Microplastics in sea ice and seawater beneath ice floes from the Arctic Ocean — TEI-tagged view

La Daana K. KanhaipersName, Katarina GardfeldtpersName, Thomas KrumpenpersName, Richard C. ThompsonpersName, Ian O’ConnorpersName (2020date). “Microplastics in sea ice and seawater beneath ice floes from the Arctic Ocean.” Scientific Reportstitle·j.

Object of study — microplastic abundanceterm·var, distribution and composition in Arctic sea iceterm·key cores and under-ice water

Research question — what microplasticsterm·dom Arctic sea iceterm·key and the water beneath it hold, as a basis for assessing threats

Contribution — sea iceterm·key holds microplasticsterm·dom at orders of magnitude above the underlying water

Full text — tagged

Microplasticsterm·dom in sea iceterm·key and seawater beneath ice floesterm·key from the Arctic Oceanterm·key Abstract Within the past decade, an alarm was raised about microplasticsterm·dom in the remote and seemingly pristine Arctic Oceanterm·key. To gain further insight about the issue, microplastic abundanceterm·var, distribution and composition in sea iceterm·key cores (n = 25) and waters underlying ice floesterm·key (n = 22) were assessed in the Arctic Central Basinterm·dom (ACBterm·dom). Potential microplasticsterm·dom were visually isolated and subsequently analysed using Fourier Transform Infraredterm·mth (FT-IRterm·mth) Spectroscopy. Microplastic abundanceterm·var in surface waters underlying ice floesterm·key (0–18 particles m −3 ) were orders of magnitude lower than microplasticterm·dom concentrations in sea iceterm·key cores (2–17 particles L −1 ). No consistent pattern was apparent in the vertical distribution of microplasticsterm·dom within sea iceterm·key cores. Backward drift trajectoriesterm·key estimated that cores possibly originated from the Siberian shelves, western Arctic and central Arctic. Knowledge about microplasticsterm·dom in environmental compartments of the Arctic Oceanterm·key is important in assessing the potential threats posed by microplasticsterm·dom to polar organisms.

Subject terms: Marine biology, Physical oceanography Introduction Within the past decade, microplasticterm·dom pollution emerged as an issue of concern in the Arctic Oceanterm·key due to the discovery of these contaminants in its sea iceterm·key 1 , 2 , surface and sub-surface waters 3 – 8 , deep sea sediments 9 – 11 , biota 4 , 6 , 12 , 13 and mostly recently its snow 14 . Of the environmental compartments in this remote oceanic basin, it was shown that sea iceterm·key can function as a temporal sink, a secondary source and a transport medium for microplasticsterm·dom 1 , 2 . Historically, however, observational records of ‘dirty ice’, examination of Arctic ice coresterm·key, laboratory-based experiments and modelling studies were the first to highlight the potential for sea iceterm·key in the Arctic Oceanterm·key to trap, transport and redistribute sediments and various contaminants (i.e. metals, organochlorines, organophosphates, polycyclic aromatic hydrocarbons) 15 – 27 . Microplasticsterm·dom (plastic particles <5 mm in diameter) were first discovered in sub-sections of 4 ice coresterm·key retrieved from various locations in the Arctic Oceanterm·key 1 . In this initial study, a total of 6 types of synthetic polymers were reported in the ice coresterm·key and microplasticterm·dom concentrations in Arctic sea iceterm·key were estimated (based on extrapolations) to be between (1.3–9.6) × 10 4 particles m −3 . A second study 2 , subsequently examined 5 sea iceterm·key cores from the Arctic Oceanterm·key, reported on the presence of smaller (<100 µm in diameter), more diverse types of synthetic polymers (n = 17) and estimated (based on extrapolations) microplasticterm·dom concentrations in Arctic sea iceterm·key to range between (1.1 × 10 6 )–(1.2 × 10 7 ) particles m −3 . Both studies 1 , 2 , while limited in scale and extent, collectively suggested that sea iceterm·key can function as a sink, source and transport medium for microplasticsterm·dom in the Arctic Oceanterm·key.

