Comparison among epilithic diatom communities from areas of invasive Caulerpa species (Caulerpa taxifolia and Caulerpa cylindracea) in the Adriatic Sea (ne Mediterranean)

Diatom community structures at locations under the influence of different invasive Caulerpa species ; C. taxifolia and C. cylindracea, were compared in a two-year interval. Samples for taxonomic study were taken from coastal rocks at three stations (Island of Hvar, island of Mljet and Dubrovnik area) at eastern Adriatic Sea from November 2008 to October 2010. Light and electron microscopy examinations were used for qualitative analysis. Taxonomic analysis of diatom communities revealed altogether 388 epilithic taxa belonging in 63 genera including Hyalosynedra, Rhopalodia, Navicula, Cocconeis, Berkeleya, Tabularia, Nitzschia and Grammatophora as the most abundant. The study confirmed differences in species composition and abundance of epilithic diatoms among two Caulerpa sites. While the average number of taxa at C. taxifolia sampling site (island of Hvar) was 77 per sample, the average number of taxa at C. cylindracea sampling sites (Dubrovnik and Mljet) was lower and amounted to 64 per sample. The highest values of Shannon- Wiener Diversity Index were during autumn and were almost the same for all three sampling sites ranging from 5.26 to 5.34. As revealed by MDS ordination, location has more impact on diatom community structure than seasonal changes. ANOSIM test performed on species abundance data confirmed that the difference between diatom communities among different sampling seasons was not significant. The results indicate that the influence of invasive seaweed of genus Caulerpa that change the habitat by competing with autochthonous algae diminishing the structural complexity, richness of species and diversity evident at the macroscopic level have impact on microflora as well. Results of this study presents the first study of the taxonomic composition and distribution of epilithic diatoms from areas of invasive Caulerpa spp., and it contributes to the broader and regional knowledge of diatom diversity in these challenged environments.