Aspects of the ecology of selected central Mediterranean sandy beaches (with special reference to seasonal changes in species composition and abundance of the arthropod assemblages on Maltese beaches)

Abstract

Four sandy beaches (White Tower Bay, Golden Bay, Ramla and Xatt L-Ahmar) on the Maltese Islands were sampled seasonally between autumn 2001 and summer 2003, whilst a number of other beaches in the Central Mediterranean were sampled only once during a spring season for comparative purposes. These included Spiaggia dei Conigli and Guitgia Beach in Lampedusa (Pelagian Islands), Lido Burrone and Porto in Favignana (Egadian Islands), Pozzallo (Sicily) and Gioa Tauro in Calabria (Italian mainland). Selected physical parameters of the beaches sampled and associated surf zone were monitored by quantifying the exposure to wave action, the sediment percentage organic content, mean sediment particle size, slope, width and the particulate organic content (POM) of nearshore water. Every beach was divided into a wet and a dry zone and sand samples were taken at regular intervals along transects with a 24-cm-diameter corer and wet sieved for any infauna. Nocturnal, surface-active fauna was collected by means of pitfall trap constellations, whilst shallow-water (<1m deep) faunal assemblages were sampled by means of a 0.5mm-mesh hand-towed net during nocturnal sweeps covering an area of ca. 25m2. Seasonal variations in pitfall trap faunal composition in habitat and in feeding preferences were monitored. Different diversity measures for the collected fauna were computed. Significant differences between the diversity of different beaches were tested for and multivariate statistical techniques were used to investigate for any relationships between abiotic and biotic parameters and to quantify inter-beach differences. For Maltese beaches, there was little inter-annual variation in the total number of faunal individuals and species collected, with total of 16260 individuals belonging to 181 species being recorded in the first sampling year and 14556 individuals belonging to 168 species being recorded in the second sampling year. Faunal assemblages recorded were almost entirely composed of arthropods, with insects being the most abundant taxon in terms of species richness and crustaceans (namely isopods and amphipods) being the most abundant in terms of individual abundance. Tenebrionids were the most abundant coleopteran family, followed by staphylinids. The species most commonly collected by pitfall traps included Phaleria acuminata, Phaleria bimaculata and Tylos europaeus. In terms of individual abundance, detritivores and omnivores were the most abundant feeding type for pitfall trap and handnet collections respectively, whilst mysids were the most abundant taxon recorded by handnets. Although differences in the supralittoral species composition between the Maltese beaches sampled were observed, the degree of compartmentalisation reported for the same beaches was less pronounced than that reported by earlier studies when the multi seasonal macrofaunal data is considered. A higher degree of compartmentalisation was observed for shallow-water assemblages. The current study confirmed the dearth of infaunal individual abundance for local beaches recorded by previous studies, although higher macrofaunal species richness were recorded from the local beaches than that quoted in literature for other Mediterranean beaches. The relatively low infaunal individual abundance densities of local beaches could not be explained solely in terms of the islands' isolation and relative lack of macrofaunal receruitment.