Floating Versus Stranded Macro-Litter in an Urban Adriatic Coastal System: A One-Year Comparison of Two Litter Compartments in Dubrovnik (Croatia)

Coastal litter accumulates along urban shorelines, yet the floating and stranded litter compartments of tourism-intensive coastal cities are rarely characterised side by side. We conducted monthly macro-litter surveys over one year (April 2023–March 2024) at two Dubrovnik sites less than 200 m apart, each representing a different litter compartment: Porat harbour, a semi-enclosed basin retaining floating litter, and Posat, an urban beach collecting stranded debris. Surveys followed the updated European MSFD marine litter monitoring guidance. In total, 1325 items were recorded at Porat and 1768 at Posat. The harbour was far more variable month to month (coefficient of variation 135% versus 77%), consistent with episodic basin retention. Artificial polymers dominated both compartments (84.9% at Porat; 79.2% at Posat), but the leading category differed: plastic fragments in the harbour and expanded polystyrene on the beach. Composition differed significantly between the two compartments (PERMANOVA, p = 0.038), though the separation was modest. Because each compartment requires a different sampling method, compartment and sampling method are fully confounded; absolute magnitudes are therefore reported separately and not compared. At Posat, where cleaning was suspended, the Clean Coast Index averaged 7.1 (moderate) but ranged from 1.5 to 21.9 between months, spanning all five cleanliness classes. These distinct signatures indicate that single-compartment monitoring may overlook urban coastal heterogeneity, and that mitigation requires compartment-specific measures.