Monitoring multi-year macro ocean litter dynamics and backward-tracking simulation of litter origins on a remote island in the South China Sea

Abstract

Ocean litter has accumulated rapidly and is becoming a major environmental concern, yet quantitative and regular observations and exploration that track litter origins are limited. By implementing monthly sample collections over five years (2012)(2013)(2014)(2015)(2016) at Dongsha Island, a remote island in the northern South China Sea (SCS), we assessed macro ocean litter dynamics, identified source countries of individual plastic bottles, and analyzed the origins of the litter by a backward-tracking model simulation considering both the effects of current velocity and windage. The results showed that large amounts of litter, which varied monthly and annually in weight and quantity, reached the island during the study years, and there were spatial differences in accumulation patterns between the north and south coasts. Styrofoam and plastic bottles were the two primary sources of macro ocean litter both annually and monthly, and most of the litter collected on the island originated from China and Vietnam, which were collectively responsible for approximately 47.5%-63.7% per month. The simulation indicated that current advection at the near-surface depths and low windage at the sea surface showed similar patterns, while medium to high windage exhibited comparable expression patterns in response to potential source regions and drifting time experiments. At either the surface with low windage or current advection at depths of 0.5 m and 1 m, macro ocean litter in the Western Philippine Sea, i.e. through the Luzon Strait between Taiwan and the Philippines, was an important contributor to the litter bulk from October to March, whereas the litter was predicted to mainly originate from the southwestern SCS from April to September. With an increasing windage effect, litter in the Taiwan Strait was predicted to be an additional major potential source. Surprisingly, a small proportion of the macro ocean litter was predicted to continuously travel in the northern SCS for a long duration (> 2 years) before drifting onto Dongsha Island. The estimated drifting time of macro ocean litter also showed monthly and directional variability. This study demonstrated that a tremendous quantity of macro ocean litter, which may cause great damage to the marine ecosystem, drifts in the ocean surface layer and is finally pushed onto beaches. Therefore, we proposed an action plan for effective ocean litter management development at regional and global spatial scales, which is vital for improving and restoring the health and sustainability of the oceanic environment.

Temporal Trends and Potential Drivers of Stranded Marine Debris on Beaches Within Two US National Marine Sanctuaries Using Citizen Science Data

“…This study has shown that monitoring data can be used to estimate marine debris abundance and temporal trends, and identify drivers of debris loads on shorelines within two US National Marine Sanctuaries: Olympic Coast (OCNMS) and Greater Farallones (GFNMS). Similar to other shoreline monitoring studies worldwide, marine debris in the two sanctuaries was dominated by plastic items, in particular, plastic fragments (Derraik, 2002;Thiel et al, 2013;Hong et al, 2014;Hoellein et al, 2015;Schulz et al, 2015;Hardesty et al, 2017a,b;Nelms et al, 2017;Araujo et al, 2018;da Silva et al, 2018;Ko et al, 2018;Ryan et al, 2018;Ambrose et al, 2019).…”

An inshore–offshore sorting system revealed from global classification of ocean litter

“…Floating litter is sorted out in turbulent conditions induced by wave and wind, with some items more prevalent on the water surface (high-buoyancy litter) and others more susceptible to submerge into the mixing layer (low-buoyancy litter) 28,29 . Thus, highly buoyant items are more affected by the wave-driven Stokes drift and windage, which results in ejection from the oceanic accumulation zones and transport to shore and beaches 8,26,30,31 . By contrast, low-buoyancy litter would escape the wind and waves sweeping the ocean surface and have a greater likelihood of entering into the Ekman convergence to the great mid-ocean gyres [5][6][7]32 .…”

“…Large-sized floating fishing nets often have a long underwater draft, being less affected by wind-driven transport processes sweeping the ocean surface and more likely trapped into the subtropical gyres 8 . By contrast, shapes including internal air volume (for example, empty bottles, buoys, Styrofoam crates) are especially prone to windage, showing the highest beach delivery rates 30,31 .…”

Understanding the Sources of Marine Litter in Remote Islands: The Galapagos Islands as a Case Study

“…Still, some of the highest densities of marine litter have been reported in remote islands, not only near oceanic convergence zones (730 items⋅100 m − 1 , 500 kg⋅100 m − 1 ; in Inaccesible Island) (Ryan et al, 2019), but in islands situated far away from these garbage patches (e.g. Henderson and Dongsha Islands) (Ko et al, 2018;Lavers and Bond, 2017), revealing the global magnitude of this environmental problem.…”