Monitoring of a Posidonia oceanica bed (Punta Manara, Eastern Ligurian Sea, Italy) and the associated molluscs twenty years after: What's new?

This research project by the Marine Biology Laboratory at Turin University determined the degree of conservation and measured the size of the Posidonia oceanica bed in the eastern Ligurian Sea off Punta Manara in 1985–1986, a bed that had also been examined in 1973. This project assessed the structure, dynamics, and organization of the foliar stratum malacofauna in relation to the most important space–time variables, the bathymetric gradients, and the seasons. In 2006 the laboratory investigated this bed with methods like the previous ones in order to evaluate the effect of anthropogenic disturbances and to assess any changes in the P. oceanica and mollusc-community dynamics. Our overall project has spanned many years, something unique in P. oceanica studies. Our structural analysis of the meadow found (i) a reduction in shoot density in all the bed's regions, particularly in the shallow and intermediate belts; (ii) a progressive reduction of the lower limit (now considered erosive–regressive). We sampled the foliar stratum molluscs using hand-towed nets, the means we used to collect semi-quantitative data. In 2006, there were 40% fewer specimens and 31.5% more species than in 1985–1986. The core-stock (sensu Russo) has remained the same over the years in terms of both the presence and the abundance of species (8 dominant species belonging to Cerithidae and Rissoidae). As in 1985–1986, Bittium latreillii is the dominant species, whose juveniles exhibited similar settlement dynamics on the foliar strata and whose samples had comparable peaks in reproduction and abundance. The multivariate approach was applied to both the 1985–1986 and the 2006 data set. The present results confirm those obtained through the Detrended Correspondence Analysis (DCA) in 1985–1986. Namely, time (the period or the season) is the main factor of variability, the one that influences the malacological assemblages the most. The composition of the stocks depends closely on the sampling period (ANOSIM, R = 0.532, P < 0.001) and hence is linked to the biological cycle of the seagrass. The development of P. oceanica affects the malacological assemblages and regulates them quantitatively, something most evident in the reproductive and settlement cycles of the two dominant families, Cerithidae and Rissoidae. Thus the spatial variations (the variations in depths or bathymetric gradient) play a major role in determining the species of the stocks, while the temporal variations – the changes in seasons – play a major role in determining the total number of molluscs.

► The 85 study determined the conservation of the Posidonia bed of Punta Manara.
► The aim was to evaluate the effect of anthropogenic disturbances.
► Our analysis found reduction of shoot density and regression of the lower limit.
► Results are decrease in the specimens number and increase in the species number.
► Time (period) is the main factor influencing malacological assemblages.