Seasonal variation in the response to a toxin-producing cyanobacteria in Daphnia

Many populations of water fleas (Daphnia) are exposed to algal blooms dominated by microcystin-producing cyanobacteria. However, the severity of these effects on Daphnia fitness remain poorly understood in natural populations. We investigated seasonal changes in body size, reproduction and survival of D. longispina 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 Daphnia collected during periods of, or from lakes with, 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 D. longispina appear to be caused partly 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 Daphnia and phytoplankton.

Automated summary

This study investigated the seasonal changes and the effects of microcystin-producing cyanobacteria on the reproduction and survival of Daphnia longispina individuals in five eutrophic lakes in southern Sweden. The results showed that the concentration of microcystin was higher during summer and autumn, and individuals collected during these periods or from lakes with high microcystin concentration had a stronger decline in reproductive output. The populations were unable to adapt quickly enough to recover from the negative effects of microcystin, indicating limited effects of seasonal tolerance on the eco-evolutionary dynamics between Daphnia and phytoplankton.