In the northern Adriatic Sea there is a high number of rocky outcrops (of which a census has not yet been taken) with dense and diversified benthic communities that have not been studied until now. We studied two of these communities, as well as two other ones, which live on artificial substrata (a naval wreck and a barrier of concrete blocks), by collection of several samplings taken by SCUBA diving. A total of 116 species were identified, 67.6 % of which were suspension feeders: ascidians, bivalves and poriferans, in decreasing order of frequency. Classification and ordination analysis, based on biomass values (ash free dry weight, AFDW), distinguished the communities on artificial structures from those on outcrops. Such a distinction is not due to the nature of the substratum but to an interaction between 1. the slope, almost horizontal in outcrops, and subvertical in artificial structures, 2. the water turbidity and 3. the consequent rate of sedimentation. An outcrop near the coast, with hydrological conditions similar to those present at stations with artificial substrata, has a lower biomass. These three environmental factors act on the relative percentage of species, of which some may become strongly dominant. They have no effect on the number of species in the community except for Porifera of which the number of species decreases as the turbidity increases. Where the slope of substrata does not exalt the negative effect of sedimentation, the biomass values (as g AFDW m2) are very high, ranging from 346 to 436 at stations with a high water turbidity, to around 195 for the station farthest from the coast, where there is both lower turbidity and sedimentation. |
