Lake re-oligotrophication causes rapid evolution of life history traits in <i>Daphnia</i> fed with toxic cyanobacteria

Many lakes across the globe have undergone eutrophication during the past century, primarily attributable to nutrient enrichment resulting from anthropogenic activities. Significant alterations have been observed in phytoplankton communities, resulting in blooms of cyanobacteria with the potential to adversely impact zooplankton populations by causing mechanical disruption, nutritional deficits, or toxicity. In Lake Greifensee (Switzerland), eutrophication began ca. 1930 and peaked in 1974, after which a continuous recovery followed until the present time. The eutrophication peak was accompanied by an increase in the potentially toxic cyanobacterium <em>Microcystis</em>. Using a resurrection ecology approach, we tested the hypothesis that <em>Daphnia</em> clones from periods of high trophy cope better with <em>Microcystis</em> (e.g., have a higher fitness in life history experiments) than clones from periods with a lower trophy. In a fully factorial life-history experiment, we evaluated the response of <em>Daphnia</em> hatched from recent and older sediment layers to the presence of a toxic <em>Microcystis</em> and a nontoxic <em>Microcystis</em> strain, as well as “good” food (<em>Acutodesmus</em>). We found that “older” clones coped better with <em>Microcystis</em> and its toxins (i.e., had a higher rate of population increase, <em>r</em>). However, older clones also had a higher <em>r</em> feeding on good food. The present study demonstrated how rapid evolution enables <em>Daphnia</em> to better adapt to changing environments.