Hitchhikers on marine plastic debris during the ‘Golden Tides’ event of seaweed Sargassum horneri in the western coast of Korea

Abstract

Massive amount of seaweed Sargassum horneri in the event of Golden Tides were washed ashore in the south western coast of Korea through oceanic currents and winds during January of 2021. The detached S. horneri are known to be dispersed from Zhejian Province of China and it had accompanied various marine plastic debris (MPD) over long distance. Large accumulation of both S. horneri and MPD damages the aquaculture industry and marine ecosystem. The MPD acts as a vector for transportation of hitchhikers which are mainly substrate-associated organisms including zooplankton as well as larvae of meiofaunal biofouling pertaining to the range between 40 to 1,000 μm. Some of the organisms may be invasive species that may threaten native species, degrade biodiversity, alter structure of food webs and may rise other problems. In order to identify the hitchhikers, we have examined MPDs including buoys, nets, containers, bottles, and shoes, and then categorized them according to the plastic material types. Five polymer types of plastics including polyethylene(PE), polypropylene(PP), polyvinylchloride(PVC), polyethylene terephthalate(PET), and others (PVC/PE) had been submersed in filtered seawater and incubated for 10 days. And then surface of the plastics were additionally scrubbed in order to get attached organisms and then detached hitchhikers were preserved for microscopic analysis (morphology). Analysis of DNA metabarcoding was also carried out to provide complementary information on species level. According to the morphological analysis, we have identified 14 taxa from 6 phyla, and the dominant taxa included nematode larvae, harpacticoids, ostracods, caprellids, polychaetes and bivalve larvae. The integrated (40-1,000 μm) abundance of hitchhikers ranged 19−118 inds. 0.1 m-2 (mean: 76 inds. 0.1 m-2) from MPD. PVC had the highest integrated abundance, where nematodes larvae were dominant species (75%), followed by ostracods (17%). Whereas, PET had the lowest integrated abundance, where nematodes larvae were also dominant (57%) and then followed by harpacticoids (36%). The metabarcoding data showed that Pellioditis marina and Halomonhystera disjuncta (Nematoda), Mytilus edulis and Crassostrea gigas (Bivalvia), Nereis heterocirrata (Polychaeta), Tisbe spp. and Tigriopus japonicus (Harpacticoida) were abundant in the average abundance of MPD. Our findings suggest that MPD may act as a vector for dispersal of hitchhikers, and polymer type of especially PVC may transport higher abundance of hitchhikers including meiofaunal biofouling than other polymer types. Further studies are needed to find out whether polymer-specific species are apparent and whether the species can be harmful to the environment.