Porifera and Bryozoa on artificial hard bottoms in the Venice lagoon: spatail distribution and temporal changes in the northern basin

The spatial distribution and temporal changes of demosponges and bryozoans in the northern basin of the Venice Lagoon were studied from samples collected from wooden piles and artificial rocks in June and October 2001. Twenty species of demosponges and 18 species of bryozoans were recorded. Sponges were found at 10 out of 11 sampling stations, while bryozoans were found at all 11 sampling stations. The studied taxocoenoses showed a high percentage cover of the lagoon hard bottoms. Eleven out of the 20 detected species of sponges were reported in a previous study on this taxocoenosis carried out on material collected in 1954-1955. Moreover, the most abundant species were also recorded during a study in 1989. These observations suggest a remarkable degree of persistence of the sponge community. Among bryozoans, the comparison with literature data showed a wider variation in species composition, with the occurrence of five new species in the studied area. The Indo-Pacific invasive Tricellaria inopinata replaced the native Bugula spp. in the role of dominant species. Finally, a significant increase in species richness along the salinity gradient, more evident for sponges than for bryozoans, was identified.