Effects of sea urchin grazing on a Posidonia oceanica meadow in the Gulf of Oristano (western Sardinia, Italy).

Abstract: Although the interactions between grazers and Posidonia oceanica have been largely investigated, there are still controversial views about the importance of Paracentrotus lividus. This study investigates the effects of different densities of P. lividus on P. oceanica at leaf and shoot scale. The experiment was conducted in September 2005 and May 2006 on a shallow meadow in the Gulf of Oristano (western Sardinia, Italy). In the field, P. lividus individual were included within cages, according to five different densities: 0, 8, 20, 40 and 80 individuals m-2. It was predicted that (i) under the cages, number and length of leaves, number of shoots and epiphyte biomass would decrease as an effect of sea urchin and that (ii) these effects would be higher at high than at low densities of sea urchins. The number of leaves and epiphyte biomass per surface unit did not change in response to sea urchin grazing, while a response was found in green and brown tissue as well as in the number of broken leaves. The number of shoots completely grazed by P. lividus varied for densities higher than 8 ind. m-2; it peaked at the highest densities of sea urchins (i.e. 80 ind. m-2) exceeding the 45% of the total number of shoots initially present. The importance of this variable to quantify the grazing effect of P. lividus on the meadow is emphasized.