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Daphnia eco-evolutionary process meets the clear water phase: seasonal plankton dynamics when consumers evolve

Nelson Hairston, Lindsay Schaffner, Steve Ellner, Eliza Fairchild, Lars Rudstam (Cornell), Brooks Miner (Ithaca College), Piet Spaak (EAWAG Switzerland), Luc De Meester, Lynn Govaert (KU Leuven, Belgium)

The seasonal change in phytoplankton and Daphnia abundance is a classic pattern in the ecology of temperate zone lakes. It includes a spring bloom dominated by edible diatoms, followed by an increase in grazing Daphnia that terminates the bloom with a clear-water phase dominated by small rapidly growing phytoplankton, at the end of which the Daphnia population declines and a summer bloom of cyanobacteria and colonial green algae dominates. We show for Daphnia mendotae in Oneida Lake, that the Daphnia population responds evolutionarily to the natural selection imposed by seasonal change in phytoplankton quality. Genetically distinct clones (identified using microsatellite DNA loci) rise to prominence at different times of year consistent with their ability to grow on phytoplankton of different edibility. Clonal performance was determined in the lab by measuring juvenile (somatic) growth rate on “spring phytoplankton” comprised of diatoms, cryptophytes and greens, and on “summer phytoplankton” comprised of cyanobacteria and greens. Clones that dominated in spring grew much better on spring than on summer phytoplankton, while the one that dominated in summer grew nearly equally well on both food types. Calculations of clonal population growth rates show seasonal changes in fitness differences, and projections of effects of clonal evolution on total Daphnia population growth rate shows the importance of seasonal evolution on lake plankton dynamics. Therefore, the classic seasonal plankton consumer-resource dynamics in this lake are eco-evolutionary in nature: underlain by consumer evolution, which affects those dynamics while they are in progress. This study was published in 2019 in Nature Ecology and Evolution and Sharon Lawler wrote in Faculty -1000: “In one of the best eco-evolutionary field studies to date, Schaffner et al. show that evolution helps drive the within-year dynamics of the zooplankton and phytoplankton of a temperate lake” and “This study featured thorough sampling of Daphnia and producers in the lake, plus careful measurement of how different producers affected the growth of many of the dominant D. mendotae clones. This enabled the authors to calculate that, without evolution, the cumulative population size of D. mendotae would have been a fraction of that observed. Hendry recently called for just such studies, and it’s gratifying to see one that has been so well done.”