1. Growing evidence shows that cyanobacterial blooms thrive in warmer temperatures, underlining the importance of understanding the relationship between cyanobacterial food, temperature and performance of the key freshwater herbivore Daphnia.
2. To evaluate potential effects of toxic cyanobacterial food and warmer temperatures, we first offered populations of Daphnia galeata a mixture of a low dosage of toxic non-colony-forming cyanobacteria [Microcystis aeruginosa (PCC7806 strain) and non-toxic, good-quality food (Scenedesmus obliquus) and S. obliquus as a sole food source]. After 2 weeks on the different diets, we exposed the daphnids to different temperatures (15 °C, 20 °C and 25 °C) for 2 weeks.
3. We expected that short-term exposure to toxins would reduce population growth, but populations would consist of larger individuals (‘bigger is better’). Upon long-term exposure, however, daphnids switched to higher temperature would be forced to mature at a smaller size and produce smaller, more vulnerable, offspring with subsequent reductions in population size (‘hotter is smaller’). Conversely, populations of daphnids switched to lower temperature should be able to cope better with prolonged exposure to toxins by producing more and larger offspring, less vulnerable to toxins (‘bigger is better’).
4. The amount of the toxin microcystin incorporated in the Daphnia tissue decreased in the first 7 days of exposure, suggesting triggering of detoxification mechanisms upon first exposure. In the remainder of the experiment, the amount of microcystin incorporated in Daphnia tissue increased to high levels, showing the differences between short-term exposure and long-term exposure to low-dosage toxins.
5. Contrary to our expectations, short-term exposure to low dosages of toxic Microcystis resulted in daphnids attaining a higher population density on the mixed diet. Furthermore, on mixed diets daphnids did not increase their body mass (‘bigger is not better’), but rather decreased strongly compared to daphnids growing on pure Scenedesmus diets. A potential explanation for this observation could be a positive hormetic response on short-term exposure to low-dosage toxin, leading to individuals living longer.
6. Prolonged exposure (>14 days) to low-dosage Microcystis resulted in a strong decrease in population densities, regardless of temperature. This decrease in population densities seemed to show a lagged response to the observed decreases in clutch size. On average, clutch size was much smaller on a low-dosage Microcystis diet than on a pure Scenedesmus diet. However, this difference in response between the two diets was smaller when daphnids were exposed to higher temperatures, as daphnids growing on pure Scenedesmus diet at higher temperatures had reduced clutch size.
7. Our observations indicate that the population-level responses to prolonged exposure to low-dosage cyanobacterial toxin mask potential responses to changes in temperatures. Warmer temperatures and toxins in diet seem to trigger similar responses in Daphnia populations: smaller individuals with smaller clutch sizes. The combined effect of toxic cyanobacteria and warmer temperatures on Daphnia populations might be additive rather than synergistic.