应对高温的一种策略。