Massive occurrence of benthic plastic debris at the abyssal seafloor beneath the Kuroshio Extension, the North West Pacific

The deep-sea is considered to be an ultimate sink for marine plastic debris. The abyssal (3500-6500 m) covers the bulk of the deep ocean floor yet little is known about the extent of plastic debris on the abyssal seafloor. We undertook a quantitative assessment of debris presents on the abyssal seafloor (5700-5800 m depth) beneath the Kuroshio Extension current system in the Northwest Pacific, which is one of the major transit points for massive amounts of debris sourced from Asia that are entering the North Pacific Ocean. The dominant type of debris was single-use plastics-mainly bags and food packaging. The density of plastic debris (average 4561 items km−2) in the abyssal zone was the highest recorded for an abyssal plain, suggesting that the deep-sea basin of the Northwest Pacific is a significant reservoir of plastic debris.

bottom of the water column with an assumed sinking speed of 1.32 x 10 -2 m/s from the 1 5 2 laboratory experiment ( Fig. 4a-f). Under this assumption, all particles were shown to 1 5 3 reach the seafloor within 7 days, even to 6,000 m water depth. Therefore, is it likely that 1 5 4 the behavior of sinking particles was not affected by subsurface ocean properties. This 1 5 5 includes the subsurface current velocity which is highly variable over both spatial and 1 5 6 temporal scales. 1 5 7 1 5 8 The model simulation also investigated whether bottom currents could move the debris 1 5 9 after they reached the seafloor therefore widening the range of debris distribution. It was 1 6 0 shown that after 5 years the distribution of the particles initially located at the surface of 1 6 1 St. 8 and 10 tended to move north-eastward with relatively higher abundance found along 1 6 2 145ºE (Fig. 4g, h).In contrast, all the particles initially located at the surface of St. 6.5 at 1 6 3 Myojin Knoll moved towards the eastern region, some of which were found along 145 o E, 1 6 4 but no particles remained around the original settling point after 5 years ( Fig. 4i).  1 9 0 1 9 1 Although comparisons between areas can reflect differences in sampling methodologies, 1 9 2 the density of plastic debris on the seafloor below the KE/KERG (average 4561 1 9 3 items/km 2 ) was higher than the world average of submarine canyons (2804 items km -2 ), 1 9 4 seamounts, banks and mounds (382 items/km 2 ), trenches (1720 items/km 2 ), shelfs (664 1 9 5 items/km 2 ), and ridges (339 items/km 2 ) ( Fig. 5, see also Table S2). Notably, the 1 9 6 macroplastic density below the KE/KERG (abyssal plain) was two orders of magnitude 1 9 7 higher than the world average for abyssal plains (88 items/km 2 ).
The density of debris below the KE/KERG was also comparatively higher than other 2 1 2 areas in proximity to Japan. The macroplastic density below the KE/KERG (average 2 1 3 4561 items /km 2 ) was higher than that found in the deep-basin off northern Japan (335 2 1 4 7 items/km 2 ) with a depth of 6000 m, that experiences no effect of the Kuroshio current 14 .

1 5
It was also 2-3 fold higher than the debris density reported for Sagami Bay, which was 2 1 6 unexpected as Sagami Bay is located off the coast of a highly populated area that includes 2 1 7 Tokyo (see Fig. 1c).
2 1 8 2 1 9 Considering the high proportion of single-use plastics, the debris found in the KE/KERG Previous studies showed that the density of man-made debris on the deep seafloor, far 2 3 5 from the coast, is lower than that of the seafloor adjacent to populated areas 14,29 . As such, 2 3 6 land-based debris density tends to decrease with increasing distance from the coast, and 2 3 7 has not been considered to transport in large quantities more than a few tens of kilometers Kuroshio current could be vertically transported to the seafloor, even to abyssal depths. is in broad agreement with the observation that no debris were found at St. 6.5. It is 2 5 4 important to note that the seafloor below St. 6.5, Myojin Knoll, is located on the 2 5 5 Izu-Ogasawara ridge along 139ºE (Fig. 1). Due to the presence of the ridge, which might  located on the Izu-Ogasawara ridge (Table 1). For comparison, we also took observations 3 2 0 at Sagami Bay (St. 2, depth 1,400 m), a bay close to a highly populated region off the 3 2 1 coast of Japan that includes Tokyo. Videos from the full HD and 4K camera were played on shore and observations of debris 3 4 6 were recorded. Each item of debris was classified into three categories: plastic, metal and 3 4 7 wood (both natural and processed). All items which could easily be identified as debris 3 4 8 were labeled as such, and those where an initial identification was not possible were 3 4 9 labeled as potential debris items. These potential debris items were further evaluated 3 5 0 several times by the authors until a consensus was reached. In cases where a consensus 3 5 1 could not be reached between authors the item was not considered to be debris 32 .
3 5 2 3 5 3 The areas observed (km 2 ) for each dive were estimated from the horizontal distance (km) was estimated using the distance from the sea bottom to the fixed camera, the angle of the 3 5 8