Small-scale changes of an arthropod beach community after hard-engineering interventions on a Mediterranean beach

Sandy beaches and their resident communities are subjected to increasing impacts of human interventions and activities. Still, very little is known about the effects of hard engineering interventions on sandy beach ecosystems. This study analysed the arthropod community on a beach stretch of about ten kilometres in thewestern Mediterranean coast of Italy. This is a microtidal and exposed sandy beach at the delta of Ombrone River, long since subjected to coastal erosion. Aiming at contrasting erosion dynamics, a seawall of about one thousand metres was constructed landwards at one side of the river mouth, and several perpendicular groynes were made in the sublittoral zone in front of the wall and one thousand metres further. The objectives of this study were to (1) identify the main drivers defining the arthropod community structure on a changing Mediterranean sandy beach and (2) evaluate whether the anti-erosion hard-engineering interventions had any effects on the arthropod community along the beach continuum, at the two sides of the river mouth. Arthropods were sampled on the beach every three months, over a two year period, during and after the hard-engineering works. Individuals were counted and identified when possible to the species level; data were analysed with multivariate factor analysis, which was used as an exploratory tool to reveal relationships among variables. The results proved a correlation of the community composition with coastal erosive–accretive dynamics, climatic (seasonal) changes and beach width. Biotic interactions seem to have assumed a major role in defining the community composition in the beach stretch where the seawall has already affected the coastal dynamics by producing beach accretion.