Rapid temporal adaptation structures tolerance to toxic cyanobacteria in a natural population of the water flea <i>Daphnia</i>

Cyanobacteria blooms pose a substantial threat to freshwater systems globally. While zooplankton grazers such as <i>Daphnia</i> can have an important role in suppressing cyanobacteria blooms, cyanobacteria can adversely impact <i>Daphnia</i> fitness and even kill them. Earlier work has shown an evolutionary increase in tolerance to cyanobacteria across years and strong genotype × genotype interactions determining the interaction between <i>Daphnia</i> and the cyanobacterium <i>Microcystis</i>. Here, we test the hypothesis that <i>Daphnia magna</i> can adapt during 1 growing season to changes in dominant strains of <i>Microcystis</i>. Over 2 consecutive years, we collected <i>D. magna</i> clonal lineages and <i>Microcystis</i> strains from a single pond early and late in the growing season and we assessed whether <i>Daphnia</i> survival differed when exposed to <i>Microcystis</i> strains from either the same or a different time point within the growth season. Our findings reveal important <i>Daphnia</i> genotype × <i>Microcystis</i> genotype interactions, with <i>Daphnia</i> survival being higher when exposed to <i>Microcystis</i> from the same time point than when exposed to <i>Microcystis</i> of a different time point. Our results extend earlier findings to variation within 1 single natural system and growth season, and suggest an important impact of rapid (co)evolutionary dynamics shaping the tolerance of zooplankton grazers to cyanobacteria.