Unraveling Benthic Diversity in Posidonia oceanica Meadows of the Southern Mediterranean : Insights from the Gulf of Gabès

Posidonia oceanica meadows are key biodiversity hotspots and ecosystem engineers in the Mediterranean Sea. However, the composition and drivers of benthic communities associated with these habitats remain poorly documented in the southern Mediterranean, where coastal ecosystems are increasingly threatened by climate change, biological invasions, and human activities.

This study assessed the diversity, seasonal dynamics, and environmental drivers of benthic macrofauna associated with P. oceanica meadows in the Gulf of Gabès (Tunisia). Seasonal surveys were carried out over two consecutive years during April 2023 to January 2025. A total of eight sampling campaigns were conducted at 20 sites in the Gulf of Gabès, spanning depths of 2–20 m. Benthic samples were collected by SCUBA diving using 20 × 20 cm quadrats.

Overall, 168 benthic species were recorded. Crustaceans constituted the dominant taxonomic group (48% of the total fauna), followed by polychaetes (36%) and molluscs (28%). The highest diversity occurred within the meadows surrounding the Kerkennah and Djerba Islands, whereas the lowest diversity was observed in industrialized sectors of Sfax, Skhira, and Gabès. Twenty-two non-indigenous species were identified, highlighting the ongoing biotic changes affecting these ecosystems. Species diversity and abundance exhibited marked seasonal variation, with the highest values recorded during spring and summer. The greatest diversity and abundance were observed in the Kerkennah Islands during summer 2024, with mean values of 52 species and 3,280 ind.m⁻², respectively. Seasonal shifts in benthic assemblages were strongly associated with variations in seagrass phenology and environmental conditions such as depth, sediment features, and physicochemical parameters (e.g. temperature, turbidity, salinity). Community analyses distinguished three major benthic assemblages, primarily structured by environmental gradients and anthropogenic pressures.

Our results demonstrate that P. oceanica meadows in the Gulf of Gabès support a highly diverse benthic fauna while revealing marked spatial differences driven by habitat quality and human disturbance. These findings provide essential baseline information for the conservation and management of Mediterranean seagrass ecosystems under global environmental change.