Across shore variability and trophodynamic features of meiofauna in microtidal beach of the NW Mediterranean

The across shore variability and trophodynamics of meiofauna were studied in a microtidal beach of the Thyrrenian Sea (NW Mediterranean). Two sites were sampled at Collelungo beach (Maremma Park, Italy) subjected to different regimes of sediment erosion and deposition. At each site, four levels were sampled in November 2002 and May 2003 along a transect from the supralittoral zone to the surf zone. Sediment cores were taken down to a depth of 10 cm and meiofaunal abundance and community structure were analyzed and related to the principal trophic resources (quantity and quality of organic matter, chlorophyll a, bacteria density and biomass). Meiofaunal abundance ranged between 14 ind. 10 cm_2 and 716 ind. 10 cm_2 in the top 0e10 cm of sediment. Abundance was lower in the surface (0e2 cm) than in the deeper (5e10 cm) sediment layers but no significant differences were found between the two sites. Multivariate BIOENV analysis showed that dryness, grain size (related to physical processes) and bacterial biomass, were the main variables explaining meiofauna distribution in these beaches. Meiofaunal densities and number of taxa were always higher at the swash level, while lower abundances were observed at the dry sampling level (þ5 m). This across shore trend was also observed for the quality of the organic matter (PRT/CHO) and bacterial densities. Nematode assemblage structure at the swash sampling level showed a dominance of non-selective deposit feeders (1B), with Xyalidae as the dominant family (56%), followed by Thoracostomopsidae (14%) and Selachnematidae (12%). According to the findings, physical and biological variables at the swash level create optimal living conditions for the meiobenthos, making the swash a key area within the beach ecosystem, with potential implications within basic and applied ecological studies.