Fish-mediated impacts highlight the conservation value of Esker kettle lakes.

Kettle lakes on eskers, formed during glacial retreat and isolated from surface hydrological networks, represent unique ecosystems that provide high-quality habitats for aquatic biodiversity and offer essential services such as drinking water. However, their ecological integrity is increasingly threatened by fish introductions and forest harvesting. We assessed the effects of fish presence on waterbird and macroinvertebrate communities in esker kettle lakes of Eastern Canada, using these taxa as indicators of ecosystem health. Comparisons were made with clay-belt lakes, the region's most common and typically fish-bearing waterbodies. Physicochemical conditions differed markedly between esker and clay-belt lakes but not between fishless and fish-bearing esker lakes. Waterbird richness and abundance varied among lake types, with several species negatively affected by fish presence. Macroinvertebrate communities were also shaped by fish, while fish assemblages differed between esker and clay-belt lakes. In esker lakes, fish richness was positively correlated with a forest disturbance index reflecting human access, forestry, agriculture, housing, and recreational activities. Our results highlight the distinctive species assemblages of fishless esker lakes and their high vulnerability to anthropogenic pressures. Protecting these ecosystems is essential for biodiversity conservation, and our findings provide a scientific basis for sustainable management integrating lake and forest ecosystems.