DEEP RHODOLITH BEDS IN THE USTICA ISLAND (SICILY, SOUTHERN TYRRHENIAN SEA): A SEDIMENTARY AND PALEOECOLOGICAL APPROACH

Abstract

The results of a multidisciplinary study conducted on the circali- ttoral soft bottom assemblages at Secchitello (Sicchiteddu) and in the neighbouring areas along the southern coast of Ustica Island (Sicily, Southern Tyrrhenian Sea) are presented. In particular, the study pro- vided useful data for a taxonomic, ecological and paleoecological revi- sion of the Coastal Detritic facies with free-living calcareous algae and for a critical analysis of their relationship with the sedimentary dynamics. During a survey, carried out by the University of Catania (1991) with a R.O.V. (remotely operated vehicle), the presence of the Coastal Detritic (DC) biocoenosis, characterized by a surprisingly high biodiversity, and of the Platform Coralligenous (C) biocoenosis, dominated by rhodolith bio-concretions with large and branched Bryozoa was recorded. The DC of Secchitello, situated at a depth of between 50 and 90 m, was mainly characterized by: prâlines, boxwork, branches and coated grains growth forms; the dominance of the Lithothamnion minervae fa- cies; coarse sediments belonging to the sand and gravel classes, with the last one mainly of biogenic nature. Where the hydrodynamism is reduced (at a depth of between 80 and 90 m), an ecotone community between the DC and the C biocoenoses, growing on boxworks, was found. The density and biodiversity of the free-living calcareous algae and of epiflora found at Secchitello are high, and the extension of the rhodolith beds are comparable to that of the C assemblage. The rough morphology of the bottom, increasing the current activ- ity, together with the presence of biodisturbance phenomena mainly due to echinoderms such as Cidaris cidaris, Echinaster sepositus and “sail” epiphytes, such as Cystoseira spinosa v. compressa and Phyllophora cris- pa, were responsible for the increase of the rhodolith overturning. The analysis of thin sections of non-geniculate coralline red algae allowed to reconstruct the paleoenvironmental evolution of the Secchitello, point- ing out three different phases of colonization: initial, intermediate and recent. The transition from one phase to another is probably due to the changes in intensity and direction of the currents in the last millennium.