Benthic and coastal macrolitter in the Greek seas : abundance, distribution and sources
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Date
2026-07-10
Authors
Κουβαρά, Κωνσταντίνα
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Abstract
Marine litter is recognized as one of the most pervasive and persistent forms of pollution affecting marine ecosystems worldwide, causing a wide range of negative impacts. The Mediterranean Sea, due to its semi-enclosed nature, intense coastal urbanization, tourism, shipping activity, and limited water exchange, is considered one of the global hotspots of marine litter accumulation. Within this context, the Greek seas are subjected to increasing anthropogenic pressures, while comprehensive and harmonized assessments of marine litter distribution, abundance, sources, and ecological impacts remain fragmented. The present doctoral dissertation investigates coastal and benthic macrolitter pollution in the Greek seas through a multi-scale and multi-methodological approach, focusing on litter abundance, composition, spatial distribution, temporal variability, and potential sources.Initially, an integrated approach combining legacy datasets with newly collected data acquired through beach surveys, diver observations, bottom trawls, Remotely Operated Vehicles (ROVs), and Towed Underwater Camera (TUC) systems was applied in the Saronikos Gulf. The average litter density across the 108 beaches examined was 2.61(±0.41) items m⁻² (range: 0-23.6 items m⁻²), with plastics being the dominant material. Small polystyrene and plastic fragments, together with cigarette butts, dominated litter composition. Spatial analysis revealed that the western part of the Gulf exhibited higher litter densities, likely due to the combined influence of the predominance of remote beaches and the surface circulation patterns within the Gulf. Based on 109 benthic surveys, mean seafloor litter density was estimated at 12,145 items km⁻² (range: 117-251,250 items km⁻²), exhibiting a decreasing trend with depth.In the Thermaikos Gulf, seafloor litter and its ecological impacts were investigated for the first time, using data from 40 ROV dives and 11 scuba dives. ROV surveys revealed litter densities ranging from 0 to 83,817 items km⁻² with an average of 14,951 (±2611) items km⁻². Shallow waters exhibited 50 times higher densities (mean 737,900 (±429,500) items km⁻² and range of 1,250 to 4,810,000 items km⁻²), highlighting the severe accumulation of litter in nearshore areas, particularly in ports. Water circulation patterns, depth, and proximity to urban areas emerged as key drivers of litter distribution. Plastics dominated the litter composition, with fishing-related items prevailing in deeper waters, while single-use plastics were more dominant in ports. Benthic animals observed in ROV footage were identified, and their abundance was recorded, providing insights into the ecosystem's exposure to litter. Significant interactions between marine litter and benthic organisms were observed, including ghost fishing, entanglement, colonization, and the use of litter for shelter, highlighting the ecological implications of litter accumulation on marine ecosystems. At a national scale, data derived from the “Typhoon Project”, a five-year coastal cleanup initiative covering the entire Greek coastline, were analyzed. A total of 10,105 unique coastal sites were surveyed, making this one of the largest and most systematic marine litter datasets of its kind worldwide. 65.7% of the surveyed sites showed no litter accumulation, while 34.3% contained measurable litter loads. The overall mean litter density for the entire study area was estimated at 0.104 ± 0.471 kg m⁻² (range: 0-15.165 kg m⁻²). Results indicated that substrate type, accessibility, beach organization, and shoreline orientation relative to prevailing winds and wave direction significantly influenced litter accumulation patterns. Plastics overwhelmingly dominated litter composition, with fragmented plastics accounting for more than 60% of all recorded items. Single-use plastics and packaging-related items were also highly prevalent. Spatial analysis revealed highly heterogeneous litter distribution, with major hotspots identified in gulfs, remote coastlines, uninhabited islands, and highly touristic areas. Litter accumulation patterns appeared to result from the interaction of multiple factors, including anthropogenic pressures, hydrodynamic conditions, geomorphological characteristics, and management practices. Finally, a reduction in litter density was observed at approximately 40% of the sites that were re-surveyed following the initial cleanup. Additionally, the impacts of the COVID-19 pandemic on marine litter pollution in Greece were investigated through 59 beach and 83 underwater cleanups. The analysis revealed the emergence of PPE (face masks and gloves) as a new category of marine litter. PPE and other COVID-19-related items were recorded in both coastal and benthic environments, while wet wipes exhibited higher densities compared to the pre-COVID period. COVID-19-related items accounted for 1.04% of the total litter. These findings demonstrate how abrupt changes in human behavior and consumption patterns can rapidly alter marine litter composition and pollution dynamics. Overall, the dissertation provides one of the most comprehensive assessments of coastal and benthic marine litter pollution conducted in the Greek seas to date. The results contribute valuable baseline information for future monitoring efforts and support the implementation of effective marine litter management strategies and environmental policies at both national and regional scales. Furthermore, the dissertation contributes to the broader field of marine litter research through the critical discussion and comparison of different methodologies and monitoring approaches, as well as through the identification of their limitations and potential biases.
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Keywords
Marine litter, Seafloor pollution, Plastic pollution, Ecological impacts, Greek seas, Anthropogenic pressures, Coastal litter