Resurrection of dormant zooplankton grazers reveals multiple evolutionary responses to toxic cyanobacteria

The cyanobacterial blooms caused by eutrophication exert strong selection on zooplankton grazers. Resurrection ecology, coupled with multivariate reaction norm analysis, can be used to parse the relative contributions of plasticity and evolution to mediating adaptation to environmental changes in natural populations. Here, we tested whether increasing levels of toxic cyanobacteria result in multivariate reaction norm changes in a natural zooplankton population. We resurrected dormant eggs of Ceriodaphnia cornuta (Cladocera) from three sediment layers in Dianshan Lake from 1988 to 2018, a period during which the abundances of cyanobacteria continuously increased, and conducted a common garden experiment to quantify the multivariate reaction norms of nine morphological, life history, and behavioral traits involved in the response to the toxic cyanobacterium Microcystis aeruginosa. The youngest subpopulation coinciding with the high-eutrophic period was characterized by smaller size at birth and maturity and fewer offspring in their second broods compared with the oldest subpopulation coinciding with the low-eutrophic period. Patterns of covariance in multivariate trait space provided strong evidence of adaptive evolution in multiple traits over time. The total trait change generated through both constitutive evolution and evolution of plasticity ranged from 1% to 64% depending on the trait and subpopulations being compared. These results suggest that multivariate phenotypic shifts in response to environmental changes in the study population are achieved by integrating plasticity, evolution of trait means, and evolution of plasticity.