The effects of nutrient availability on tolerance to herbivory in a brown seaweed

1. Plants can resist herbivory through mechanisms of resistance (traits that reduce the amount of tissue lost to herbivores) and tolerance (traits that reduce the negative effects of herbivory on plant fitness). The expression of such traits requires the allocation of plant resources, and consequently, trade-offs between tolerance and resistance can occur in plants. For marine algae, little is currently known about the expression of tolerance, the relationship between tolerance and resistance, and the impact of resource availability on these traits. 2. We manipulated herbivory (simulated or real) and nutrient availability in a series of laboratory and field experiments to test for tolerance to different types of damage under varying resource conditions in the brown alga Sargassum cf. podacanthoides. In addition to growth and photosynthetic responses of the alga, we measured the concentration of phlorotannins (secondary metabolites associated with resistance in many brown algae) to test for possible phenotypic trade-offs between traits of tolerance and resistance. Furthermore, we assessed whether the limiting resource model (LRM) could predict algal tolerance under different resource conditions. 3. Sargassum cf. podacanthoides expressed traits of tolerance (increased growth rates and reallocation of resources) that were specific to both the type and severity of herbivory encountered. Consistent with the predictions of the LRM, nitrogen enrichment did not alter the compensatory growth responses of the alga. Correlations between growth and phlorotannin content were variable, dependent on the measure of growth analysed, and only weakly influenced by nutrient availability. Nutrient enrichment, however, consistently changed within-alga patterns of tissue composition and photosynthetic activity. 4. Synthesis. Herbivory in the marine environment is often severe, and, similar to higher plants, tolerance to herbivory may be an important mechanism that maintains the productivity of marine algae. We demonstrate the potential applicability of the terrestrially derived LRM to predict algal tolerance under eutrophic conditions. Nutrient-induced changes to within-alga patterns of tissue composition should also be considered when evaluating the effects of nutrient enrichment on marine communities. Furthermore, as in higher plants, the relationships between traits of tolerance and resistance are variable in algae, and trade-offs cannot be assumed.