Assessing marine litter and its ecological impact on the seafloor of Thermaikos Gulf (NE Mediterranean Sea, Greece): Insights from ROV and diver surveys

This study investigates marine litter and its ecological impacts on the seafloor of the Thermaikos Gulf (NE Mediterranean Sea, Greece) for the first time, using data from 40 ROV dives (15-90 m depth) carried out between 2020 and 2024, and 11 scuba dives (0-5 m depth) conducted primarily at fishing ports during the same period. Plastics dominated the litter composition (80 %), with fishing-related activities contributing to 65 % of the litter at depths from 15 to 90 m, while single-use plastics comprised 46 % of the litter at nearshore locations. ROV surveys revealed litter densities decreased with depth and were likely influenced by water circulation patterns and human activities, particularly fishing. Shallow waters exhibited 50 times higher densities, highlighting the severe accumulation of litter in nearshore areas, particularly in ports. Benthic animals observed in ROV footage were identified, and their abundance was recorded, providing insights into the ecosystem's exposure to litter. Moreover, documented faunal interactions, including ghost fishing, entanglement, colonization, and the use of litter for shelter, highlight the ecological threats posed by marine litter. These findings surpass established Mediterranean litter density thresholds for the seafloor, underscoring the urgent need for stricter waste management at ports and offshore, stronger enforcement of EU directives, and targeted educational campaigns. This research establishes benchmark levels for setting environmental objectives under the Marine Strategy Framework Directive and for conducting targeted research to address litter pollution in the Thermaikos Gulf and similar coastal regions.