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

A soft-bottom community (6–8 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 AFDW m−2 (May 1991) and 2399.8 ± 1073.2 mg AFDW m−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.