A belt transect setting strategy for mark-recapture experiments to evaluate the 1D diffusion coefficient of beached litter in the cross-shore direction

Deposition rates and residence time of litter varies among beaches in the Lofoten archipelago, Norway

A considerable portion of marine litter pollutes the world's coastlines. Its accumulation on beaches represents the product of deposition and retention, processes which are not well understood. A mark-recapture study was performed with a two-week sampling interval at three sites in Lofoten, Norway. Deposition and retention vary over relatively small spatial scales (approx. 13 km radius). No correlation was found among sites in the timing of high and low deposition events, suggesting these are governed by local factors. Contrastingly, the correlation in the timing of high and low retention events was generally stronger among sites, suggesting these may be affected by regional factors. The results underline the importance of customising cleanup frequency for different beaches as spatiotemporal variation in the relative importance of deposition and retention dictate the optimal frequency for maximal removal of litter from circulation in the local marine environment, which cannot be discerned from accumulation (i.e., standing stock) alone.

Buoyancy affects stranding rate and dispersal distance of floating litter entering the sea from river mouths

Rivers are a major source of litter entering the sea but our understanding of the transport and fate of plastics in estuarine environments is poor. Marked blocks of varying buoyancy were released at three river mouths in South Africa. Of the 1400 blocks released, 80% were recovered on nearby beaches, with a higher recovery rate for more buoyant blocks. Dispersal distances increased with decreasing buoyancy at all sites; median dispersal distance of stranded items ranged from 20 to 90 m for expanded polystyrene (EPS) to 70-90 m for wood and 60-1042 m for high density polyethylene (HDPE) blocks. Floating litter in estuaries is subject to bidirectional flow and export is largely controlled by hydrodynamic conditions such as tides, winds, and wave action, as well as coastal structure and vegetation. Cleaning beaches around river mouths will help to reduce leakage of plastic and other litter into the sea.

The transport and fate of marine plastics in South Africa and adjacent oceans

South Africa is thought to be one of the worst contributors of plastic into the sea globally. Although some plastic items derive from offshore sources (mainly fishing and other maritime activities, but also long-distance transport), the importance of local, land-based sources is indicated by the composition of beach debris and the concentration of macro-, meso- and microplastics close to urban source areas. Some 60–90% of plastic from land-based sources is expected to strand on beaches, but plastic standing stocks on beaches are much lower than global model predictions of land-based pollution. Burial in beaches and transport into backshore vegetation are significant sinks, although this plastic is likely to be released as the climate crisis leads to rising sea levels and more extreme storms. Most buried items are fairly small, while many larger items, which account for most of the mass of plastic, are removed from beaches by cleaning efforts. However, even daily accumulation rate estimates – which exclude the effects of cleaning – fall well short of model predictions of plastic leakage from land-based sources. Oceanographic models predict that plastics entering the sea from South Africa are exported to the South Atlantic and Indian Oceans, with the proportion depending on source location and item density. At sea, floating macroplastic is concentrated close to urban centres. Farther offshore, plastic items tend to be large and buoyant because biofouling causes small, low buoyancy items to sink. Size-selective removal of plastics by biota might also contribute to the paucity of floating microplastics (smaller than 1 mm). The seabed is likely to be the main long-term sink for waste plastics, but the limited data available indicate low levels of plastics on the seabed off South Africa. Only a small proportion of plastic predicted to leak into the sea from South Africa can be accounted for. However, this should not delay the implementation of effective mitigation measures to limit plastic leakage. Significance: • High densities of waste plastic around urban centres indicate that most macro- and microplastics come from local, land-based sources and do not disperse far at sea. • Beach clean-ups remove up to 90% of the mass of stranded plastic, largely found in macroplastic items (larger than 25 mm). • The seabed is a long-term sink for marine plastics, but densities of plastic on the seabed around South Africa are still modest. • The global model prediction of plastic leakage from South Africa into the sea probably is a gross overestimate.

An estimation of the average residence times and onshore-offshore diffusivities of beached microplastics based on the population decay of tagged meso- and macrolitter

Residence times of microplastics were estimated based on the dependence of meso- and macrolitter residence times on their upward terminal velocities (UTVs) in the ocean obtained by one- and two-year mark-recapture experiments conducted on Wadahama Beach, Nii-jima Island, Japan. A significant linear relationship between the residence time and UTV was found in the velocity range of about 0.3-0.9ms^-1, while there was no significant difference between the residence times obtained in the velocity range of about 0.9-1.4ms^-1. This dependence on the UTV would reflect the uprush-backwash response of the target items to swash waves on the beach. By extrapolating the linear relationship down to the velocity range of microplastics, the residence times of microplastics and the 1D onshore-offshore diffusion coefficients were inferred, and are one to two orders of magnitude greater than the coefficients of the macroplastics.