Compensation of nutrient pollution by herbivores in seagrass meadows

The capacity of natural systems to absorb disturbances without undergoing major change is a critical aspect of stability. While human-driven increases in resources are well known to cause profound change to habitat-forming perennials, often as an indirect effect on enhanced growth of opportunistic species, less is known about the natural capacity for consumption to compensate for this excess production. Nutrient inputs to seagrass systems increase the production of epiphytic algae and lead to seagrass decline however, it is relatively unclear whether grazers can compensate for this elevated production. Small invertebrate mesograzers are important herbivores in these systems, grazing predominately on epiphytic algae, potentially increasing the survival of seagrass during periods of nutrient enrichment. Using a cage-free technique and catchment-based nutrient concentrations, we experimentally manipulated mesograzer abundance and nutrient concentrations simultaneously in a subtidal Posidonia angustifolia meadow to test for trophic compensation. We tested the hypotheses that nutrient enrichment not only stimulates mesograzers to increase consumption, but that this increase in rate of consumption also matches the rate of increase in the production of epiphytes. Consecutive increases in nutrient addition caused consecutive increases in epiphyte production in treatments of reduced herbivory, but not in treatments of natural herbivory. Mesograzers fully compensated for the effects of minor nutrient addition and nearly compensated for the effects of moderate nutrient addition by decreasing the accumulation of epiphytic algae on seagrass leaves. Although nutrient addition alters food availability and quality of epiphytes, the abundance of grazing amphipods did not increase in treatments of natural herbivory, suggesting that the greater consumption of epiphytes was due to an increase in per capita grazing. These results suggest that herbivores may not only counter the effects of resource enhancement but may fully compensate minor to moderate events of nutrient pollution by consuming the additional productivity of opportunistic species that can cause loss of community dominants.