Within this remote oceanic basin, microplasticterm·dom entrapment within sea iceterm·key potentially occurs during sea iceterm·key formation and drift while microplasticterm·dom release occurs upon the melting of sea iceterm·key 1 , 2 , 14 . In the Arctic Oceanterm·key, while sea iceterm·key formation occurs over the central Arctic Oceanterm·key, the most important regions for this are the shallow Siberian shelves in the Eurasian Basin and the Beaufort Sea in the Amerasian Basin 15 . Microplasticsterm·dom entrapped within sea iceterm·key during its formation can potentially be reflective of microplasticsterm·dom present in seawater from known ice formation regions in the Arctic 1 , 2 . While the majority of sea iceterm·key melts in close proximity to its formation zone, some sea iceterm·key (i.e. that which forms over the Siberian shelves) is advected off the coast, joins the Transpolar Driftterm·key with some eventually exiting through the Fram Strait 15 , 16 . Sea iceterm·key from the Beaufort, Chukchi and East Siberian seas may become incorporated into the Beaufort gyre with some sea iceterm·key having the potential to exit this gyre and eventually join the Transpolar Driftterm·key 17 . Within the Arctic Oceanterm·key, sea ice dynamicsterm·dom are one of the key factors that potentially influences contaminant fateterm·dom. Mobile sea iceterm·key floes in the Arctic Oceanterm·key are capable of entrapping contaminants along their drift pathways and playing a role in the redistribution of contaminants due to their eventual release upon melting of the ice 2 , 16 , 19 . Estimating backward sea iceterm·key trajectories is an important tool that facilitates an examination of sea iceterm·key sources, drift pathways, thickness changes and atmospheric processes acting on the ice cover 2 , 28 – 30 . When utilized in microplasticterm·dom studies, backward sea iceterm·key trajectory data (i.e. sea iceterm·key sources and drift pathways) can be used to make inferences about potential regions where microplasticterm·dom entrapment in sea iceterm·key occurred. The AWI ICETrack application, a lagrangian approach that traces sea iceterm·key backward in time using a combination of satellite-derived low-resolution drift products, is one available modelling approach that was utilised in the most recent study on microplasticsterm·dom in Arctic sea iceterm·key 2 . In the Arctic Oceanterm·key, sea iceterm·key has been identified as a temporal sink and secondary source of microplasticsterm·dom since contaminant release is projected to occur upon the melting of sea iceterm·key 1 , 2 . In the context of a changing climate, projections for Arctic sea iceterm·key include decreasing sea iceterm·key extent, reductions in sea iceterm·key thickness (less multi-year ice), alterations in the rate of sea iceterm·key drift, intensified melting of sea iceterm·key in the marginal zones and interruption of its Transpolar Driftterm·key 28 , 31 – 33 . These changing conditions will inevitably influence the dynamics of contaminant fateterm·dom and transport in the Arctic Oceanterm·key, especially if the contaminants of interest are capable of being entrapped within, transported and subsequently released by sea iceterm·key.

Given that the 18 Arctic Large Marine Ecosystemsterm·dom (LMEsterm·dom) support a diverse array of marine life 34 and sea iceterm·key provides a range of microhabitats for numerous species 35 – 37 , an understanding of microplasticterm·dom contamination in sea iceterm·key and surface waters of the Arctic Oceanterm·key is vitally important. The specific objectives of this study were to (i) provide a more spatially comprehensive assessment of microplasticterm·dom concentration and composition in sea iceterm·key cores from the Arctic Oceanterm·key, (ii) assess the vertical distribution of microplasticsterm·dom in entire sea iceterm·key cores, (iii) estimate backward drift trajectoriesterm·key and identify source areas of sampled sea iceterm·key cores and (iv) assess microplastic abundanceterm·var, distribution and composition in surface waters (beneath ice floesterm·key) in the Arctic Oceanterm·key.

Results Quality control Of the shipboard air contamination checks (n = 13) that were carried out, a single fibre (blue polyesterterm·key fibre, 0.47 mm) was found. This indicates a low likelihood that microplasticsterm·dom were introduced to the samples as a result of airborne contamination. Of the method blanks (n = 15), 7 were free of contamination while the remainder contained either a single fibre (n = 6) or 2 fibres (n = 2). Fibres in the method blanks were polyesterterm·key (n = 6), polyamideterm·key (n = 1) or a polyamideterm·key blend (n = 3). In order to account for any contamination that might have been introduced during ice coreterm·key processing, a blank correction was applied whereby a single fibre was removed from each sub-section total. Additionally, if there were any matches between synthetic polymers found in the samples and those that came into contact with the samples (either during sampling or laboratory processing), these particles were excluded from the results.

