Seasonal and small-scale spatial variability of herbivory pressure on the temperate seagrass Posidonia oceanica

Despite increasing evidence that herbivory on seagrasses can be substantial in nearshore systems, there is only scarce experimental data on the direct quantification of seagrass removal by herbivores. To help fill this gap, and to evaluate the hypothesis that herbivory on the Mediterranean seagrass Posidonia oceanica (L.) Delile is generally low, we assessed the patterns of grazing pressure variability and its effects on seagrass in an undisturbed meadow in the NW Mediterranean. We quantified herbivore density and grazing pressure through both direct (tethering experiment) and indirect (through marks of herbivore attacks) measurements. Although grazing varied greatly both temporally and spatially, our results show that whereas consumption by the sea urchin Paracentrotus lividus is relatively minor, P. oceanica is intensely grazed by the fish Sarpa salpa in summer. During this period, fish are very abundant at a depth of 5 m, with consumption rates that temporarily exceed seagrass production, which is at its yearly minimum. This imbalance between consumption and production causes the appearance of mowed patches that can be seen from early summer to September; seagrass biomass is reduced by 50 % in such patches. Through direct measurements of consumption, our study has revealed that P. oceanica consumption by herbivores can be substantial with respect to the total annual production and much higher than previously estimated through indirect measurements. Thus, it becomes apparent that estimation of consumption rates by indirect methods can grossly underestimate the importance of herbivory in seagrass ecosystems, which leads us to strongly advocate the use of direct methods whenever possible.