The untold story of Sea Caves: Lost and found anthropogenic debris in confined marine environments

Anthropogenic debris poses an increasing threat in the marine environment due to its persistence and adverse effects on marine biota and ecosystem services. Previous anthropogenic debris research has centred on shorelines, surface waters, the water column, coastal seafloor and deep-sea domain, leaving sea-caves largely overlooked. This study introduces a novel quantitative approach tailored for sea cave systems, broadening the scope of marine litter research beyond previously targeted environments. All surveyed sea caves contained debris, with densities ranging from 0.044 to 96.839 items m^-2. Most debris was primarily transported via sea-based pathways, and consisted mainly of fragmented plastics (66 %). Occasionally debris was found embedded within tar layers, producing "plastitar" deposits on cave walls. Analyses revealed that wave exposure and available beach area influenced debris retention, while differences in cave architecture resulted in distinct interior spatial deposition patterns across two cave types, leading to the formation of accumulation hotspots. Findings demonstrate that semi-submerged sea caves serve as important reservoirs for anthropogenic marine debris alongside toxic chemical components and potentially associated harmful microbiomes. Such contamination poses risks to sea cave biota and likely compromises the conservation efforts for the Mediterranean monk seal (Monachus monachus).