Nearshore and offshore distribution of floating marine macro-litter in Romanian waters inferred from observations and Lagrangian backtracking

This study examines the distribution, transport pathways, and potential source regions of floating marine macro-litter (FMML) in Romanian waters of the western Black Sea, combining ship-based observations with Lagrangian backtracking simulations. Visual surveys were conducted during an oceanographic cruise in July 2024 along eight transects spanning nearshore and offshore waters between the Danube Delta and the Northwestern Shelf.

The observations revealed a pronounced spatial contrast in FMML densities across nearshore and offshore domains. Transects located closer to the coast exhibited relatively low concentrations (80–649 items km^-2), whereas offshore transects beyond the Northwestern Shelf showed exceptionally high densities, exceeding 9000 items km^-2. Offshore counts were dominated by elongated white plastic strips, a debris type not previously reported as prevalent in this sector of the Black Sea.

To explore transport pathways and potential source regions, Lagrangian backtracking simulations were performed using the LOCATE model, a multiscale framework for floating marine litter transport, forced by high-resolution surface currents from the NEMO ocean circulation model. The simulations indicated two distinct transport regimes. Trajectories associated with nearshore observations remained largely confined to the northwestern sector of the basin and were consistent with circulation patterns influenced by major riverine systems, while offshore debris consistently traced back toward the Crimean Peninsula. Complementary analysis of mesoscale circulation using satellite-derived altimetry and the Q parameter identified a persistent cyclonic gyre near 32° E–44° N, acting as a retention zone that favors offshore accumulation of floating debris.

By integrating in situ observations, Lagrangian modeling, and circulation diagnostics, this study documents offshore FMML hotspots in Romanian waters and highlights the role of mesoscale circulation and coastal–offshore connectivity in shaping the distribution of floating debris. These findings underscore the transboundary nature of floating marine litter in the Black Sea and emphasize the need for coordinated, basin-scale management strategies that account for remote source regions and offshore retention processes.

This work received financial support from the TRAP project (EsTRAtegias participativas para la gestión de la contaminación por Plástico del litoral transfronterizo) (EFA147/03), funded by the POCTEFA Program / Interreg VI-A 2021–2027.