Microplasticterm·dom contamination in sea iceterm·key cores A total of 2031 particles were isolated from the 25 sea iceterm·key cores and analysed using FT-IRterm·mth spectroscopy. Of these, 501 particles were rejected due to (i) poor spectral matches, (ii) matches with polymers used during sample collection or processing and, (iii) identification as being natural or semi-synthetic polymers. A further 117 synthetic polymers were excluded from further analyses since they were categorized as being macroplastics (>5 mm). Of the 1413 confirmed synthetic polymers, 223 were removed during the blank correction processes. Subsequent analyses are therefore based on 1190 synthetic polymers <5 mm from the sea iceterm·key cores.

Microplasticterm·dom concentration in sea iceterm·key cores (n = 25) from the Arctic Central Basinterm·dom ranged between 2–17 particles L −1 with particle size ranging between 0.10 mm–4.99 mm (Fig. 1 , Supplementary Table 1 ). Of the sampled cores, the majority had microplasticterm·dom concentrations <8 particles L −1 (Fig. 1 ). The two cores with the highest microplasticterm·dom concentrations were cores 1 (17 particles L −1 ) and 3 (15 particles L −1 ), (Fig. 1 ). In terms of polymer composition, a total of 9 types of synthetic polymers were found in the ice coresterm·key with an overall predominance of polyestersterm·key (57%) and polyamidesterm·key (19%) followed by polyurethane (6%), styrene/acrylates (6%), polyacrylonitrile (6%), polyvinyl chloride (5%) and other polymers (1.3%) which included polypropylene and polyethylene (Supplementary Fig. 1 ). The majority of the microplasticsterm·dom that were found in the ice coresterm·key were fibres (79%) with the remainder (21%) being fragments. Size class distributions of microplasticsterm·dom in the ice coresterm·key were as follows: 0.1–0.5 mm (32%), 0.5–1 mm (20%), 1–2 mm (23%), 2–3 mm (13%), 3–4 mm (7%), 4–5 mm (4%), (Supplementary Fig. 1 ). In terms of colour, the majority of microplasticsterm·dom recorded in the sea iceterm·key cores were blue (53%) followed by red (10%), pink (9%), yellow (7%), black (5%), green (3.5%), transparent (3.5%), white (3%), grey (3%) and orange, purple and brown (3%).

Microplasticterm·dom concentration in sea iceterm·key cores from the Arctic Central Basinterm·dom by sampling location ( a ) and potential origin ( b ). ( a ) generated using Ocean Data View 50 .

Vertical distribution of microplasticsterm·dom in sea iceterm·key cores Overall, there appears to be no consistent pattern in the vertical distribution of microplasticsterm·dom within sea iceterm·key cores (Fig. 2 ), as well as no overall correlation between sub-section depth of the ice coreterm·key and microplasticterm·dom concentration (Spearman’s Rank Correlation, p-values > 0.05). However, when individual ice coresterm·key were considered, core 8 was the only ice coreterm·key for which there was a significant negative correlation between microplasticterm·dom concentration and sub-section depth (Spearman’s Rank Correlation, rho = −0.74, p-value = 0.001). For certain ice coresterm·key (3, 6, 7, 8, 23) it was also apparent that microplasticterm·dom concentration was comparatively higher in certain upper sub-sections of the core (Fig. 2 ). Microplasticsterm·dom were also shown to be pervasive throughout the majority of the ice coresterm·key. In 4% (n = 9, total sub-sections = 232) of all ice coreterm·key sub-sections, no microplasticsterm·dom were found (relevant to cores 8, 10, 14, 15, 17, 19, 23).

Vertical distribution of microplasticsterm·dom in sea iceterm·key cores from the Arctic Central Basinterm·dom [In all cases (except cores 2, 8, 17, 20, 23), last bar indicative of sub-section of ice coreterm·key in contact with the underlying seawater].

