Of rocks and hard places: comparing biotic assemblages on concrete jetties versus natural rock along a microtidal Mediterranean shore

Abstract

The species composition and structure of littoral assemblages on concrete jetties were compared with those on natural rocky shores at two localities on the microtidal NE coast of Malta. Algal species richness and total algal biomass were significantly lower on jetties. Faunal species richness and faunal abundance were also lower on jetties, albeit not significantly so. Jetties and natural rocky shores differed in relative species composition. The algae Palisada perforata/ Laurencia obtusa and several mobile animals, mostly crustaceans, molluscs, and polychaetes, were more abundant on rocky shores. In particular, the supralittoral gastropod Melarhaphe neritoides had a density ranging between 8 ind./m2 to 98 ind./m2 on rocky shores but was never recorded on jetties. Conversely, the algae Cladophoropsis spp., the barnacle Chthamalus stellatus, and the limpets Patella caerulea and P. ulyssiponensis were more abundant on jetties. The rocky shores of sites with jetties were compared with those of a reference site (Dahlet ix-Xmajjar) that lacked jetties. No significant differences occurred in algal biomass and faunal abundance; however, faunal species richness was significantly higher at the reference site. These results may be linked to a number of abiotic factors inclusive of increased exposure to waves, decreased area for colonisation, and habitat homogeneity. The present case study has implications for coastal zone management.