1 Introduction

Mesoscale eddies, one of the most energetic forms of flow in the oceans, are almost everywhere in the oceans and have time scales of days to months and spatial scales of tens to hundreds of kilometers (e.g., Wunsch 1981). They can also last longer, up to a couple of years (Hamad et al. 2005; Chelton et al. 2011b; Mkhinini et al. 2014). Traveling long distances, they may act as significant transporters of microplastics, as has been shown for surface drifters and plankton (Chelton et al. 2011a, b; Early et al. 2011; Miladinova et al. 2020; Miladinova et al. 2020).

In the Northern Hemisphere, cyclonic eddies (anticlockwise vortices with low sea surface height around the center) are conventionally thought to expel material at the sea surface due to the outward surface flows during the developing stage, while anticyclonic eddies (clockwise vortices with high sea surface height around the center) attract material via inward surface flows. For instance, a previous report supports that anticyclonic eddies accumulate more microplastics than cyclonic ones in the North Atlantic subtropical gyre interior region (Brach et al. 2018). Their observation is considered to have been carried out during the developing stage of eddies. Even cyclonic eddies may cause flows toward the eddy center through the convergence induced by the downward displacement of the main pycnocline during attenuation periods, resulting in the capture of material. Although the previous study provided valuable information on the abundance of microplastics in both cyclonic and anticyclonic mesoscale eddies with moderate intensity of approximately 0.08 m (based on the relative height between the interior and exterior regions) (Brach et al. 2018), there is scarce knowledge on the microplastic abundance in eddies.

Knowledge on the microplastic abundance in eddies with stronger intensity in western boundary currents, such as Gulf Stream and Kuroshio Extension (KE) rings, which shed from the Gulf Stream and the KE, respectively, are quite limited. Such strong eddies have substantial swirl current and can change the distribution of materials (e.g., Chelton et al. 2011a, b). As for particles other than plastic debris, it has been reported that cyclonic eddies effectively accumulate particles in their centers (Lobel and Robinson 1988). These patterns of drifting particle accumulation by mesoscale eddies are not unequivocal, requiring further investigation.

In addition to the above-mentioned processes, there may be processes which expel and attract materials if attenuation of eddies and other factors are taken into account. Regardless of wind direction, different wind speeds relative to alternating currents due to a strong cyclonic eddy can cause a convergence of the Ekman current toward the eddy center (McGillicuddy 2016). Furthermore, if an eddy is sufficiently strong, it can keep material in the interior (Nagano et al. 2016). Therefore, intense mesoscale eddies may retain materials in their inner region for longer and transport microplastics across long distances, regardless of their rotation directions, making intense mesoscale eddies an important player in the transport of microplastics.

In the Northwest Pacific, the Kuroshio turns southeast of Japan, flows eastward as the KE, partly recirculates as the Kuroshio Extension recirculation gyre (KERG), and eventually feeds into the North Pacific subtropical gyre (NPSG) interior flow (Hasunuma and Yoshida 1978; Aoki and Imawaki 1996; Qiu and Chen 2010b; Nagano et al. 2016). The KE current system is one of the major transit points of plastic debris that have sources in the Asian regions and enter the North Pacific, carrying debris to the NPSG or the notorious Great Pacific Garbage Patch (GPGP) (Lebreton et al. 2018). In KERG, energetic cyclonic mesoscale eddies, i.e., cold-core rings, are frequently observed (e.g., Kouketsu et al. 2016; Nagano et al. 2016) and are considered to greatly modify the transport and redistribution of plastic debris by the KE current system. As such, investigation into the processes influencing the distribution and transport of floating plastic in the KE system is crucial to better estimate the abundance and dynamics of plastic debris in the North Pacific.

Here, we present the study of floating microplastics in a cold-core ring in the local KERG, which detached from the KE in June 2019 with a sea surface height (SSH) depression around the eddy center approximately 0.8 m lower than the eddy edge. We targeted floating microplastics because the abundance of plastics on the ocean surface is recommended for use as one of the indicators to measure the level of achievement of SDG 14 (United Nations 2023). Microplastics in the adjacent waters, including the Kuroshio, were also investigated to facilitate a comparison. Our study specifically addressed two key topics regarding microplastics associated with mesoscale eddies through shipboard and satellite observations and data assimilations: (1) How great is the floating microplastic abundance in a mesoscale cyclonic eddy with high intensity? (2) Can a cyclonic mesoscale eddy capture the surrounding particles and retain them while traveling the ocean? Our findings will contribute to our understanding of the fluxes, pathways and fate of plastic debris on the ocean surface that provides insight to better tackle the challenge of plastic pollution.

