First record of the alien Macroalgae, Rivularia atra and Polysiphonia opaca, on the Libyan coastline

This investigation documents the presence of the alien macroalgal species Rivularia atra Roth ex Bornet & Flahault (Rivulariaceae: Nostocales) and Polysiphonia opaca (C. Agardh) Moris & De Notaris (Rhodomelaceae: Ceramiales) for the first time on the Libyan coastline. Of 7 investigated sites along Al-Jabel Al-Akhdar coastline during summer 2013 and winter 2014, three sites were invaded by R. atra and one site by P. opaca. The percent cover and relative frequency – as measures of abundance -of R. atra (15-24% and 40-70% respectively) were higher than those of P. opaca (8% and 17 % respectively). The investigation revealed that R. atra and P. opaca inhabited not only the rocky substrates but also the limpet Patella caerulea and that the two species do not thrive in polluted habitats. Statistical analysis showed a highly significant effect of season, site and their interaction on the physico-chemical characteristics of water. The results concluded that the nature of substrate as well as the extent of pollution may be major modifiers of algal distribution, since the non-polluted bare rocky shore was the favorite habitat for the alien species R. atra and P. opaca. Therefore, these species are promising indicators for water quality.