Rapidly evolving zooplankton in a salinizing world: To what extent does microevolutionary adaptation to one salt increase tolerance to another one?

Abstract

AbstractSalinization is a worldwide emerging threat to freshwater systems. It can be caused by various salts with different toxicity to freshwater organisms. Populations of freshwater organisms have been shown to rapidly evolve adaptations to salinity, yet it remains unclear whether adaptation to one salt increases tolerance or, conversely, susceptibility to other salts. Using an experimental evolution approach, we subjected replicate populations of a planktonic monogonont rotifer species (Brachionus calyciflorus Pallas) to non‐saline and NaCl‐enriched environments. A subsequent multigenerational common garden (CG) transplant experiment demonstrated rapid microevolutionary adaptation to salinity. A second CG experiment provided evidence that adaptation to NaCl increases tolerance mainly to CaCl2 but to a much lesser extent to Na2SO4. Increased tolerance to CaCl2 was associated with increased tolerance to Ca2+ ion toxicity. In contrast, acquired tolerance to Na2SO4 could not be shown to involve an improved ability to cope with ion toxicity. Our results highlight that the evolution of cross‐tolerance may expand the temporal and spatial dimensions under which the ecological consequences of salt adaptation apply.