Nutrient load and epiphytes are drivers of increased herbivory in seagrass communities

ABSTRACT: Eutrophication is one of the major threats facing seagrasses, promoting effects in different compartments of the community (e.g. plants, epiphytes, fauna). In this study, we researched how in situ nutrient enrichment modified the consumption rates of Cymodocea nodosa plants during a period of 3 mo, by creating a set of mesocosm feeding choice experiments with the generalist herbivore Paracentrotus lividus. Nutrient enrichment intensified the consumption of C. nodosa leaves by increasing the palatability of their tissues at different levels. At a first level (i.e. the individual plant response), nutritional quality of the tissues increased (i.e. nitrogen content), while both biomechanical (i.e. absolute force-to-tear, FTA, and specific force-to-tear, FTS) and mechanical (i.e. fiber content) traits were reduced. At a second level, the presence of epiphytes coating the leaves increased their nutritional quality without actually increasing their biomechanical resistance, which rendered higher consumption rates. However, the presence of Ulva sp. (a highly palatable macrophyte) reduced the direct consumption of C. nodosa leaves, mainly when coated by epiphytes, a fact fully endorsed by the compensatory feeding theory. Therefore, the nutritional quality of the C. nodosa leaves is a key factor regulating their susceptibility to be consumed, although mechanical and biomechanical factors also gained importance when nutritional quality was low. In addition, the presence of other components of the community (epiphytes and macroalgae) may increase or reduce herbivore pressure, highlighting the complex nature of herbivore-plant interrelationships.