Climate warming and nitrate pollution are pervasive aquatic stressors that endanger the persistence of fishes prevailing in anthropogenically disturbed habitats. Individually, elevated nitrate and temperature can influence fish energy homeostasis by increasing maintenance costs and impairing oxygen transport capacity. However, it remains unknown how fish respond to simultaneous exposure to elevated temperature and nitrate pollution. Hence, we examined the combined effects of nitrate and elevated temperatures on aerobic scope (AS, maximum-standard metabolic rates) and cardiorespiratory attributes (haemoglobin H_B, haematocrit H_CT, relative ventricle mass RVM, and somatic spleen index SSI) in a freshwater salmonid, Thymallus thymallus. A 3 × 2 factorial design was used, where fish were exposed to one of three ecologically relevant levels of nitrate (0, 50, or 200 mg NO₃⁻ l^-1) and one of two temperatures (18 °C or 22 °C) for 6 weeks. Elevated temperature increased AS by 36 % and the improvement was stronger when coupled with nitrate exposure, indicating a positive synergistic interaction. H_B was reduced by nitrate exposure, while H_CT was independent of nitrate pollution and temperature. Stressor exposure induced remodeling of key elements of the cardiorespiratory system. RVM was 39 % higher in fish exposed to 22 °C compared to 18 °C but was independent of nitrate exposure. SSI was independent of temperature but was 85 % and 57 % higher in fish exposed to 50 and 200 mg NO₃⁻ l^-1, respectively. Taken together, these results highlight that simultaneous exposure to elevated temperatures and nitrate pollution offers cross-tolerance benefits, which may be underscored by cardiorespiratory remodeling.

气候变暖和硝酸盐污染是普遍的水生压力源，危及人为干扰栖息地中普遍存在的鱼类的持久性。个别而言，硝酸盐和温度升高会通过增加维护成本和削弱氧气的运输能力而影响鱼类能量的体内平衡。然而，仍然未知鱼类对同时暴露于高温和硝酸盐污染的反应如何。因此，我们研究了硝酸盐和高温对淡水中有氧范围（AS，最大标准代谢率）和心肺功能（血红蛋白H _B，血细胞比容H _CT，相对心室质量RVM和躯体脾指数SSI）的综合影响鲑科，Thymallus thymallus。使用3×2析因设计，将鱼暴露于三种与生态相关的硝酸盐水平（0、50或200 mg NO ₃ ^- l ^-1）和两种温度（18°C或22°C）之一）的6周。升高温度会使AS升高36％，并且与硝酸盐接触时，增强作用更强，表明存在积极的协同作用。硝酸盐暴露可降低H _B，而H _CT可降低与硝酸盐污染和温度无关。应激源暴露引起心肺系统关键要素的重塑。与18°C相比，暴露于22°C的鱼的RVM高39％，但与硝酸盐暴露无关。SSI是不受温度影响，但为85％和57％在鱼更高暴露于50和200毫克NO ₃ ^-升^-1分别。综上所述，这些结果突出表明，同时暴露于高温和硝酸盐污染下可提供交叉耐受性，这可通过心肺重构来强调。