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Nearly a century of road salt use in the snowbelt region of North America has led to substantial increases in salinity levels in freshwater habitats. Salt pollution in lakes and rivers is well characterized. Lacking are broad insights for seasonal ponds. As critical habitats for many endemic species, these small and often poorly flushed surface waters are especially vulnerable to accumulating high levels of salts and other pollutants. Here, we measured salinity in 165 seasonal ponds, characterizing salt pollution patterns across space, through time, and over depth within ponds. We found that 70% of ponds within 37 m of a road contained salinity levels exceeding Canadian federal guidelines. 54% of ponds within 25 m exceeded less conservative US federal guidelines. Within ponds, the water column was stratified due to the combined density effects of salt and temperature. Bottom waters of polluted ponds were about 57% saltier than near-surface waters, though many were much saltier than this. Compared to lakes and rivers, far more seasonal ponds appear to be compromised by deicing salt, and overall, the concentration of salt appears to be substantially higher. Among aquatic habitats, seasonal ponds are experiencing the most severe impacts of freshwater salinization, with consequent impacts on sensitive aquatic organisms.
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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 (Rana sylvatica), 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. © The Author(s) 2025. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology.
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Phenotypic variation is common across life history and among populations occupying different environments, yet the molecular mechanisms underlying these axes of divergence remain poorly understood. Much work has focused on gene expression as a link between genetic variation, environmental variation, and phenotypes, but post-transcriptional processes such as alternative splicing—which affect how transcripts are assembled rather than how much of a transcript is produced—are increasingly recognized as additional modulators of plasticity and adaptation. Here, we examined gene expression and alternative splicing together in the wood frog (Rana sylvatica), an amphibian with a complex life cycle whose populations differ across replicated gradients of road adjacency and associated pollution. We found extensive transcriptomic differences between hatchlings and adults, with thousands of genes differentially expressed or spliced. Individuals clustered strongly by population for both expression and splicing. Differences at the habitat level were less extensive, but revealed two differentially expressed genes (HSP70 and Gpsm2) and one differentially spliced gene (Cd82) that consistently distinguished roadside and woodland populations. Overall, genetic differentiation between populations was low, suggesting that phenotypic and transcriptomic differences likely emerge in the presence of gene flow and reflect plastic responses. Together, these results highlight transcriptomic plasticity as an important mechanism shaping variation across both development and population differentiation. © 2025 The Author(s). Ecology and Evolution published by British Ecological Society and John Wiley & Sons Ltd.
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