Tracking the pumice rafts from the Fukutoku-Okanoba submarine volcano with Satellites and a Lagrangian Particles trajectory model.

On August 13th, 2021, the Fukutoku-Okanoba, a submarine volcano in the Northwest Pacific Ocean, erupted. Satellites detected various pumice rafts that had drifted westward to reach southern Japan over two months. To cope with the potential danger from pumice rafts, predicting their trajectories is crucial. Using a Lagrangian particle tracking model, the trajectories of the rafts were investigated. The model results showed strong sensitivity to the windage coefficient of pumice rafts, which is uncertain and could cause significant errors. An optimal windage coefficient was estimated by comparing the model results with satellite images using a skill score based on the distance between simulated particles and the nearest observed rafts divided by the travel distance of the particles. The optimal windage coefficients ranged between 2 and 3 % and produced pathways comparable to the observations from satellites. The simulation results showed that the pumice rafts moved from Fukutoku-Okanoba toward the Ryukyu Islands for approximately two months prior to being pushed by the north-easterly wind toward Taiwan against the Kuroshio. The methods presented here may become a valuable tool in managing coastal hazards due to diverse marine debris.

Underwater hidden microplastic hotspots: Historical ocean dumping sites.

Three ocean dumping sites located in the Yellow Sea (YS) and East Sea (ES) of South Korea have accumulated terrestrial waste from 1988 to 2015. Most of this waste comprised industrial wastewater and sewage sludge, which are sources of microplastics. In this study, we investigated the spatiotemporal distribution and characteristics of microplastics in surface and core sediments of the YS, South Sea (SS) and ES, including at dumping sites (YDP and EDP). The mean abundance of microplastics in surface sediments was ranked in order of EDP (59,457 ± 49,130 particles/kg d.w.), ES (5,047 ± 9,404 particles/kg d.w.), YDP (3,965 ± 3,213 particles/kg d.w.), SS (314 ± 488 particles/kg d.w.) and YS (288 ± 400 particles/kg d.w.). EDP and YDP showed about 14- and 12-fold higher microplastic abundances, and more diverse polymer compositions, than the ES and YS, respectively. The historical trend of microplastic pollution in age-dated core sediments from EDP and YDP aligned well with the amount of historical ocean dumping. As the level of ocean dumping has gradually reduced since 2006, and was finally banned in 2015, the microplastic abundance decreased accordingly. Interestingly, spherical polystyrene (PS) primary microplastic was the dominant type in EDP sediments (78%) and other surface sediments in the ES (52%). More than 60 million tons of sewage and wastewater sludge were dumped at EDP, and extremely high abundances of up to 130,000 particles/kg d.w. were observed in EDP surface sediments. PS primary microplastics were continuously present in the EDP and ES sediment cores in the dumping period and are suspected to have originated from industrial wastewater sludge. The particle transportation model results showed that PS was dispersed throughout the ES during ocean dumping. In addition, deep circulation can contribute to the dispersion of particles after sinking. These results indicate that ocean dumping sites represent an underwater hotspot and source of microplastics in seafloor sediments.

Prevalence of small high-density microplastics in the continental shelf and deep sea waters of East Asia.

Microplastics are widely distributed throughout aquatic environments. Information about the vertical distribution and fate of microplastics in seawater remains limited. To elucidate the vertical distribution of microplastics, three to six vertical water column layers were sampled based on the thermocline depth, from which the vertical distribution and characteristics of microplastics larger than 20 µm were investigated in continental shelf and deep-sea waters around South Korea. In addition, microplastics incorporated into marine aggregates (aggregated fraction) were investigated to determine the contribution of aggregates to vertical transport of microplastics. The abundance of microplastics was in the range of 15-9,400 particles/m3. No consistent trend was observed in the overall vertical profiles. The size, shape and polymer compositions of microplastics at each station were generally comparable throughout the water column. Unexpectedly, high-density (HD; > 1.02 g/cm3) polymers accounted for an average of 73% of total microplastics. As polymer density increased, the proportion of microplastics less than 100 µm in size increased. HD polymers also accounted for 68% of the aerosol samples collected together with water samples. Due to the relatively high proportion of HD polymers in far-offshore waters, high-density solution should be used to extract microplastics, even from surface seawaters. The aggregated fraction accounted for 0-28.6% (average, 3.4%) of total microplastics. Marine aggregates are considered an important mechanism of transport for microplastics less dense than seawater to the deep-water column, but they showed lower proportions than expected in continental shelf and deep-sea waters around South Korea.

Tracking flood debris using satellite-derived ocean color and particle-tracking modeling.

Flood debris associated with Typhoon Lionrock from the Tumen River at the border between Russia and North Korea was traced using ocean color and a Lagrangian particle-tracking model. As debris is transported along with discharged water during floods, a means of tracing floodwater should also allow any associated debris to be tracked. By analyzing the anomalous distribution of colored dissolved organic matter (CDOM) and total suspended sediments (TSS) from the Geostationary Ocean Color Imager (GOCI), the southward movement of the floodwater was tracked along the eastern coast of the Korean Peninsula. This movement was driven by the North Korean Cold Current and was consistent with model results. The similarity between the satellite-derived and modeled datasets shows that CDOM and TSS can be used to track flood-derived debris for several hundreds of kilometers and locate hotspots of debris accumulation.

Destination of floating plastic debris released from ten major rivers around the Korean Peninsula.

Using a Lagrangian particle tracking model the coastal accumulation of debris from 10 major Chinese and Korean rivers discharging to the seas around the Korean Peninsula is investigated. The amount of debris from each river is proportional to the population over the catchment area of the river and the mismanaged plastic waste (MPW) ratio of the country the river belongs to. The debris generally washes up on the coast near the originating river mouth. When only the debris originating from the rivers on the Korean Peninsula is considered the model results are consistent with the observations for the Korean coast reported in previous studies in that the amount of microplastic and macroplastic are greater near the Han and Nakdong Rivers. The amount of debris from Chinese rivers calculated based on the MPW ratios is 75 times greater than that from the rivers on the Korean Peninsula and if the Chinese rivers are added the model the results differ from the observed patterns. Macroplastic distribution also exhibits a similar discrepancy between the model and observation results if debris from the Chinses Rivers are taken into account. Observed data from Korea's Marine Litter Information System show that the accumulation of foreign-originating macroplastic most of which is Chinese is highest around the southwestern corner of the Korean Peninsula. Debris from the Chinese Yangtze and Yellow Rivers reproduces this observed distribution. Based on the observational data more debris originates from Korea than from foreign sources but Chinese-originating debris is dominant in the model. There are two possibilities for this discrepancy. The first is the presence of sinking due to biofouling; if 50% of the plastic sinks every 20 days or so the model results become similar to the observations. The second is the large difference in the MPW ratios between China and Korea employed in the model. Reducing the Chinese MPW ratio to 25% or lower the model results more closely reflect the observations made along the Korean coast.