Effects of herbivores on a Posidonia oceanica seagrass meadow:: importance of epiphytes

There is increasing evidence that herbivory in seagrasses is more important than previously thought. However, little experimental data evaluating the importance of seagrass grazing and the factors involved in these interactions is yet available. We carried out an experimental evaluation of the effects of macroherbivores (i.e. the sea urchin Paracentrotus lividus and the sparid fish Sarpa salpa) in a temperate seagrass (Posidonia oceanica) meadow by means of 2 macroherbivore density manipulation experiments in which several measurements of plant and epiphyte vitality, and abundance were carried out. The results show that P. oceanica can withstand high densities of sea urchins over long periods, as no significant effect on shoot density, shoot size, leaf growth or carbohydrate reserves was detected during the time course of both experiments. Conversely, epiphyte load was significantly higher in herbivore exclusion cages; even though this increase did not appear to reduce plant vitality. Furthermore, epiphyte biomass was greatly reduced when urchins were present even at relatively low densities (5 ind. m(-2)), suggesting that epiphytes are the most limiting food resource for sea urchins in P. oceanica meadows. Whereas epiphyte load was only reduced 30% by the presence of fish alone, sea urchins at low densities (5 ind. m-2) decreased epiphyte load ca. 60%, reaching values as high as 80% in high density treatments, in which case the effect of fish was negligible. This study shows that in P. oceanica meadows, seagrass-herbivore interactions are further complicated by an underlying network of seagrass-epiphyte-herbivore, in which epiphytes appear to play a crucial role.