Grant Emerson Haines, Joseph Phillips, Bjarni Kristjansson, Camille A.-L. Leblanc

Thermal stratification is common in lentic freshwater systems, and has extensive effects on ecosystem function and the interactions between aquatic organisms and their surroundings. Although thermal regimes in temperate systems are well-characterized, small arctic and subarctic lakes and ponds can have irregular thermal regimes, and the conditions leading to stratification and mixing are less predictable. Nevertheless, they can be important habitats for northern fish species, and their ecological processes, including thermal stratification and dissolved oxygen dynamics, may determine whether they can support fish populations. Where such populations do persist, temperature and dissolved oxygen determine where and when fish are active, including foraging and spawning behavior, and consequently may shape trajectories of local adaptation. We studied differences in thermal stratification regimes in two adjacent ponds in a unique cave pond system in Iceland, where conditions shaped by thermal stratification may be acting as selective agents for phenotypic divergence of Arctic char. In particular, we characterize differences in thermal stratification regimes in relation to the orientation of cave openings and the highly irregular catchment topography, describe the association of the thermal regimes with dissolved oxygen dynamics, and describe potential consequences for the ponds’ resident charr populations