Potential origin of sea iceterm·key Backward trajectories of the ice coresterm·key indicated that they possibly originated from the (i) Siberian shelves in the Eurasian basin (i.e. Laptev Sea, East Siberian Sea), (ii) western Arctic (i.e. Beaufort Sea, Chukchi Sea) and, (iii) Central Arctic Oceanterm·key (Fig. 3 , Supplementary Table 1 ). Of all the cores (n = 25), only 12 had a >75% match between measured sea iceterm·key thickness and model-predicted sea iceterm·key thickness (Fig. 3 , Supplementary Table 1 ). Based on the backward trajectories of the sea iceterm·key cores, those cores which had the highest estimated microplasticterm·dom concentrations i.e. >8 particles L −1 possibly originated in the Laptev, East Siberian and Chukchi Seas (Figs. 1 , 3 ). Sea iceterm·key age estimations indicated that the majority of the sea iceterm·key cores were at least second year (SYI, n = 16, survived at least 2 melt seasons) and first year ice (FYI, n = 7, survived at least 1 melt season) with only two cores being classified as multi-year ice or greater than 3 years old (MYI, n = 2, survived at least 3 melt seasons), (Supplementary Table 1 ). Although univariate analyses indicated that there was a significant difference in microplasticterm·dom concentrations between the different cores (Kruskal-Wallis, df = 24, p-value = 5.93 × 10 −8 ), multivariate analyses (Principal Components Analysis), was unable to provide any useful discrimination between sea iceterm·key cores.

Backward trajectories derived using the AWI Ice Track application indicate formation zones for sampled sea iceterm·key in the (i) Laptev Sea (cores 1, 23, 25), (ii) East Siberian Sea (cores 5, 10, 17), (iii) Chukchi Sea (cores 7, 8, 11), (iv) Central Arctic Oceanterm·key (cores 9, 21, 13). [Trajectories shown here were for cores with a >75% match between model-predicted and field-recorded sea iceterm·key thickness, colours rep. months].

Microplasticsterm·dom in seawater beneath ice floesterm·key A total of 189 particles were isolated from surface water beneath ice floesterm·key samples in the ACBterm·dom. Of these, 47 were rejected for the above-mentioned reasons (i.e. those stated for the ice coreterm·key samples). A further 17 synthetic polymers were excluded since they were categorized as macroplastics (>5 mm). Subsequent analyses were based on 125 synthetic polymers <5 mm from the surface water samples. Microplastic abundanceterm·var in seawater beneath the ice floesterm·key (0–18 particles m −3 , Fig. 4 ) were orders of magnitude lower than those reported for sea iceterm·key, (2 × 10 3 ) to (1.7 × 10 4 ) particles m −3 (extrapolated from particles L −1 ). Of the 22 sites where surface waters were sampled, only one site did not have microplasticsterm·dom detected. In terms of polymer composition, the majority of polymers detected in surface waters of the ACBterm·dom were polyestersterm·key (70%) and polyamidesterm·key (23%) with a minority of polyvinyl chloride (7%). Microplasticterm·dom size class distributions were as follows: 0.25–0.5 mm (16%), 0.5–1 mm (18%), 1–2 mm (34%), 2–3 mm (14%), 3–4 mm (11%), 4–5 mm (6%), with the majority of microplasticsterm·dom (68%) being <2 mm. Overall, fibrous microplasticsterm·dom predominated (89%). In terms of colour, the majority of microplasticsterm·dom were blue (58%) and red (18%), with other colours such as transparent (5%), pink (4%), yellow (4%), grey (4%), purple (3%), green (2%), black (2%) and brown (1%) also being recorded.

Microplastic abundanceterm·var in surface waters beneath ice floesterm·key in the Arctic Central Basinterm·dom. ( a ) generated using Ocean Data View 50 .

