River flow regime and proximity as drivers of beach marine litter accumulation: A case study from the Neretva River Delta, Eastern Adriatic.

Rivers are major transporters of land-based litter to coastal environments, yet the extent to which river flow and distance from the river mouth shape beach litter accumulation remains insufficiently understood. The Neretva River, the largest river system in the eastern Adriatic, and Mali Ston Bay, a semi-enclosed coastal area into which it flows, were used as a case study to investigate the influence of seasonal river discharge and spatial proximity to the river mouth on beach litter dynamics. Monthly surveys conducted over two years (2021-2022) at beaches located near and far from the river mouth quantified litter abundance, weight, and composition in relation to seasonal hydrological conditions. Increased river discharge during the wet season led to a significant rise in beach litter loads, with both abundance and weight more than doubling compared to the dry season. Proximity to the river mouth also played a key role in debris distribution, with higher litter loads at deltaic beaches than those farther away in the bay. Plastics dominated the litter composition across all sites and seasons, with single-use plastics particularly prevalent. Despite pronounced seasonal and spatial variability, interannual litter accumulation remained stable, suggesting persistent pollution sources. These findings emphasize the importance of upstream waste management, riverine litter interception, and seasonally targeted beach cleanups, particularly near river mouths, as well as the need for regulatory actions, such as restrictions on single-use plastics, and long-term monitoring to mitigate river-driven coastal litter pollution.