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.