Algal defense, grazers, and their interactions in aquatic trophic cascades

Freshwater phytoplankton exhibit a range of levels of resistance to grazing by zooplankton. Defense theory developed for algal-grazer interactions predicts that there should be a trade-off between resistance and growth. To test this aspect of the theory, the growth rates of several putatively resistant and edible algal species that are present in systems where aquatic trophic cascades are known to occur and where algal resistance is assumed to be minimal were compared. After demonstrating that there is an inverse relationship between resistance to grazing and growth rate, I reviewed the evidence concerning the role of resistant algae in aquatic trophic cascades. Both defense theory and aquatic trophic cascade theory suggest that resistant algae should be favored in an environment with heavy grazing, while edible algae should be favored in the absence of grazing. However, by definition, four- and two-level trophic cascades are characterized by runaway consumption resulting in near elimination of the primary producers. Data from the well-studied cascading freshwater systems seem to contradict some of the predictions of trophic cascade and defense theory; relatively resistant algae can bloom in the absence of heavy grazing and relatively edible algae can bloom after grazer pressure is increased. Based on these observations, I hypothesized that size-specific grazing by zooplankton may partially explain blooms of resistant algae in the absence of grazing and blooms of edible algae in the presence of heavy grazing. In spite of the complex interactions between phytoplankton and zooplankton, the strong top-down signal, which is the signature of the aquatic trophic cascade, often persists. (C) Elsevier, Paris.