While many laboratory and mesocosm studies have shown rapid evolution can occur on an ecologically relevant timescale leading to eco-evolutionary dynamics, these interactions are rarely documented in nature. This study is one of the first to demonstrate these processes in a natural lake. We used an important planktonic consumer, Daphnia mendotae, and the quality of its resource, phytoplankton, to demonstrate this eco-evolutionary process. We observed seasonal changes in phytoplankton species composition (an ecological process) drive changes in the frequency of consumer genotypes (evolution), which in turn has the potential to affect the consumer population's somatic growth rate (ecology). Genotypes predominant in spring, when edible phytoplankton dominated, grew well in the lab when fed spring algal taxa, but poorly on a diet containing relatively inedible cyanobacteria typical of summer. Conversely, genotypes that dominated in late summer, or showed no seasonal frequency pattern, were relatively resistant to dietary cyanobacteria.