Soft-bottom macrobenthic community composition and biomass in a Posidonia oceanica meadow in the Ligurian Sea (NW Mediterranean)

A soft-bottom community (6e8 m depth) within a seagrass bed in Prelo Bay (Eastern Ligurian Sea) was studied between April 1990 and August 1991. Samples were collected using a suction device and were sieved through a 1-mm mesh size. Cluster analysis showed seasonal fluctuations in the macrofauna density with highest values in summer 1990 (maximum 2870.0 2029.4 ind. m 2 in August 1990) and lowest in late spring 1991 (100.0 37.8 ind.m 2 at the end of May 1991). On the whole, the community was characterised by a high number of species (184 species plus 17 higher taxa) but only 12 accounted for 78% of the total density, so diversity was relatively low. Dissimilarity between seasons was high (on average 74.7 8.3) and the principal discriminating species was the amphipod Siphonoecetes dellavallei, whose successful recruitment dominated community density in summer 1990. Total macrofauna biomass ranged between 166.1 67.8 mg AFDWm 2 (May 1991) and 2399.8 1073.2 mg AFDWm 2 (June 1991); molluscs being the major contributors (ca. 61%) with single large individuals. Secondary production reached on average 0.4 0.2 g C m 2 y 1. Notwithstanding the relative low biomass, deposit-feeders were the major contributors to secondary production (38%), indicating their high efficiency in utilizing their food resource (detritus-associated bacteria). The seagrass bed seems to play only an indirect role in the structure and function of macrofaunal assemblages.