Rapid Evolutionary Response to Salinity Fluctuations in a Coastal Daphnia Population

AbstractSalinization poses a widespread threat to freshwater ecosystems. Land use practices and sea level rise contribute added salt, while climate change may drive increasing fluctuations in salinity. Evidence of local adaptation to salt stress indicates that evolution may mitigate some of the ecological harm that salinization would inflict. However, the extent to which populations may adapt to fluctuating salinity conditions remains poorly understood. We performed a common-garden experiment examining the evolutionary response of a Daphnia population to interannual variation in salinity conditions observed in a coastal Maine lake. Daphnia isolated during peak salinity conditions (2022) and several months after another salinity peak (2019) showed substantially increased survival and reproduction under chronic sea salt stress relative to Daphnia isolated during a period of relatively low-ion conditions (2021). We hypothesize that Daphnia egg bank dynamics and a fitness cost of salt tolerance under low-ion conditions may explain these evolutionary dynamics.