2 Materials and methods

2.1 Study location and mesoscale eddy

This study was conducted on the ocean surface in the KERG and in the adjacent waters including the main current of the Kuroshio (Fig. 1a, b). In June 2019, a cyclonic mesoscale eddy with a diameter of approximately 100 km was generated by detachment from the KE at 35˚N;148˚E, characterized by a depression in the SSH in the central region of approximately 0.8 m. The cyclonic eddy was advected clockwise in the anticyclonic KERG for several months, then merged into the KE in December 2019 (Supplemental figure S1). The daily SSH anomalies and geostrophic velocity anomalies were obtained from the Copernicus Marine and Environment Monitoring Service (CMEMS) at a horizontal resolution of 0.25° × 0.25° (Global Ocean Gridded L4 Sea Surface Height and Derived Variables Reprocessed (1993-ongoing).

Fig. 1
figure 1

Maps of the study area. (a) The averaged Kuroshio and adjacent currents around the Japanese archipelago from 1993 to 2018. Color shades denote current speed (m/s). (b) Sea surface height (SSH) map (color contours, m) on 1 September 2019 and locations of the sampling sites (black circles). The Kuroshio and Kuroshio Extension (KE) are shown by a sharp southward increase in SSH, and the recirculation gyre is illustrated by a high-SSH (> 1.6 m) region. Panels (a) and (b) were generated using E.U. Copernicus Marine Environment Monitoring Service (CMEMS) dataset at a horizontal resolution of 0.25° × 0.25° (Global Ocean Gridded L4 Sea Surface Height and Derived Variables Reprocessed (1993-ongoing))

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Floating microplastic was collected during a research cruise aboard the R/V Yokosuka (YK-1911) in August 29 -September 4, 2019 (Fig. 1b). We collected floating plastic particles within (St. 8.5) and northern edge (St. 9.5) of the mesoscale eddy. To enable a comparison, collections were also made at points southwest of the eddy with a relatively high SSH (St. 7.5), just southeast of the Kuroshio current (St. 4), and in the regional recirculation (St. 11) larger than KERG. The date, GPS location, and Beaufort number of each sampling site is reported in Table S1.

2.2 Sampling and analysis of microplastics

The details of the method for the sampling and analysis of plastic particles are given in the Supporting Information Materials. Briefly, floating plastic particles, including microplastics (< 5 mm) and mesoplastics (5–25 mm), were collected using a neuston net (mesh size, 333-µm, mouth opening size, 1.0 m width and 0.75 m height) equipped with a pre-calibrated flow-meter. The net was dragged for 20 min at a speed of 1–2 knots in the upper 0.4 m of the sea surface, at a distance of 4 m from the ship’s starboard side. Triplicate samples were collected at each site by repeating the net towing.

The net samples were purified using 30% H2O2 at room temperature for 7 days (Hurley et al. 2018). All potential plastic particles were visually inspected using a dissecting stereo microscope, manually sorted using forceps, and photographed; the length and width of each particle were measured using image software (Olympus, cellSens Dimension 2.1). Textile microfibers were excluded from the analysis due to the large mesh size of the net and to minimize external contamination.

The polymer type of each of the sorted plastic-like particles was identified using Fourier Transform Infrared (FT-IR) with a single reflection diamond Attenuated Total Reflection (ATR) (Nicolet iS5, Thermo Fisher Scientific). A hit quality > 60% was used as the threshold for polymer type (La Daana et al. 2017). The particles identified as plastic were pooled by each net tow sample and transferred to a pre-weighed glass vial, and then the weights of the plastic particles were measured. After the individual plastic particles in each sample had been counted, the density (pieces/km2) and mass (g/km2) were calculated from the towed distance measured by the flow-meter.

2.3 Particle-tracking simulation

In order to examine the behavior of floating microplastics by surface flows in the mesoscale eddy associated with time variation, we conducted in-time particle-tracking simulation experiments using Parcels (Probably A Really Computationally Efficient Lagrangian Simulator) (Lange and van Sebille 2017). Particles were advected either backward or forward in time by the surface ocean currents and Stokes drift. We used the daily surface ocean current velocity of Global Ocean Physics Reanalysis GLORYS12V1 (DOI:https://doi.org/10.48670/moi-00021), a product of Copernicus Marine Service. Details of the assimilation system are described in Lellouche et al. (2021).

