Weather and catchment morphology drive thermal regime variation among subarctic ponds, and possible effects on resident Arctic charr

Abstract Thermal stratification, which is a common feature of lentic freshwater systems, has extensive effects on ecological interactions and ecosystem function, including processes that may determine which systems can support fish populations and affect growth, phenology, and metabolism where populations exist. Because these habitats are important for Northern freshwater fishes, improvement of our ability to forecast thermal stratification and associated ecological processes, like dissolved oxygen dynamics, could increase the accuracy of occupancy and distribution modeling, inform conservation strategies, and predict contemporary evolutionary patterns. Although thermal regimes in temperate systems are well-characterized, the irregular thermal regimes that are often present in small Arctic and subarctic lakes and ponds are more poorly understood. In a unique cave pond system near Mývatn Iceland, where conditions shaped by thermal stratification may be acting as selective agents on divergence of Arctic charr populations, we found differences in thermal stratification regimes related to the orientation of cave openings and the highly irregular catchment topography. In particular, while greater exposure to warm air temperatures can facilitate summer stratification and results in more temporally and spatially variable temperatures, exposure to wind—which is modulated on a small scale by the terrain—can facilitate mixing. These patterns caused only the more sheltered of the two ponds remain continuously isothermic. We also found that growth rates and body condition in the ponds’ Arctic charr populations ( Salvelinus alpinus ) are consistent with constraints on growth and metabolism imposed by low temperature in the cooler, continuously isothermal pond, although we cannot rule out the effects of prey limitation.