Epiphytic flora and vegetation on Posidonia oceanica (L.) DELILE leaves in the Hvar Island area (middle Adriatic, Croatia)

Abstract

     This paper presents the results of seasonal and depth distribution of epiphytic flora and vegetation on Mediterranean seagrass Posidonia oceanica (L.) DELILE leaves. The investigations were carried out seasonally, in 1984 at 5, 15 and 25 m depth in one Posidonia bed on the west coast of Hvar Island (middle Adriatic). A total of 103 epiphytic algal taxa were determined (Rhodophyta 72 taxa, Phaeophyta 16 taxa and Chlorophyta 15 taxa). The highest number of taxa was observed during winter (75 taxa) and at 5 m depth (78 taxa), and the lowest during autumn (63 taxa) and at 15 m depth (73 taxa). The SØRENSEN’s coefficient of similarity, R/P quotient and Cluster Analysis were calulated for similarity epiphytic floras of different Mediterranean and Adriatic areas. In epiphytic vegetation the mean total covering of sample (Rt%) decreased along the depth gradient (21.56% at 5 m and 11.03% at 25 m). Within qualitative dominance (DN%) predominated the filamentous algal taxa of Phsl and Ssl ecological supergroups, and D ecological group. Regarding quantitative dominance (DR%), in this vegetation dominated the calcareous, filamentous and foliose algal taxa of Phsl and Ssl ecological supergroups, and HP and D ecological groups. The Cluster Analysis show that a high significant (about 87%) of epiphytic vegetations exist between the investigated vegetation (present paper) and group of areas which are formed of Sicily, Dubrovnik, Ischia and the French coast. Within the structure of epiphytic vegetation it is possible to separate the algal taxa for which the high values of RMi% 3 0.50% along with high values of P (3, 4 or 5) were recorded, that the most significantly participated in structure of this vegetation. The following species were present at all depths: Chondria tenuissima, Dictyota linearis, Pneophyllum fragile and Fosliella farinosa var. farinosa, and at different depths there were also found: Spyridia filamentosa, Herposiphonia secunda f. tenella, Laurencia obtusa, Ceramium nodosum and Myrionema orbiculare at 5 m depth; Polysiphonia fruticulosum, Spyridia filamentosa, Herposiphonia secunda f. tenella, Laurencia obtusa and Polysiphonia sp.3 at 15 m depth; Antithamnion cruciatum var. profundum and Polysiphonia sp.3 at 25 m depth.