Epilithic Algae from Seven Megaliths in the Vicinity of Topolovgrad (Haskovo District, Southeast Bulgaria)

The present work focuses on seven megaliths sampled for the first time as a continuation of our studies on the biodiversity of algae on megaliths in Bulgaria. A total of 55 species from four divisions were identified (Chlorophyta was the richest with 31 species), of which 14 species are new for Bulgaria. Two species were of conservation concern, but six were potential toxin producers that could affect the health of visitors to the megaliths. Despite the general granitic character and relatively close location of the studied sites, their floristic similarity was low, with most algae (51 species) being rarely distributed (except <i>Stichococcus bacillaris</i> and <i>Mychonastes homosphaera</i>). The largest cult complex Paleokastro, furthest from populated areas, had the highest species diversity (22 species), while the lowest (8 species) was found in the highly exposed Kalinkin Kamuk, located in a village. The NMDS analysis tool showed the existence of four distinct ecological groups and that temperature and light are the most important drivers for the distribution of the epilithic algae on the investigated megaliths. Correlations with temperatures estimated to identify potential indicators or future survivors under global warming conditions were low, except for the Streptophyta. It was represented by four species of the genus <i>Klebsormidium</i>, which is known for its high ecological tolerance and drought resistance. Extending the studies on lithophytic algae to megaliths is important for a better knowledge of their biodiversity and ecology, but also for the protection of megaliths and for understanding the impact of climate change on these fragile monuments.