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A 6-month macrograzer exclusion field experiment was conducted in an altered  Posidonia oceanica  meadow affected by organic effluents from a fish farm. This experiment was carried out in order to ascertain whether or not overgrazing had contributed to the meadow degradation observed, because higher densities of macrograzers (mainly the sea urchin  Paracentrotus lividus , 20 to 60 times higher density) were found in the degraded meadow than in an undisturbed reference meadow located nearby. The plant variables measured (shoot density, shoot biomass, leaf growth rate and epiphyte load) had consistently lower values in unmanipulated plots of the degraded meadow than in plots of the undisturbed reference meadow, with the exception of herbivore pressure, which was 2 to 5 times higher. In the herbivore exclusion plots established in the degraded meadow, shoot biomass and epiphyte load increased to values similar to those in the reference plots in the undisturbed meadow. At the end of the experimental period, shoot density had significantly decreased by up to 30–50% of the initial values in the control (unmanipulated) plots, while no changes were observed in the exclusion plots or in the reference meadow. Our results show that the impact of macrograzers on shoot features made a significant contribution to the reported decline of the  P. oceanica  meadow next to the fish farm. It is also suggested that organic effluents generated as a result of aquaculture activities may help to increase grazing activity in these seagrass communities, although the mechanisms by which nutrient loads control the behaviour of herbivores remain unclear.

The importance of herbivory in the decline of a seagrass ( Posidonia oceanica ) meadow near a fish farm: an experimental approach