Discussion Microplasticsterm·dom, as evidenced by the present study, were ubiquitous in both sea iceterm·key cores and seawater underlying ice floesterm·key from the Arctic Central Basinterm·dom (ACBterm·dom). Microplasticterm·dom concentrations in sea iceterm·key (estimations based on extrapolations from 25 sea iceterm·key cores, (2 × 10 3 ) to (1.7 × 10 4 ) particles m −3 ) were orders of magnitude higher than those reported for seawater beneath ice floesterm·key (0–18 particles m −3 ). While these findings corroborate those of previous studies 1 , 2 that identified sea iceterm·key as a temporary sink for microplasticsterm·dom in the Arctic Oceanterm·key, it is collectively apparent that to date microplasticterm·dom concentrations in Arctic sea iceterm·key have been underestimated. Analytical techniquesterm·mth (visual identification followed by FT-IRterm·mth spectroscopy of potential microplasticsterm·dom) employed in the present study led to the exclusion of particles <100 µm from analysis and as such would have resulted in an underestimation of microplastic concentrationterm·mth in sea iceterm·key cores (number of cores = 25; range 2–17 particles L −1 ; extrapolated range (2 × 10 3 )–(1.7 × 10 4 ) particles m −3 ). This is particularly relevant in light of the fact that the majority of microplasticsterm·dom in Arctic sea iceterm·key reported by a previous study 2 using Imaging FTIRterm·mth were <50 µm (number of cores = 5; extrapolated range (1.1 × 10 6 ) – (1.2 ×10 7 ) particles m −3 ). Although the previous study 2 reported the highest microplasticterm·dom concentrations thus far in sea iceterm·key, fibres were excluded from the analysis. By contrast, the present study reported a pre-dominance of fibrous microplasticsterm·dom (79%) in sea iceterm·key from the Arctic Oceanterm·key. It is therefore apparent that if either fibres or particles <100 µm are excluded from the findings, microplasticterm·dom concentrations in sea iceterm·key will be underestimated. Furthermore, both the present study and a previous study 2 showed that there was no consistent pattern in the vertical distribution of microplasticsterm·dom within sea iceterm·key cores. If isolated sub-sections of ice coresterm·key are used to estimate microplasticterm·dom concentrations in Arctic sea iceterm·key as was done in a previous study (number of cores = 4, meltwater volumes 50–100 mL, extrapolated range 1.3–9.6 × 10 4 particles m −3 ) 1 , those extrapolations will either be underestimations or overestimations of microplasticterm·dom concentrations. In the present study, meltwater volumes in ice coresterm·key ranged between 3–12 L. Extrapolations from particles L −1 to particles m −3 are relevant for comparison to microplasticterm·dom concentrations in surface and sub-surface waters. However, when microplasticterm·dom concentrations from <20 L of meltwater from a single ice coreterm·key is used to estimate microplasticterm·dom concentrations in 1,000 L or 1 m −3 of meltwater, such estimations are based on numerous assumptions and limited datasets (few ice coresterm·key, low meltwater volumes). It is therefore suggested that microplasticterm·dom concentrations in sea iceterm·key should be reported as particles L −1 .

The identification of microplasticterm·dom type (fibre, fragment, etc) and polymer composition is a critical component of microplasticterm·dom studies as it provides investigators with some insight about the potential sources of microplasticsterm·dom in the environment. In the present study, 9 different types of synthetic polymers were reported in the sea iceterm·key cores while 3 were reported in surface waters underlying the ice floesterm·key. In both environmental compartments, the majority of microplasticsterm·dom were comprised of fibrous polyestersterm·key (57%- sea iceterm·key cores, 70%- surface waters) and polyamidesterm·key (19%- sea iceterm·key cores, 23%- surface waters). Both polyestersterm·key and polyamidesterm·key have a higher density than seawater 38 , thus raising the question as to why these particles were prevalent in both the sea iceterm·key cores and surface waters of the Arctic Central Basinterm·dom. These findings were comparable to the first published study on microplasticsterm·dom in Arctic sea iceterm·key 1 whereby it was reported that the most common synthetic polymers were polyestersterm·key (21%) and polyamidesterm·key (16%). By contrast, although another study 2 on microplasticsterm·dom in Arctic sea iceterm·key did report on the presence of polyesterterm·key and polyamideterm·key, the most dominant synthetic polymer in the examined cores was polyethylene (48%) with the difference in findings possibly occurring due to the exclusion of fibres. In the western Arctic at the Bering and Chukchi Seas 8 , polyestersterm·key predominated in surface waters. In the present study, the majority of microplasticsterm·dom in sea iceterm·key cores (79%) and surface waters (89%) were fibrous. Similarly, in the Bering and Chukchi Seas 8 , fibrous microplasticsterm·dom were predominant in surface waters. In the marine environment, fibres may be originating from fishing gear, textiles due to laundering fabric and cigarette filters 38 . Laboratory experiments have indicated that the input of textile fibres into the marine environment can occur following the discharge of wastewater from domestic washings 38 – 43 . Recently, it was reported 14 that both microplasticsterm·dom and microfibers were present in snow from ice floesterm·key in the Fram Strait which suggests that these particles could have been transported into the region by winds and been deposited onto ice floesterm·key and surface waters via snow. For the remote Arctic Oceanterm·key, it is difficult to pinpoint the exact origin of fibrous microplasticsterm·dom. Although definitive statements cannot be made about the origin of microplasticsterm·dom in surface waters or sea iceterm·key of the ACBterm·dom, potential sources of these contaminants may include (i) riverine discharge from the Siberian and Canadian rivers 7 , (ii) influx of contaminated Pacific and Atlantic waters 1 , (iii) grey water discharge from vessels operating in the Arctic 7 and, (iv) atmospheric deposition 14 .

