Longitudinal water temperature gradients in streams are widespread and are associated with changes in fish community structure. A major cause of these changes is variation in the thermal tolerances and preferences of coldwater, coolwater, and warmwater fish species. Much less is known about water temperature gradients in lake outflow coldwater streams and associated variation in fish communities. Our main objective was to assess longitudinal temperature gradients in lake outflow streams and to measure the corresponding variation in fish community structure. We quantified temperature gradients and the fish communities in two lake outflow streams in central Wisconsin from June through October 2019. We measured longitudinal water temperature gradients spanning up to 6 °C or more during the summer but the gradients weakened in late summer and were not present in autumn. Warmwater and coolwater species such as rock bass (Ambloplites rupestris), bluegill (Lepomis macrochirus), and creek chub (Semotilus atromaculatus) were more common in reaches nearest to lake outflows in both streams. Coldwater species such as brown trout (Salmo trutta), brook trout (Salvelinus fontinalis), and mottled sculpin (Cottus bairdii) were more common in downstream reaches. Increasing water temperatures in lakes due to global climate change will likely increase the magnitude of longitudinal temperature gradients in coldwater streams that are fed by warmwater lakes, which will probably impact the distributions of coldwater fish species in these systems.

溪流中的纵向水温梯度很普遍，并且与鱼类群落结构的变化有关。这些变化的一个主要原因是冷水、冷水和温水鱼类的耐热性和偏好的变化。关于湖泊流出冷水流中的水温梯度和鱼类群落的相关变化知之甚少。我们的主要目标是评估湖泊流出流的纵向温度梯度并测量鱼类群落结构的相应变化。我们量化了 2019 年 6 月至 10 月威斯康星州中部两条湖泊流出流的温度梯度和鱼类群落。我们在夏季测量了高达 6 °C 或更高的纵向水温梯度，但梯度在夏末减弱，而在秋季则不存在。温水和冷水物种，如摇滚鲈鱼（Ambloplites rupestris )、蓝鳃鱼 ( Lepomis macrochirus ) 和小河鲢 ( Semotilus atromaculatus ) 在两条溪流中最靠近湖泊流出的河段中更为常见。冷水种类，如褐鳟（Salmo trutta）、溪鳟（Salvelinus fontinalis）和斑杜鱼（Cottus bairdii）在下游更常见。由于全球气候变化导致湖泊水温升高，可能会增加由温水湖泊供给的冷水溪流中纵向温度梯度的幅度，这可能会影响这些系统中冷水鱼类的分布。