Ingestion of microplastics by mesozooplankton in the East Sea, Korea: possible vertical movement of microplastics by mesozooplankton

Abstract

Microplastics (MP) can be ingested by mesozooplankton which is a potential threat for the organisms in the marine environment. The ingestion of MP by mesozooplankton in the East Sea, Korea was studied for the first time to evaluate the amount of MP that is ingested by mesozooplankton and possible vertical movement of MP in the water column through mesozooplankton. The 24-hour sampling at 3-hour intervals [Time(T) T1: 09:00, T2: 12:00, T3: 15:00, T4: 18:00, T5: 21:00, T6: 24:00, T7: 03:00, T8: 06:00, T9: 09:00], at fixed station (37°03.3868’N, 130°56.3621’E) from five different layers within 0-200 m was conducted using an opening-closing net (mesh size: 200μm). The mesozooplankton samples were identified according to the taxonomical groups (15-30 groups) that mainly contributed to 70~90% of total mesozooplankton abundance for each sampling time and depth. The total abundance of mesozooplankton did not exhibit clear diel vertical migration pattern but most dwelled in the surface mixed layer (SML, 0-20 m) (T1~T9 mean: 3,095 inds. m-3) and the abundance was lower in the deeper layers (150-200 m) (T1~T9 mean: 288 inds. m-3 ). The numerically major taxa which appeared in this study were dinoflagellates Pyrocystis noctiluca (56.9%), immature copepods (16.6%), adult copepods (15.6%), chaetognaths (6.3%), gelatinous plankton (including ctenophores, cnidarians, hydroids, thaliaceans, 1.4%), and others (including foraminiferans, radiolarians ostracods and cladocerans, 1.1%). The ingested MP abundance were 202-855 particles m-3 (mean: 437 particles m-3) in all the layers. In all the layers, paint particles (94.8%) were the highest ingested MP and fragment and fibers increased in the deeper layers. Mesozooplankton that showed slight diel vertical migration (DVM) were adult copepods of Metridia pacifica (female and male) and copepodites stages IV-V. Abundances of adults and immature copepodites IV-V of M. pacifica were higher during the night time (T5~T7) in the 0-200 m (T5~T7 mean: 29 inds. m-3), and no individuals appeared during the day time in the 0-200 m, which implies that Metridia species may migrate into the deeper waters (>200 m) during the day time. In the SML and depth layer of 20-60 m, adult species of M. pacifica and copepodites IV-V had ingested MP (mean: 17.6 particles m-3) in which paint particles were dominant type of MP (96.8%). In this study, higher abundance of mesozooplankton and ingested MP concentrations were apparent in the SML where the major type of MP was paint particles for all layers, and fragments and fibers for particular depth layers. This study suggested that mesozooplankton ingests MP mostly in the SML and may be moved upwards and downwards within the studied water column by the DVM species.