Resource competition during seasonal succession toward dominance by cyanobacteria

An annual replacement of eucaryotic algae by nostocalean cyanobacteria was studied during 3 growing seasons in Pleasant Pond, Minnesota. Competition for N together with grazing on inferior competitors occurred. Nitrogen was more limited than P. During the transition to dominance by Nostocales, Nostocales did not usually respond to N additions, whereas many other species did, suggesting that some of the species unable to produce heterocysts were being outcompeted for N. Daphnia pulex grazers consumed Anabaena flos-aquae, Fragilaria sp., gelatinous greens, pennate diatoms, and Scenedesmus spp. Algal responses to added N and to grazers were often interdependent. A behavioral indirect effect via the grazers' functional response could explain this dependence. -from Author