Hard-bottom benthic communities in two semi-submerged marine caves (Adriatic Sea): an updated baseline with insights on short-term recreational boating impacts

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We investigated two semi-submerged marine caves within the Tremiti Islands Marine Protected Area (South Italy, Adriatic Sea), integrating geomorphological mapping, video-transects, qualitative sampling, and an assessment of recreational boating. High-resolution 3D surveys were used to characterize cave morphology. Benthic assemblages were analysed from SCUBA video-transects performed at two depths (3 and 7 m) in May and November 2021 (before and after the tourist season, respectively), and species inventories were refined through qualitative sampling. A total of 118 taxa were recorded. Benthic cover was consistently high (98–99%) across caves, depths, sectors, and periods. Assemblages were dominated by encrusting sciaphilic Rhodophyta and Porifera, and showed clear spatial gradients both horizontally and vertically. Recreational boating pressure was high, with >5000 boat transits and approximately 77,000 passengers recorded in summer 2021. Despite this intensity, no detectable short-term changes in benthic composition were detected at the community level. Large excursion boats – quick visits, regulated, and professionally operated – had minimal impact, whereas small, rented motorboats, with longer and less controlled visits, represent a potential risk. These findings provide the first integrated ecological and geomorphological baseline for Tremiti semi-submerged caves and support targeted management of recreational boating within MPAs.