For this simulation, we uniformly aligned 15 million model particles at the sea surface every 1/72° in the geographical region of 20–39°N / 120–155°E. We traced the model particles for 26 days until 3 September 2019 when the physical plastic sampling was made at the target eddy to monitor the behavior (forward) of the microplastics around the eddy. Numbers of particles included in 1/12° x 1/12° squares were normalized by dividing with the initial particle number in each square (36). Furthermore, the model particles retained in the target eddy were traced further forward until 12 November 2019 to obtain the subsequent retention rate of microplastics in the eddy. To investigate the retention rate of the number of particles in the targeted eddy, we counted the number of particles present in the cyclonic eddy. In this study, we defined the cyclonic eddy interior as a region of the modeled SSH lower than 0.82 m.

2.4 Statistical analysis

The difference in the concentration of plastic particles between the different sampling sites was determined using one-way analysis of variance (ANOVA), and the differences among the means were analyzed using Tukey-Kramer multiple comparison tests. The normality of the data and homogeneity of variance were examined and verified before the procedure of ANOVA using a Shapiro-Wilks test and a Bartlett test, respectively. A difference at P < 0.05 was considered significant.

Spatial similarities of the microplastic community structure at the study sites with different numbers of particles and polymer types were graphically depicted using non-metric multidimensional scaling (MDS), and group average clustering was carried out. MDS is a technique in which the distance between samples is expressed as a (non -) similarity index of a community, the mutual relationship is projected on a two dimensional plane, and sample clusters are identified and matched with environmental conditions by a rank-based non-parametric-type test. The similarity matrix obtained from the density values was calculated by the Bray-Curtis index (Bray and Curtis 1957) with square-root transformed data. To test for spatial variation in community density, analysis of similarities (ANOSIM) was then performed (Clarke and Warwick 1994). All multivariate analyses were conducted with the software PRIMER v. 7 (Plymouth Marine Laboratory).

3 Results and discussion

3.1 Density and mass of plastic particles in the eddy

In total 5863 plastic particles were examined for their polymer type and size. The highest densities of microplastics (458 ± 108 × 104 pieces/km2, mean ± SD) and mesoplastics (10 ± 3 × 104 pieces/km2) were found around the center of the cyclonic eddy (St. 8.5) (Fig. 2a, b. Table S2). These were one or two orders of magnitude higher than those found in the other sampling sites, with statistically significant differences (Tukey-Kramer, P < 0.01 in both microplastics and mesoplastics). The mass (g/km2) of plastic particles (sum of micro- and mesoplastics) showed a similar trend as for the density data, with on average 4098 ± 1093 g/km2 (mean ± SD) at the eddy center (St. 8.5), which was significantly higher than the densities of other sites (Tukey-Kramer, P < 0.01) (Fig. 2c). Following the eddy center, the density of microplastics in the Kuroshio (St. 4) showed a relatively higher density (63 ± 45 × 104 pieces/km2), while relatively higher densities of mesoplastics were found also at St. 4 (1.0 ± 0.33 × 104 pieces/km2) and St. 7.5 (0.99 ± 0.67 × 104 pieces/km2). The Kuroshio current is known for its higher density of microplastics compared to the adjacent waters (Yamashita and Tanimura 2007) (Shiu et al. 2021), because it is the main stream that brings plastic debris leaked from East Asia and parts of Southeast Asia, both major regions producing mismanaged plastic waste that ends up in the ocean (Jambeck et al. 2015). The density of microplastics found in the Kuroshio (St. 4) in the present study was in the same order of magnitude as the values previously reported in the Kuroshio off southwest Japan, namely a mean of 17.4 × 104 pieces/km2 (Yamashita and Tanimura 2007) and a mean of 15.6 × 104 pieces/km2 (Thushari et al. 2023). The lowest density and mass of micro- and mesoplastics were found at St. 11 in the southern region (2.9 ± 1.0 × 104 pieces/km2 in microplastics; 0.080 ± 0.13 × 104 pieces/km2 in mesoplastics; 84 ± 78 g/km2 in micro- and mesoplastics).

