Influence of anthropogenic modification of rocky shore habitat on associated macrobenthic biota

Abstract

Artificial hard structures deployed on the shore may support different biotic assemblages compared to natural rocky shores. Despite that shore artificial structures, including jetties, quays and slipways, are very common in local bays and inlets, the influence of such features on the rocky shore habitat and the associated biota has not been investigated. The main aim of the present study was to characterise the biota associated with jetties ('altered shores') and assess whether these assemblages differed from those found on natural rocky shores ('unaltered shores'), and to characterise any such differences that exist. Four localities on the northeastern coast of the Maltese Islands, having a similar Thomas Exposure Index and geomorphology, and representing a gradient of modified shores ranging from pristine to highly rnodified were chosen for the present study. Four stations at each locality, two situated on jetties and two on natural rock, were sampled by scraping the biota within three replicate 10 cm-wide belt transects each of which extended from the lower to the upper mediolittoral zone. The supralittoral zone was also sampled using random discontinuous sampling. All sampling was made between August and September 2011. Identification of the zone boundaries proved difficult on 'altered shores' due to high variability in the presence and abundance of reference algal species (example Cystoseira spp.) and the impoverished biota. Most algal species were recorded on both 'altered' and 'unaltered shores', whilst 27 out of 47 macrofaunal species were only recorded on 'unaltered shores'; no species was solely found on 'altered shores'. Algal and macrofaunal abundance were lower on 'altered shores', but not significantly so. The number of macrofaunal species was significantly lower on 'altered shores'. Laurencia spp. was abundant on 'unaltered shores' and sparse on 'altered shores'. Crustaceans contributed more, and molluscs less, to the total macrofaunal abundance on 'altered shores' (55% and 23% respectively) in contrast to 'unaltered shores' (36% and 38% respectively). Some macrofaunal species, common to 'unaltered shores', including crustaceans (Maera sp.), molluscs (Acanthochitona spp.), polychaetes (Lumbrineridae spp.) and echinoderms (Amphipholis squamata), and which are typical of algal turf and crevices, were absent on 'altered shores'. Melarhaphe neritoides was less dense or absent in the supralittoral zone on 'altered shores' in contrast to 'unaltered shores'. Overall, therefore, the biotic assemblages on 'altered shores' were not the same as those that are typical of 'unaltered shores', specifically, in terms of the total number of species, the relative abundance of the most abundant taxa, and frequency of occurrence of the less abundant taxa. The present findings are deemed important to help better management and conservation of shore habitats around the Maltese Islands.