COS 187-4 - Conceptions and determinants of freshwater pond thermal refuges

Abstract: Cold thermal refuges at spatially relevant scales are posited to provide physiological and demographic benefits to organisms amidst climate warming. Identifying the features that promote thermal refuges is vital, both as a means of identifying habitats that may allow the long-term persistence of populations, and also to comprehend the feasibility of managing ecosystems to promote such attributes. Despite being appreciated and well-characterized in lotic ecosystems, thermal refuges in pond environments have not yet been sufficiently operationalized– despite providing important ecosystem services that may be compromised by climate warming. Here, we put forward different conceptions of what thermal refuges may entail for pond environments based on pond persistence, mean temperature, and the internal relationship between pond mean temperature and thermal spatial heterogeneity. Subsequently, we draw from theory to intuit the ecological significance of each refuge type during both typical summer and heatwave conditions. Finally, by collecting landscape-scale and spatially resolved thermal data for a series of 20 ponds in the Mount Saint Helens region (Washington, USA) during summer 2022, we identify the shared and divergent factors that are associated with promoting each refuge conception.

We observed substantial amounts of thermal spatial heterogeneity during summer months, even among shallow pond environments. We predict that such within-pond thermal refuges, by allowing organisms consistent access to a sub-lethal water environment, can allow for the persistence of herbivore and consumers during periods of surface warming. Our recorded summer water temperature measurements captured a one-week heatwave event from July 26 to August 2 where many pond surface temperatures exceeded 30^OC. During this period when mean pond temperatures were elevated to create stressful conditions, internal thermal refuges remained prominent in certain ponds. Yet, both pond mean temperatures and within-pond thermal refuge quality varied considerably among ponds. The abiotic characteristics associated with more substantial thermal refuges were greater lake surface area, increased water clarity, and decreased average temperature; while higher mean pond temperature was associated with decreased canopy cover. A negative relationship existed between within-pond refuge quality and mean temperature across the summer months, as well as during the heatwave, albeit to a lesser extent. While multiple approaches exist for operationalizing how thermal refuges are measured in pond ecosystems, pond thermal refuges have the potential to mitigate detrimental consequences to organisms amidst extreme heat.