Sedimentological Parameters and Seagrasses Distributions as Indicators of Anthropogenic Coastal Degradation at Monterosso Bay (Ligurian Sea, NW Italy)

Abstract

This paper illustrates an integrated physical-biological approach to investigate the effects on the coastal environment of the construction of a large embankment along the shore of Monterosso Bay, NW Italy. The embankment protrudes from the natural shoreline for more than 100 meters and has deflected the longshore current toward the offshore, as indicated by grain-size and mineralogical areal distributions of bottom sediments, and by the orientation of bedforms. Consequently, the down current portion of the beach relative to the embankment has become sediment-starved and the site of finer-grained sedimentation, and severe shoreline retreat has occurred, as indicated by sequential analysis of topographic maps and by the comparison of non-parametric photographs. This trend is recorded by the areal distribution of two seagrasses (marine phanerogams) along the bay: Posidonia oceanica - whose slow growth requires stable environmental conditions and a preference for coarse-grained sandy substratum-is present as dense and healthy meadows in the upcurrent portion of the bay whereas Cymodocea nodosa-an opportunistic, pioneer species capable of surviving in stressful environments is present only along the downcurrent (eastern) portion of the bay where it is replacing a progressively retreating Posidonia meadow. This is substantiated by (i) direct underwater observation of the remnants of a dead P.oceanica meadow along the eastern portion of the bay and (ii) interviews with the local population pointing to a more extensive Posidonia meadow along the eastern portion of the bay prior to the construction of the embankment. Thus, biological parameters such as the density and health of marine phanerogams match physical parameters (grain-size distribution patterns) more traditionally employed in coastal studies and can provide significant clues on both natural and anthropogenic medium-term coastal dynamics.