First insight into the composition and structure of macrozoobenthos in the area of the 'underwater petrified forest', Sozopol Bay (Black Sea)

The “Underwater Petrified Forest” in Sozopol Bay (Fig. 1) is one of the most remarkable marine habitats along the Bulgarian coast. This paleobotanical phenomenon consists of numerous well-preserved swamp cypress trunks and stumps dating back over 20 million years to the Miocene epoch. It is a globally rare underwater site of scientific importance, with only one other similar site documented so far, in the Gulf of Mexico. Despite its geological importance and distinctive physical characteristics, the biological communities associated with this habitat have until now remained poorly studied. The present study provides the first data on the taxonomic composition and quantitative structure of macrobenthic communities inhabiting the fossilized wood formations as a substrate and microhabitat. During field campaigns in 2023 and 2024, SCUBA divers collected and processed qualitative and quantitative benthic samples following the best scientific diving practices and standards. The analysis revealed a rich and functionally diverse macrobenthic fauna, including Porifera, Polychaeta, Bivalvia, Gastropoda, Decapoda, Bryozoa and Chordata. The highest abundances were recorded for polychaete worms (subclass Sedentaria), mussels (primarily Mytilus galloprovincialis Lamarck, 1819) and crustaceans; other taxa also contributed to the overall structural diversity (Figs 2, 3). The petrified wood provided a unique heterogеnеous hard substratе supporting both sessile and mobile invertebrates, including filter feeders, grazers, tube-building species, and small cryptic invertebrates. The intricate microtopography of the trunks additionally created varied microhabitats, further enhancing species coexistence. In contrast, the surrounding sandy seabed was markedly poorer in sessile and hemisessile taxa, highlighting the role of the petrified structures as localized biodiversity hotspots. Beyond the macrobenthic assemblages, field observations also indicated higher fish abundance and species richness around the fossilised structures. These findings further highlight the broader ecological influence of this habitat These initial results emphasise the ecological significance of the underwater petrified forest as a distinctive habitat that contributes to the local and regional biodiversity of benthic communities (Fig. 4). Given the anthropogenic pressure on the coastal ecosystems of the Black Sea, the site represents an invaluable natural archive and a baseline for future monitoring, conservation planning, and multidisciplinary research integrating ecology, geology, and paleobotany.