Effects of dredging activities on the growth and population dynamics of Posidonia oceanica (l.) Delile in the Mediterranean Sea: the case study of Capo Feto (SW Sicily, Italy).

Between 1981 and 1993 a methane pipeline was deployed between Sicily (Italy) and Tunisia. This involved the construction of a pipeline trench, which damaged the Posidonia oceanica (L.) Delile meadow at Capo Feto (SW Sicily, Italy) and disturbed the surrounding meadow. Seagrass growth and population dynamics were examined at different depth ranges and at increasing distances from the construction site outer limit (5, 15, 30, 50 m). Results showed significant differences between the shallow (10 +/- 3.3 m) and the deep (20 +/- 4.6 m) meadow as well as differences among distances. The age structure of P. oceanica varied along the distance gradient and with depth. The mortality rate decreased with distance from the trench at all depth ranges, showing that the plants close to the excavation suffered a higher level of disturbance. Turnover and annual gross shoot recruitment rate (R(gross)) were higher in the shallow portion of the meadow than in the deep range. Forecast of future meadow development (R(net)) close to the trench indicates that, if present conditions are maintained, shoot density will be reduced by 50% over the next 6 to 17 yrs.