The ecological consequences of unpalatable prey: phytoplankton response to nutrient and predator additions

Recent theoretical and laboratory studies have suggested that unpalatable prey modify the degree to which trophic levels are regulated by bottom-up or top-down forces such that primarily edible communities are regulated by predation and primarily unpalatable communities are regulated by resource supply. Despite the hypothesised importance of prey edibility, experiments have only rarely examined the response of primary producers to nutrient enrichment or the addition of a top trophic level following the reduction or elimination of inedible species. I performed two experiments in aquatic enclosures in which the prey (phytoplankton) had been manipulated to create treatments composed either nearly exclusively of only edible phytoplankton or with both edible and inedible phytoplankton. In the first experiment, the two types of phytoplankton community were subjected to either high or low nutrient concentrations. In the second experiment, the two types of phytoplankton community were present in enclosures with either 2 or 3 trophic levels. I found that the impact of both the nutrients and the number of trophic levels on total phytoplankton biomass was modified by the edibility of the phytoplankton community. Although enclosures with only edible phytoplankton were able to increase with enrichment, there was a greater overall phytoplankton biomass in enclosures with both edible and inedible phytoplankton. The addition of a third trophic level had a positive effect on phytoplankton biomass when only edible phytoplankton were present, but had no effect on phytoplankton biomass when both edible and inedible phytoplankton were present. These results therefore provide support for the hypothesis that the proportion of inedible phytoplankton determines the degree to which communities are regulated by top-down or bottom-up forces.