Altered Phenotypic Responses of Asexual Arctic <i>Daphnia</i> After 10 Years of Rapid Climate Change

Understanding the fates of organisms and ecosystems under global change requires consideration of the organisms' rapid adaptation potential. In the Arctic, the recent temperature increase strongly impacts freshwater ecosystems which are important sentinels for climate change. However, a mechanistic understanding of the adaptive capacity of their key zooplankton grazers, among them polyploid, obligate parthenogenetic Daphnia, is lacking. Theory suggests low adaptation potential of asexual animals, yet examples exist of asexuals persisting through marked environmental changes. Here, we studied asexual Daphnia pulicaria from a meromictic lake in South-West Greenland. Its oxycline hosts purple sulfur bacteria (PSB), a potential food source for Daphnia. We tested two key phenotypic traits: (1) thermal tolerance as a response to rapid regional warming and (2) hypoxia tolerance tied to grazing of PSB in the hypoxic/anoxic transition zone. To assess Daphnia's adaptive capacity, we resurrected Daphnia from dormant eggs representing a historical subpopulation from 2011, sampled modern subpopulation representatives in 2022, and measured phenotypic variation of thermal (time to immobilization-T_imm) and hypoxia tolerance (respiration rate and critical oxygen limit-P_crit) in clonal lineages of both subpopulations. Whole genome sequencing of the tested clonal lineages identified three closely related genetic clusters, one with clones from both subpopulations and two unique to each subpopulation. We observed significantly lower T_imm and a trend for higher P_crit and respiration rates in the modern subpopulation, indicating a lower tolerance to both high temperature and hypoxia in comparison with the historical subpopulation. As these two traits share common physiological mechanisms, the observed phenotypic divergence might be driven by a relaxed selection pressure on hypoxia tolerance linked to variation in PSB abundance. Our results, while contrary to our expectation of higher thermal tolerance in the modern subpopulation, provide evidence for phenotypic change within a decade in this asexual Daphnia population.