Fig. 2
figure 2

Boxplots of the density (pieces/km2) and mass (g/km2) of plastic particles from different study sites. (a) Microplastic density, (b) mesoplastic density, and (c) the total mass of micro- and mesoplastics. The line in the middle of each box represents the median value; the tops and bottoms of the boxes denote the 75 and 25% quartiles, respectively, and the top and bottom of the error bars show the maximum and minimum values. Crosses denote the mean data

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The abundance of microplastics in the eddy center was also one or two orders of magnitudes higher than the previously reported values in cyclonic (mean 2 × 104 pieces/km2, max 2.5 × 104 pieces/km2) and anticyclonic (mean 17 × 104 pieces/km2, max 36 × 104 pieces/km2) eddies in the interior region of the subtropical gyre of the North Atlantic (Brach et al. 2018). The maximum microplastic density found in the eddy center in this study (570 × 104 pieces/km2) was also higher than any of the maximum records in the Kuroshio region published in previous studies (352 × 104 pieces/km2, (Yamashita and Tanimura 2007); 202 × 104 pieces/km2, (Thushari et al. 2023), but rather comparable to the maximum number found in the GPGP (655 × 104 pieces/km2, (Goldstein et al. 2013).

3.2 Origin of the microplastics in the eddy

The MDS and ANOSIM analyses clearly indicated that the compositions of microplastic assemblages in the eddy center (St. 8.5) and two samples from the Kuroshio (St. 4) were similar, while they differed significantly from those of the other sampling sites (Global R = 0.83, P = 0.001) (Fig. 3, see also Fig. S2 for polymer composition). This supports the proposition that the microplastics found in the eddy center to a certain extent originated in the Kuroshio and the KE, known for its high microplastic concentration (Yamashita and Tanimura 2007), as the detached eddy acquired microplastics from the KE and kept in the interior of the strong eddy due to the nonlinear effect, as reported previously (Nagano et al. 2016). Indeed, the model also showed that most of the particles in the eddy derived from the Kuroshio and KE (Fig. S3), supporting the incorporation of particles from the KE into the eddy.

Fig. 3
figure 3

Non-metric multidimensional scaling (MDS) plots. MDS plots showing similarity of microplastic community in different sites. Bray-Curtis similarities were calculated based on the square-root of density. The legends next to each symbol indicate the sampling station (Sts. 4, 7.5, 8.5, 9.5, and 11) and number of net samples (#1–3)

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However, it should be noted that attributing the abundant microplastics to only the Kuroshio and the KE is insufficient to explain the large number of microplastics in the eddy center (mean 458 × 104 pieces/km2), which is an order of magnitude higher than in the Kuroshio (mean 63 × 104 pieces/km2). Although our particle tracking model was unable to fully reproduce the time evolution of the targeted eddy obtained from the satellite SSH measurements, the forward-particle tracking analysis based on the assimilation data showed that the eddy entrained particles from the surrounding regions south of the KE (Fig. 4, see also the movies available in the figure caption). This horizontal entrainment of particles by the eddy may also contribute to the high abundance of microplastics in the center of the cyclonic eddy in the KERG.

Fig. 4
figure 4

Forward in-time particle-tracking simulation. In total 15 million tracers were uniformly released every 1/72° in the geographical region of 20–39°N / 120–155°E and their surface transport was modeled for 26 days until 3 September 2019 when the physical microplastic sampling was conducted in the target eddy. Allows indicate the location of the target eddy. (a) 8 August, (b) 15 August, (c) 22 August, (d) 29 August, (e) 1 September, and (f) 3 September 2019. Each figure panel shows an enlarged cut-off area close to the target eddy from the original region of the particle-tracking simulation. Units are expressed as the normalized number of tracer particles in each grid of 1/12° horizontal resolution. The particle number 10 indicates that the number of particles in a grid is 10 or more. Contours below 1.0 m are indicated with dashed lines. The animation is available here: https://doi.org/10.5281/zenodo.11383821 or https://doi.org/10.5281/zenodo.11533267

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On the basis of the analysis of the observed and simulated SSH, while the cyclonic eddy captured particles inward from the surrounding waters, the eddy intensity was found to be reduced (Fig. S1). As a result of the weakening of the cyclonic eddy due to the downward displacement of the main thermocline, there should be a convergence toward the eddy. A similar observation was previously reported in an anticyclonic mesoscale eddy off Gran Canaria island, where a convergence occurs during the strengthening of the intensity due to increased SSH and the concurrent downward displacement of the main thermocline (Sangra et al. 2005). Thus, together with the eddy’s entrapment of microplastics from the microplastic-rich KE water, the convergence due to the eddy attenuation can explain the very high density of microplastics observed around the center of the cyclonic mesoscale eddy in the KERG.

