Aspects of the ecology of the faunal assemblages associated with wrack stranded on the Maltese shores

Abstract

Wrack stranded along the Maltese coast composed mainly of Posidonia oceanica was sampled for macrofauna from three sandy beaches (including one in Gozo), two shingle beaches and a muddy channel. Stations distributed on the banquettes were seasonally sampled by core samples and pitfall trapping. A preliminary core sampling exercise was supported by a second year of intensified sampling. Physical parameters of the wrack and beaches under study were measured. Thirty six species were recorded from two years of core sampling, while twenty nine species were captured in pitfall traps. Twenty one species were present on sandy beach wrack compared to the twelve species recorded from shmgle beach wrack. Coleoptera, Amphipoda, Tsopoda and Gastropoda were the most abundant and this agrees with results of studies on other types of wrack investigated elsewhere outside the Maltese Islands. The highest number of species was that for Coleoptera while the gastropod Truncatella subcylindrica had the highest population density with 68,070 individuals/m3 recorded at Salini. The amphipod Orchestia stephenseni was observed to be the first coloniser of freshly beached wrack and this again is in accordance with other studies which point to the colonisation ability of amphipods on wrack. High diversity indices were calculated for Salini and Xatt l-Ahmar which suggests that beaches on which wrack accumulates for a number of years register a higher wrack macrofaunal diversity than shorter term accumulations. This implies that grooming practices that take place during the summer tourist season on some of the Maltese sandy beaches, may be affecting the ecology and biodiversity of these beaches. Exposure seems to be the most important physical characteristic that influences directly wrack macrofaunal communities. No clear distinction between wrack fauna on different substrata emerged. lt was concluded that wrack presence increases macrofauna diversity both on shingle and sandy beaches as a number of wrack species are not commonly found on 'clean' beach areas. Another important result is the compartmentalisation shown by Maltese beaches whereby each individual beach has its own particular suite of species. The conservation implications of these results are discussed