The Mediterranean Sea, a major global tourist destination, is heavily impacted by marine litter, a persistent problem stemming from various human activities. The influx of tourists seasonally increases waste generation, particularly affecting beaches, which act as primary gateways for litter entering the marine environment. This study focuses on the Mediterranean islands, highly vulnerable due to their seasonal population swells. Beaches are crucial ecosystems, and their cleanliness is vital for both environmental health and the local economy. Tourist satisfaction is strongly linked to beach cleanliness, impacting visitor choice and return rates. Therefore, understanding the seasonal dynamics of tourist-generated waste on island beaches is essential for developing effective mitigation strategies and promoting sustainable tourism. This research addresses this need by monitoring marine litter on Mediterranean island beaches during both low and high tourist seasons, analyzing the seasonal variation and evaluating the effectiveness of pilot mitigation actions implemented.

Marine litter, encompassing any persistent solid material discarded in the marine and coastal environment, has gained considerable attention since the late 1960s. Initiatives like the International Coastal Cleanup Programme and the Ocean Cleanup Project highlight global efforts to address this issue. While various strategies (preventive, mitigating, removal, and behavior-changing) exist, the amount of litter reaching the marine environment, primarily plastic pollution, remains alarmingly high. The Mediterranean Sea, with its dense coastal population, significant shipping activity, and considerable tourism influx, is recognized as one of the most affected regions. Its semi-enclosed nature and anti-estuarine circulation contribute to litter accumulation, creating concentrations comparable to subtropical gyres. The adverse effects on marine ecosystems, public health, and socio-economic factors are substantial. Beaches globally represent major land-based sources of marine litter due to inadequate waste management and littering. The Mediterranean is no exception, especially during summer months when beaches become leisure hotspots. While tourism supports local economies, it can negatively impact the environment, particularly in island settings with seasonal population increases, demanding effective waste management adaptations.

To assess the seasonal variation of marine litter, 24 beaches across eight Mediterranean islands (Mallorca, Sicily, Rab, Malta, Crete, Mykonos, Rhodes, and Cyprus) were monitored in 2017. Three beach types were selected per island: highly touristic (Tbeach), locally used (Lbeach), and remote (Rbeach). A fixed 100m portion of each beach was periodically monitored (monthly during high season, once before and after). All anthropogenic items were collected, categorized (including mesoplastics (MePs), large microplastics (MPs), and pellets), and quantified. Accumulation rates (AR) were calculated considering the time elapsed since the last cleaning. An accumulation index (AI) was developed, categorizing AR from extremely low to extremely high. In 2019, pilot actions were implemented on 11 beaches (Tbeach and Lbeach from five islands, and Tbeach from Mallorca). These involved awareness campaigns (flyers, posters), improved waste bins, and visitor interaction. Post-intervention surveys assessed the impact on AR. To extrapolate findings to all Mediterranean islands, the total coastline length and beach proportion were determined using Google Earth and the JRC/IES CCM River and Catchment Database. Data correction for distance variation in the 100m monitoring sections was applied using established formulae.

A total of 162,320 items were collected in 2017. While the overall number of items didn't show clear seasonal variation, AR revealed a significant high-season increase (up to 4.7 times) for all beach types. Tbeach had the highest AR (329.6 ± 444.9 items/1000 m²/day during high season), followed by Lbeach and Rbeach. Artificial polymer materials dominated (94.2%). The five most frequent items were MePs, cigarette butts, MPs, pellets, and macroplastics. A strong correlation existed between AR of MePs/MPs and items from the ‘shoreline’ category (likely left by visitors), suggesting on-site fragmentation of larger plastics. The AI effectively assessed marine litter dynamics, showing higher values during high season across all beach types. Pilot actions in 2019 demonstrated a substantial AR reduction (average -52.5% for items from the shoreline category) across most sites. Cigarette butt AR decreased by 54.5% on average for beaches with ashtrays, and 57.8% on those without. Extrapolating the findings to all Mediterranean islands suggested a daily accumulation of approximately 40.6 × 10⁶ items/day during peak season attributable to visitor activity.

The study's findings directly address the research question by quantifying the significant impact of tourism on marine litter accumulation on Mediterranean island beaches. The seasonal increase in AR, coupled with the strong correlation between visitor-related items and smaller plastic fragments, strongly supports the hypothesis that tourism is a key driver of beach litter. The development and application of the AI provides a valuable tool for monitoring and managing marine litter dynamics. The success of the pilot actions demonstrates the efficacy of awareness campaigns in mitigating litter generation. The study's findings contribute significantly to the understanding of marine litter sources and management in tourism-dependent areas. The large-scale extrapolation, though based on assumptions, provides a realistic estimate of the potential scale of the problem and underscores the urgency for sustainable tourism initiatives.

This research highlights the substantial contribution of tourism to marine litter generation on Mediterranean island beaches. The seasonal increase in litter accumulation is significantly correlated with tourist influx. The developed AI is a valuable tool for assessing litter dynamics. The successful implementation of pilot mitigation actions demonstrates the potential of awareness campaigns to reduce litter. Further research could focus on developing more cost-effective mitigation strategies, investigating long-term effects of the pilot actions, and refining the AI for broader application. Ultimately, promoting sustainable tourism practices is crucial for mitigating the environmental consequences of marine litter.

The study's extrapolation to all Mediterranean islands involves assumptions regarding beach proportion and litter distribution, potentially affecting accuracy. The single survey per site in 2019 limited the assessment of MPs and pellets' AR after interventions. While the correlation between visitor-related litter and microplastic accumulation suggests on-site fragmentation, direct observation of this process would strengthen the conclusion. Despite the use of various beach types, regional variations in factors like beaching processes could influence the generalizability of the results.