29/03/2007

Stefania COPPA , Ivan GUALA, Maura BAROLI, Monica BRESSAN, Serena COMO, Giuseppe PIERGALLINI, Giovanni DE FALCO

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 individuals 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 urchins 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.

PROCEEDINGS OF THE 3RD MEDITERRANEAN SYMPOSIUM ON MARINE VEGETATION