Distribution and species richness of seagrass meadows in the Sea of Marmara

Comprehensive data on seagrass distribution and species richness in the Sea of Marmara (SoM) are absent from global seagrass mapping, hindering long-term monitoring and practical protection efforts for these vital meadows. This study aims to assess the species richness, spatial distribution patterns, and fundamental ecological aspects of seagrass meadows in the SoM, which serves as a crucial transition zone between the Black Sea and the Mediterranean Sea. The data set was obtained through a series of underwater surveys conducted between June and September 2024 at 140 surveyed stations, covering a total area of about 0.534 km². Spatial and species-specific surface area (m²), cover percentage (%), and depth limits (m) of seagrass meadows were estimated from underwater records collected along line transects. Seagrass meadows constituted 51.9 % of the area surveyed in the SoM, covering an area of 0.277 km². Results revealed a rich diversity of seagrass species within the SoM, identifying four species: Cymodocea nodosa, Posidonia oceanica, Zostera marina, and Z. noltei. C. nodosa was the most common species, occupying 82.8 % (0.230 km²) of the total surface area. The lower depth limits of seagrasses in the SoM were shallower than in other Mediterranean regions, with P. oceanica at 15.7 m and C. nodosa at 11.1 m. This limitation attributed to reductions in light penetration caused by high primary production and excessive pollution loads in the SoM. In conclusion, this dataset includes the first underwater observation-based mapping and identification of new areas for seagrass species in the SoM, contributing to distribution maps for the Mediterranean basin.