Seasonal variation in the response to a toxin‐producing cyanobacteria in <i>Daphnia</i>

Abstract Many populations of water fleas ( Daphnia ) are exposed to algal blooms dominated by microcystin‐producing cyanobacteria. However, the severity these effects on fitness remain poorly understood in natural populations. We investigated seasonal changes body size, reproduction and survival D . longispina individuals from five eutrophic lakes southern Sweden. tested whether collected before, during or following differed their when experimentally The concentration microcystin was significantly higher summer autumn compared spring, but there were substantial differences between lakes. reproductive output declined consistently over season, this decline stronger for periods of, with, high concentration. There little evidence that adapted toxin season. strong appear be caused partly variation abundance toxin‐producing Populations unable adapt sufficiently quickly recover negative microcystin. therefore suggest increases tolerance cyanobacteria have limited eco‐evolutionary dynamics phytoplankton.