Similar to the situation for sediments and other contaminants 15 – 27 , microplasticterm·dom entrapment in Arctic sea iceterm·key potentially occurs during its initial formation over both the shallow marginal shelves of the Arctic Oceanterm·key and the deep central basin. In the present study, backward drift trajectoriesterm·key estimated that the sea iceterm·key floes from which the ice coresterm·key were sampled originated from the Siberian shelves in the Eurasian Basin, western Arctic and central Arctic. If microplasticsterm·dom were entrapped during sea iceterm·key formation in these areas and ice floesterm·key survived subsequent melt seasons, microplasticterm·dom composition in sampled sea iceterm·key cores could be reflective of the microplasticterm·dom composition in surface waters from these areas. At the marginal shelves of the Arctic, freshwater discharge from Siberian rivers; Dvina and Pechora (Barents Sea), Ob and Yenisei (Kara Sea), Lena (Laptev Sea) and Kolyma rivers (East Siberian Sea), and the Canadian Mackenzie river (Beaufort Sea) 34 , 44 , could be potential contributors to the microplasticterm·dom load in Arctic sea iceterm·key. Since anthropogenic activities occur within the catchment areas of these rivers, the influx of freshwater into the Arctic basin is a potential pathway via which contaminants could enter this marine ecosystem 45 , 46 . Presently however, the contribution of riverine discharge to the microplasticterm·dom load in Arctic sea iceterm·key remains uncertain due to the absence of field data. Pacific water influx into the Arctic Oceanterm·key through the Bering Strait also represents a potential source of contamination 2 in that Pacific waters are influenced by anthropogenic activities that occur in northern America and eastern Russia. The Chukchi, Beaufort and East Siberian Seas which border the central Arctic are influenced by Pacific waters 8 and thus any sea iceterm·key formed in these seas could potentially reflect microplasticsterm·dom that are present in the surface waters in these areas. The backtracking results that were presented in the present study must be interpreted with caution since there was a significant mismatch (>75%) between field and model-predicted ice thickness for 10 of the 25 ice coresterm·key (Supplementary Table 1 ). Although the AWI IceTrack application which generated these backward trajectories was validated using reconstructed pathways of real buoys, matches between ice thickness in the field and model-predicted ice thickness generally provide an indication of the reliability of the findings. The ice thickness mismatches could have been influenced by the fact that the tracking algorithm reconstructs the movement and evolution of sea iceterm·key that is mainly found in the Arctic Oceanterm·key and does not resolve dynamics and formation of new ice in leads (which is possibly where these specific ice coresterm·key were retrieved).

