Shifts in competition outcomes between two Daphnia species in response to algal phosphorus content.

Competition is one of the key drivers of cladoceran community dynamics. Competitive abilities can be measured as the Threshold Food Concentration (TFC), i.e., concentration of food at which population growth rate equals zero. Species with lower TFC should be superior competitors. However, population TFC can vary with environmental conditions. We hypothesized that algal phosphorus (P) content influences TFC and can alter the outcome of competition between large and small-bodied cladocerans. We conducted competition and life table experiments with single clones of large Daphnia magna and small Daphnia longispina to assess how algal P content affected their TFC and competitive interactions. We also conducted computer simulations to further explore competition between the small and large species. Our experiments showed that TFC varied with the algal P content. P limitation increased the TFC of both species, but this increase was more pronounced for the smaller D. longispina. For this reason, D. magna was a superior competitor at low P content, while D. longispina was superior at high P content. We also found that enhanced food abundance gave an additional advantage to the large-bodied D. magna due to higher reproduction potential at high food concentrations.