Data and code for: Freshwater salinization leads to sluggish, bloated frogs and small, cramped embryos but adaptive countergradient variation in eggs

Freshwater salinization is an emerging threat to aquatic ecosystems across the planet, degrading habitats and negatively impacting wild populations. Deicing practices are a leading cause of freshwater salinization, particularly in the snowbelt region of North America where a variety of salts are widely applied to roads and other surfaces to melt snow and ice. Seasonal pools near roads are considered the most severely impacted aquatic habitats. Runoff into these low water-volume ponds can generate high salinity. Impacts of salt pollution are numerous, ranging from toxicity to population decline to impaired ecosystem function. Here, we investigate a suite of physiological consequences of salinization across multiple life history stages of the wood frog (<i>Rana sylvatica</i>), a pool-dwelling amphibian. Previous work has shown that salinized populations have diverged from unpolluted populations for a suite of physiological, morphological, and reproductive traits, and can experience severe edema (bloating) during the breeding season. Here, we measured swim performance before and after aspirating edema in wild captured wood frogs to show that edema compromises adult aquatic locomotion during breeding. We also found that wood frog mothers from salinized ponds produce ova with inherently higher rates of water uptake compared to mothers from unpolluted pools, consistent with countergradient adaptation, but the ova are smaller. Finally, we found that exposure to road salt inhibits expansion of vitelline membranes in developing embryos and is associated with reduced embryo growth. Together, these results reveal the complexity of population level responses to freshwater salinization, highlighting that impacts occur across multiple life history stages, and that local populations might be evolving adaptations to cope with anthropogenic salinity gradients in freshwater habitats.

淡水盐化（freshwater salinization）是全球范围内威胁水生生态系统的新兴威胁，其破坏栖息地并对野生种群产生负面影响。除冰作业是引发淡水盐化的主要诱因，尤以北美积雪带区域为甚——该区域广泛使用各类盐类对道路及其他路面进行融雪化冰。道路附近的季节性池塘被认为是受盐污染影响最严重的水生栖息地，径流汇入此类低容水量池塘后会形成高盐环境。盐污染的危害多种多样，涵盖毒性效应、种群衰退以及生态系统功能受损等多个方面。
本研究针对栖息于池塘的两栖动物林蛙（wood frog，*Rana sylvatica*）多个生活史阶段的盐化生理效应展开探究。已有研究表明，盐化种群在一系列生理、形态及繁殖性状上已与未受污染种群产生分化，且在繁殖季会出现严重水肿（肿胀）现象。本研究通过对野外捕获的林蛙在抽吸水肿液前后的游泳能力进行测定，证实水肿会损害成体在繁殖期的水生运动能力。此外，研究发现相较于来自未受污染池塘的雌性林蛙，盐化池塘中的雌性林蛙所产出的卵天然吸水率更高，这符合反梯度适应（countergradient adaptation）特征，但其卵的体积更小。最后，本研究发现道路盐暴露会抑制发育胚胎的卵黄膜（vitelline membranes）扩张，并与胚胎生长受限存在显著关联。
综上，本研究结果揭示了种群响应淡水盐化的复杂性，表明盐污染的影响贯穿多个生活史阶段，且当地种群可能正演化出适应性机制以应对淡水生境中的人为盐度梯度。