Potential loss of cold-water stenothermal diatoms (Bacillariophyta) from their southern refugia in the western United States

Abstract

A large number of cold-water stenothermal diatom taxa live between 39 and 49°N latitude at the southern limits of their respective ranges in the mountainous northwest sector of the United States. These taxa are either endemics, alpine taxa with disjunct populations in Europe and Asia, or boreal taxa that are more common at higher latitudes. The southern populations are vulnerable to extirpation from the effects of climate change and other stressors. A simple model was created to assess relative risk of extirpation from the northwest United States for a subset of 35 cold-water stenothermal taxa. The model has five input variables: number of populations south of 49°N latitude, mean population size, distance of the southernmost population from 49°N, mean water temperature at the time of sampling, and lowest elevation of a refugium population. Nine taxa, including three local endemics, were found to be at a high risk of extirpation or extinction resulting from climate change. Eight taxa were rated at a low risk of extirpation and risk to the remaining 18 taxa was rated as moderate. Type localities, images of voucher specimens, and latitudinal spectra are provided for each of the 35 selected taxa. The fate of rare and sensitive diatoms will depend on how the global community addresses (or does not address) climate change and how the lands and habitats that support these species are managed (or not managed) to protect larger and more charismatic species.