Fish herbivory leads to shifts in seagrass Posidonia oceanica investments in sexual reproduction

Although the dominant ecological paradigm considers herbivory to play an insignificant role in seagrass ecosystems, past herbivore densities were high enough to result in significant reduction of seagrass growth. To study the long-term impact of sustained and intense herbivory on seagrass meadows, we compared morphological, population and reproductive (flowering) parameters of Posidonia oceanica meadows inside a Marine Protected Area (MPA; where herbivore fish populations are very high) with unprotected meadows. In addition, we evaluated short-term seagrass responses by manipulating herbivore access to seagrass plots with caging experiments conducted inside the MPA. The density and individual sizes of the herbivorous fish Sarpa salpa were greater in the MPA, with a biomass 10 times higher than in unprotected areas. Fish bite marks on leaves were 50% more abundant inside the MPA. Shoot surface, rhizome sugar content and flower density were 80, 20 and 70% lower in the MPA, respectively, but shoot density was 30% higher in protected meadows than in unprotected meadows. The caging (fish exclusion) experiment generally corroborated these results, although the caging period was probably too short to produce changes in shoot density. P. oceanica responded to severe biomass removal by herbivores (80% of the photosynthetic biomass) and consequent reductions in carbon storage in the rhizome by reducing sexual reproduction (flowering intensity) and by gradually increasing clonal growth (increasing shoot density). This plasticity suggests an evolutionary adaptive mechanism to deal with historically high herbivore numbers and is evidence of the importance of herbivory as a controlling process in the structuring and functioning of seagrass meadows in the past.