Réponse du macrozoobenthos aux déversements de la rivière Ombrone sur la plage sableuse du parc régional de la Maremma (Toscane, Italie)

On the sandy beach of the Maremma Regional Park, the benthic macrofauna was studied at five stations at a distance of 2, 3, 4, 5 and 6 kilometres to the south of the Ombrone river mouth. Sampling of the fauna, together with the environmental data recordings, occurred during autumn 2002 and spring 2003. Median grain size showed a decreasing gradient as the distance from the river mouth increased but no significant differences between periods were registered. No evident gradients were found for the other environmental parameters (conductivity, organic matter, intertidal width, slope and pH). Except for the organic matter, the other parameters presented significant differences between the two periods. Spring was characterised by a strong variability of the environmental parameters that was related to the great instability of the beach during this period. The biocenosis identified on the studied beach was that typical of the midlittoral sands and was characterised by polychaetes such as Ophelia radiata and Scolelepis squamata and by the crustacean species Eurydice affinis. Species richness increased at increasing distance from the river mouth and the community was quantitatively dominated by O. radiata. In autumn, species richness together with H' and J' indices changed at the five sampling stations, whereas in spring the variations of H' were related to the changes in species richness while J' remained more or less the same. Long-shore distribution patterns showed that the most abundant species preferred the stations farther away from the river mouth. The vertical zonation of the fauna of the midlittoral zone was established and can refer to models already known for the Mediterranean.