Microplasticterm·dom incorporation into sea iceterm·key floes can also potentially occur during its drift across the central Arctic. Backward drift trajectoriesterm·key produced for sea iceterm·key sampled in the present study indicated that 24 of the 25 sea iceterm·key cores survived at least one summer melt and endured at least one winter period and that there was either drift across or initial sea iceterm·key formation within the ACBterm·dom. These findings suggest that microplasticsterm·dom found in the sampled sea iceterm·key cores were potentially reflective of microplasticsterm·dom in surface waters of the ACBterm·dom. Once present in surface waters of the central Arctic, periods of freezing will facilitate microplasticterm·dom entrapment in newly formed ice or on the underside of existing ice floesterm·key. Microplasticterm·dom presence in waters beneath ice floesterm·key (0–18 particles m −3 ) and sub-surface waters of the central Arctic 7 indicates the availability of these particles for incorporation into sea iceterm·key, whether that is first year ice forming over the central basin or vertical ice growth on the underside of the floes. In the western Arctic 8 , microplastic abundanceterm·var in surface waters of the Bering Sea ranged between 0.035–0.26 particles m −3 and in the Chukchi Sea between 0.086–0.31 particles m −3 . Outside the central Arctic, microplastic abundanceterm·var in surface Arctic waters south and southwest of Svalbard 3 ranged between 0–1.3 particles m −3 and east of Greenland 4 it ranged between 0.2–2.6 particles m −3 in 2005 and 0.8–4.5 particles m −3 in 2014. While conclusive statements cannot be made about differences in the reported microplastic abundancesterm·var, it is plausible that sea iceterm·key functions as a secondary source of microplasticsterm·dom in the central Arctic and contributes to higher microplastic abundancesterm·var. During transport, atmospheric deposition of microplasticsterm·dom unto the surface of an ice floeterm·key may also occur. A recent study 14 reported that microplasticsterm·dom and microfibers were present in snow retrieved from ice floesterm·key in the Fram Strait. These findings suggested that these particles were potentially transported into the Arctic by winds and deposited unto the ice floesterm·key via snow 14 . Atmospheric fallout of microplasticsterm·dom suggests that these contaminants can be transported through the atmosphere and reach remote areas 14 , 47 , 48 . It is possible that microplasticsterm·dom can be transported into the Arctic region by winds and that these particles can be deposited either unto ice floesterm·key during transport or directly unto surface waters 14 .

Arctic sea iceterm·key is not the ultimate but an intermediate sink for microplasticsterm·dom. From a temporal perspective, seasonal environmental conditions (spring/summer melt seasons) influence sea iceterm·key melting and subsequent microplasticterm·dom release in the Arctic Oceanterm·key. During summer, snow and ice melt off the upper portion of the ice floeterm·key with meltwater either running off the ice floeterm·key, percolating into the surface of the floe, accumulating in melt ponds or refreezing on the underside 16 . Summer melt periods can therefore facilitate the redistribution of microplasticsterm·dom within an ice floeterm·key in that particles that were present in the lower layers of an ice floeterm·key may eventually make their way to the top layer of an ice floeterm·key. In the present study, there was no consistent pattern in the vertical distribution of microplasticterm·dom within the sea iceterm·key cores. Some of the sea iceterm·key cores, however, reflected comparatively higher concentrations of microplasticsterm·dom in the upper sub-sections and it is postulated that re-distribution processes may have resulted in this pattern. Of note is the fact that sea iceterm·key melting in the Arctic Oceanterm·key usually coincides with bursts of biological activity, i.e. ice algal blooms in spring and phytoplankton blooms in summer 35 . Sea iceterm·key in the Arctic Oceanterm·key functions as a key habitat for numerous species of marine organisms 32 – 37 . Presently, it remains uncertain whether microplasticsterm·dom are incorporated within the ice crystal structure or brine channels of sea iceterm·key and whether microplasticsterm·dom may pose a threat to sea iceterm·key meiofauna or in-ice fauna that inhabit sea iceterm·key. Marine organisms that live in close association with sea iceterm·key, specifically under-ice or sea iceterm·key macrofauna and sub-ice fauna, are likely to interact with microplasticsterm·dom once these particles are released into the underlying seawater from melting ice. In the Arctic Oceanterm·key, dominant under-ice fauna are the gammarid amphipods while sub-ice fauna include various species of copepods and fish such as the polar cod ( Boreogadus saida ) and Arctic cod ( Arctogadus glacialis ) 37 . Recently, microplasticterm·dom fragments were reported in the stomachs of polar cod sampled from waters in the Eurasian Basin of the Arctic Oceanterm·key and north of Svalbard 13 and in the digestive tracts of polar cod sampled in Arctic waters east of Greenland 6 . Based on the fact that interactions can potentially occur between marine organisms and microplasticsterm·dom in the Arctic Oceanterm·key, laboratory experiments are needed to elucidate the impact of those interactions with polar organisms. From a spatial perspective, regions which could be most at risk from an influx of contaminants (e.g. microplasticsterm·dom) from melting sea iceterm·key include nearshore areas over the marginal shelves, the central Arctic basin and the marginal ice zone e.g. Fram Strait, Barents Sea 16 , 19 , 21 , 31 .

