Nutrient status, plant availability and seasonal forcing mediate fish herbivory in temperate seagrass beds

Among mechanisms for herbivore choice, palatability and abundance of food resources appear to have a central role in foraging ecology. Hence, nutrient supply due to eutrophication may cause profound alterations to both nutritional and quantitative aspects of primary producers, leading to changes in herbivory pressure and enhancing overall functional changes. We present an experiment of water column enrichment in an oligotrophic seagrass bed of Posidonia oceanica (northwestern Mediterranean Sea). In this study we measured monthly seasonal changes in fish herbivore behavior following alterations in qualitative (nutrient content, species composition) and quantitative (biomass availability) features of primary producers (seagrass, epiphytes and macroalgae) as a result of nutrient fertilization. Nutrient addition induced changes in both seagrass (enhanced plant N content and leaf growth) and epiphytes (enhanced N content, biomass load and altered species composition) throughout late spring and summer. This nutritional alteration of food organoleptic properties concurred within the seasonal peak of herbivory by the fish Sarpa salpa and caused a disproportionate herbivore pressure on enriched plots, until shoots were reduced to around 50% of their original leaf area. When plant biomass on fertilized plots was substantially reduced, herbivores fed mostly on control plots until the end of the summer period. During the rest of the year (autumn and winter), moderate nutrient additions could not disrupt the seasonal inertia of the ecosystem. Hence, our results show evidence that the foraging behavior of herbivorous fish is the result of chemical and visual sensing of the environment. Seasonal forcing, however, tightly controls ecosystem processes and the biological cycles of species, adding a further step to functional alterations in temperate seagrass meadows subjected to eutrophication.