Abundance and orientation responses of the sandhopper Talitrus saltator to beach nourishment and groynes building at san rossore natural park, Tuscany, Italy

Beach nourishment and groynes building were implemented to counteract erosion in sandy beaches located at San Rossore natural park (Tuscany, Italy), near the mouth of Arno river. From 2000 to 2003, nine groynes were built along 3.6 km of coastline at intervals of ca. 400 m, and two of the eight beach segments were filled with marble gravel. Here, we analysed the effects of these beach changes on the abundance and behaviour of the amphipod Talitrus saltator, using field and laboratory observations. Sampling with pitfall traps in order to use the capture frequency as a proxy of abundance was performed bimonthly from September 2004 to January 2006, and orientation experiments were carried out in autumn (2004 and 2005), and spring and summer 2005. Physical variables (beach width, swash width, beach slope, sand penetrability, mean grain size and salinity) were also recorded. The abundance of T. saltator increased with the distance from the river mouth, towards sites with: negligible amounts of marble locally used for nourishment; higher beach width and salinity; lower slope and penetrability values; medium grain sizes, and during the spring/summer seasons. A Generalized Linear Model with a predictive power of 64.5% considered three main descriptors in the model as significant: distance from the river mouth, sand penetrability and a seasonal factor. Orientation experiments showed a highly variable behaviour among sites, depending on coastal stability: at the site stabilized by the concurrent actions of nourishment and groynes protection measures, sandhoppers were oriented to the shoreline direction by using a sun compass; alternatively, at a site situated only 2 km from the nourished sites, they showed scattered orientation. These between-site differences in orientation, described through Spherically Projected Linear Models, were consistent throughout the study period. Different responses obtained at the individual (orientation) and population (captures) levels stress the need to account for several bioindicators to characterize biotic responses to both natural and anthropogenic changes in sandy beaches.