Sedimentological parameters and seagrasses distributions as indicators of anthropogenic coastal degradation at Monterosso Bay (Ligurian Sea, NW Italy)

This paper illustrates an integrated physical-biological approach to invest igate the effects on the coastal environment of the construction of a large embankment along the shore of Monterosso Bay, NW Italy. The embankment pro trudes from the natural shoreline for more than 100 meters and has deflecte d the longshore current toward the offshore, as indicated by grain-size and mineralogical areal distributions of bottom sediments, and by the orientat ion of bedforms. Consequently, the downcurrent portion of the beach relativ e to the embankment has become sediment-starved and the site of finer-grain ed sedimentation, and severe shoreline retreat has occurred, as indicated b y sequential analysis of topographic maps and by the comparison of non-para metric photographs. This trend is recorded by the areal distribution of two seagrasses (marine phanerogams) along the bay: Posidonia oceanica-whose sl ow growth requires stable environmental conditions and a preference for coa rse-grained sandy substratum-is present as dense and healthy meadows in the upcurrent portion of the bay whereas Cymodocea nodosa-an opportunistic, pi oneer species capable of surviving in stressful environments-is present onl y along the downcurrent (eastern) portion of the bay where it is replacing a progressively retreating Posidonia meadow. This is substantiated by (i) d irect underwater observation of the remnants of a dead P. oceanica meadow a long the eastern portion of the bay and (ii) interviews with the local popu lation pointing to a more extensive Posidonia meadow along the eastern port ion of the bay prior to the construction of the embankment. Thus, biologica l parameters such as the density and health of marine phanerogams match phy sical parameters (grain-size distribution patterns) more traditionally empl oyed in coastal studies and can provide significant clues on both natural a nd anthropogenic medium-term coastal dynamics.