Data and code from: Seasonal variation in the response to a toxin-producing cyanobacteria in Daphnia

Data and code accompanying: Hegg, Radersma &amp; Uller. 2022. Seasonal variation in the response to a toxin-producing cyanobacteria in Daphnia. Freshwater Biology, accepted. <strong>Abstract</strong> Many populations of water fleas (<em>Daphnia</em>) are exposed to algal blooms dominated by microcystin-producing cyanobacteria. However, the severity of these effects on <em>Daphnia</em> fitness remain poorly understood in natural populations. We investigated seasonal changes in body size, reproduction and survival of <em>Daphnia</em> <em>longispina</em> individuals from five eutrophic lakes in southern Sweden. We tested whether individuals collected before, during or following algal blooms differed in their reproduction and survival when experimentally exposed to microcystin-producing cyanobacteria. The concentration of microcystin in the lakes was significantly higher during summer and autumn compared to spring, but there were substantial differences between lakes. The reproductive output of individuals declined consistently over the season, and this decline was stronger for <em>Daphnia</em> collected during periods of, or lakes from, high microcystin concentration. There was little evidence that individuals adapted to the toxin over the season. The strong seasonal changes in body size, reproduction and survival in these <em>Daphnia</em> <em>longispina</em> appears to be partly caused by variation in the abundance of toxin-producing cyanobacteria. Populations were unable to adapt sufficiently quickly during summer and autumn to recover from the negative effects of microcystin. We therefore suggest that seasonal increases in tolerance to microcystin-producing cyanobacteria have limited effects on the eco-evolutionary dynamics between <em>Daphnia</em> and phytoplankton.