3.3 Fate of the entrained plastic particles

How eddies redistribute microplastics seems to depend on the eddy propagation route and the evolution of the intensity. However, we did not conduct a chasing survey of microplastic abundance in the eddy following the movement, and it would be difficult to examine how long the captured particles could be retained within the eddy using physical sampling. Instead, we examined the time variation in the microplastic abundance in the eddy by conducting a forward particle-tracking simulation. Our simulation showed that most particles captured by the eddy on 3 September 2019 remained in the center for several months while the eddy moved clockwise in the KERG (Fig. 5). The eddy merged into the KE and eventually ceased to be identifiable as an eddy after 12 November 2019. During the period, the relative percentage of particles retained inside the eddy was 94.5% on September 12, 71.7% on September 25, 63.7% on October 12, 61.3% on October 25, and 28.4% on November 12, taking the percentage on September 3 as 100%. Our results strongly suggest that mesoscale eddies are of importance in transporting and redistributing microplastics on the ocean surface of western boundary current regions such as the Kuroshio and KE regions.

Fig. 5
figure 5

Forward in-time particle-tracking simulation. Tracers were released at the targeted eddy, and their surface transport was modeled for the forward 72 days. (a) 4 September, (b) 12 September, (c) 25 September, (d) 12 October, (e) 25 October and (f) 12 November 2019. Units are expressed as the number of tracer particles in each grid of 1/12° horizontal resolution. The particle number 50 indicates that the number of particles in a grid is 50 or more. Arrows indicate the concentrated particles within the targeted eddy. The animation is available here: https://doi.org/10.5281/zenodo.11392835

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Mesoscale eddies appear to capture and accumulate floating debris and could contribute to the downward transportation of microplastics, as has been found in the North Pacific Subtropical Gyre (Egger et al. 2020; Zhao et al. 2023). Note that the floating microplastics are thought to be subducted into the subsurface water after a certain period of time for several reasons, such as entrainment in marine snow, incorporation into sinking fecal pellets, and biofouling leading to a loss of buoyancy (Isobe et al. 2019; van Sebille et al. 2020). As the lifetime of standing eddies increases, the microplastics may have a higher chance of being brought down into deeper water. This highlights the need for further studies from a three-dimensional perspective to investigate the transportation of surface microplastics into the deep sea in regions in which strong mesoscale eddies frequently occur. The KE is one of the most dynamic regions in the North Pacific with rich strong mesoscale eddies (Qiu and Chen 2010a; Ma et al. 2016). If there is an intense downward transportation process of plastic debris in the KE region, this presents a plausible explanation for the massive occurrence of large macroplastics (Nakajima et al. 2021) and microplastics (Tsuchiya et al. 2023) previously reported on the abyssal seafloor of the KERG.

3.4 Limitations of the study

It should be noted that the number of sampling sites in this study was insufficient to fully capture the variation of microplastic within a mesoscale eddy and this will require future studies with a more comprehensive design of sampling sites in different sections (Zhou et al. 2023). Yet this study examined the special representative of microplastic in a mesoscale eddy using data assimilation to complement the limited shipboard observations. It is also important to note that while only one mesoscale eddy was analyzed we believe that the knowledge on a single cyclonic mesoscale eddy obtained in this study is applicable to other intense cyclonic eddies; eddies capture and accumulate microplastics on the sea surface within their interior, as evidenced by the highest microplastic density observed in the eddy as described above. It should also be pointed out that our density counts are necessarily an underestimating of the true density of entire microplastics in the area, as small microplastics were not collected due to the large mesh size of the net (330 μm).

4 Conclusions

Whereas the earlier study by Brach et al. (2018) documented microplastic abundances in relatively weak anticyclonic and cyclonic eddies with small SSH displacements (approximately 0.08 m) in the North Atlantic subtropical gyre interior region, we highlight the degree of microplastics found in an intense cyclonic mesoscale eddy with a large SSH depression (approximately 0.8 m) in the Kuroshio Extension region. The present observations provide evidence that an intense cyclonic mesoscale eddy is one of the physical processes capturing large amounts of floating microplastics and retaining them for several months in the KERG. These data support the view that mesoscale eddies are significant aggregators and transporters of microplastics at the ocean surface. Investigating the enrichment of microplastics around the center of intense mesoscale eddies provides a unique opportunity to assess the microplastic risk to biota at realistically high concentrations, which are supposedly becoming common in the future ocean (Isobe et al. 2019).