COLONISATION PATTERNS ON THE CORALLIGENOUS OUTCROPS OF THE NORTHERN ADRIATIC SEA

A long-term colonisation field experiment was carried out to investigate the ecological processes that could lead to the differentiation of the epibenthic assemblages on northern Adriatic subtidal biogenic outcrops. Early recruitment appears to be related to local larval supply, while post-settlement processes and lateral invasion from the surrounding assemblages play a major role in structuring the mature assemblages. The results of this study stress the complexity of the colonisation dynamics leading to the heterogeneity of the assemblages dwelling in the northern Adriatic biogenic outcrops. Key-words: recruitment, benthos, species diversity. Introduction Coralligenous subtidal outcrops occur in soft silty-sandy bottoms of the northern Adriatic continental shelf offshore Chioggia and Venice between 19 and 27 metres in depth. Their epibenthic assemblages, mainly characterised by calcareous algae, algal turf, sponges and colonial ascidians, can be distinguished in three main typologies. Previous studies showed that spatial heterogeneity of assemblages, at local and regional scales, prevailed over temporal variation. Spatial variability was related to the geo-morphological features of the outcrops and to environmental variables (Ponti et al., 2011). However, complex recruitment processes and species interactions play a key role in structuring the assemblages and in maintaining their diversity (Connell, 1985; Osman and Whitlatch, 2004). In order to investigate the role of the recruitment processes in determining such variability, patterns of colonization have been studied in a 3-years field experiment. Materials and methods One outcrop belonging to each of the three main assemblage typologies found in the northern Adriatic Sea was randomly chosen (Fig. 1, Tab. 1). In August 2005, forty-eight travertine tiles (15.0×11.5×1.0 cm) were deployed at each study site in three random plots on the natural rocky bottoms. Tiles were photographically sampled using an Olympus Cammedia C-7070 WZ underwater digital camera (7 Mpixel) equipped with a TTL strobe and two 50 Watt halogen lights. Samples were collected in August 2006, August 2007 and August 2008. Four random samples for each plot and sampling date were analysed. Percent cover of sessile organisms was quantified by superimposing a grid of 100 equal sized squares and identifying all taxa visible within each quarter of these squares, corresponding to 0.25% of the total sampled area (Bianchi et al., 2004). Percent cover was related to the total readable area of each image, obtained subtracting dark and blurred zone or portion covered by motile organisms. Organisms were identified to the lowest possible taxonomic level and assigned to morphological and ecological groups. The endolithic bioeroder bivalve Gastrochaena dubia (Pennant, 1777) was quantified by counting the siphon holes. Differences in the colonisation pattern were analysed among dates (3 levels, random: 2006, 2007, 2008), sites (3 levels, fixed: P204, MR08, P213) and plots (3 levels, random, nested in sites). Distance-based three-way permutational analysis of variance (PERMANOVA; Anderson and ter Braak, 2003) was applied to the experimental F. FAvA, M. PonTi, M. ABBiATi 78 design to test for differences related to the investigated factors. Univariate analyses were based on the Euclidean distances on untransformed data, while multivariate tests were based on Bray-Curtis similarities on squared root transformed data. Similarities among assemblages were represented by principal coordinate analysis plot (PCO, Gower, 1966). The DISTLM forward procedure provided a selection of the species that better explain the observed similarity patterns (McArdle and Anderson, 2001). Fig. 1 Map of the study area, showing the three investigated sites. Mappa dell’area di studio con indicati i siti investigati. Tab. 1 Location (geographic coordinates, datum wgs84) and geo-morphological features (depth, distance from the coast, extent and height of relief) of the study sites. Posizione (coordinate geografiche, datum wgs84) e caratteristiche geo-morfologiche (profondità, distanza da costa, estensione ed altezza dell’affioramento). Site Lat N Lon E Depth Coast Dist. Extent Height (m) (km) (m2) (m) MR08 45° 13.831’ 12° 29.354’ 22.2 14.6 2096 1.5 P204 45° 12.674’ 12° 23.076’ 20.2 6.6 276297 2.2 P213 45° 10.270’ 12° 31.013’ 25.4 15.