The Arctic Oceanterm·key is a dynamic ecosystem where projections for Arctic sea iceterm·key in the context of a changing climate can inevitably influence contaminant fateterm·dom and transport, especially if the contaminants of interest are capable of being entrapped within, transported and subsequently released by sea iceterm·key. Corroborating previous studies 1 , 2 , the present study evidenced that sea iceterm·key is capable of functioning as an intermediate sink, secondary source and a transport medium for microplasticsterm·dom in the Arctic Oceanterm·key. For the first time, microplasticterm·dom presence in seawater underlying ice floesterm·key was reported for the central Arctic. Understanding the presence, sources, transport pathways and fate of microplasticsterm·dom in the Arctic Oceanterm·key is critical to determining the potential threats posed by such contaminants to marine organisms that inhabit or depend upon different environmental compartments in this ecosystem.

Methods Sample collection This study was conducted onboard Swedish icebreaker Oden during the Arctic Oceanterm·key 2016 expedition (August 8 th to September 19 th 2016). At 25 ice stations, sea iceterm·key cores (n = 25) were retrieved and seawater (n = 22) was pumped from beneath the ice and filtered onsite for microplasticsterm·dom. At each ice station, a suitable location upwind of all other site activities was selected and overlying snow was removed from an area of approximately 0.5 m 2 . Ice coresterm·key were collected using a Nordic ice drill with an attached Husqvarna X-series 326A125 motor and a stainless-steel core barrel of 12.5 cm diameter. At all sites, the goal was to completely penetrate the ice by drilling and reach the underlying seawater. A single ice coreterm·key (n = 1) was retrieved at each site, placed into clean bags (polyethylene) and transported to the laboratory onboard the vessel for further processing. Once the sea iceterm·key core was retrieved at a site, water was then pumped from under the ice floeterm·key. A pre-cleaned polyvinyl chloride (PVC) hose (approximately 2 m in length) was inserted into the drilled hole. This hose was then connected to the inlet of a manual JABSCO Amazon Warrior pump (Model Number 29280-0000). Another hose of approximately 1 m in length was then connected to the outlet of the pump. Prior to any further connections, seawater was pumped from beneath the ice to flush the system of any contaminants. At this point, the flow rate of water through the system was manually checked by pumping seawater into a 1 L measuring cylinder. This was performed in triplicate in order to estimate the length of time needed for pumping the relevant volume of water. Following this, the hose from the outlet of the pump was positioned into the cover of a wooden stand containing a stainless-steel sieve (250 μm). This wooden stand with the sieve was positioned over a bucket into which filtered water flowed. The water that entered the bucket exited the site via a hose that was approximately 3 m in length. At 22 (of the 25) ice stations, water was manually pumped for at least 40 minutes thus ensuring that 1200 L of water was pumped at all sites, with the exception of two sites at which lower volumes (780 L, 1036 L) were pumped. Once pumping of water from beneath the ice was completed, the sieve was covered with aluminium foil, secured in the wooden stand and transported to the laboratory for further processing. At 3 (of the 25) ice stations, seawater could not be pumped from beneath the ice floeterm·key due to the incomplete penetration of the ice floeterm·key during drilling.

Laboratory processing and analyses The outer surface of each ice coreterm·key was scraped off using a boomerang scraper. A stainless-steel hand saw was then used to cut each ice coreterm·key into 10 cm vertical subsections. Subsections were placed into individual clean Ziploc bags (polyethylene) and allowed to melt for 24–48 hours. Once melted, the water from each subsection was transferred to a graduated cylinder and its volume measured. Each Ziploc bag was rinsed in triplicate with Milli-Q water to ensure that all particles were transferred out of the bag. Water from each sub-section was filtered under vacuum onto glass microfiber paper (GF/C); Whatman: 47 mm, pore size: 1.2 μm, using a Buchner funnel and a vacuum flask. Each filter paper was folded, placed into an aluminium foil packet and stored in a freezer (−20 °C) until further processing. The sieve that was used for filtering water from beneath the ice was rinsed in triplicate and all water was also processed in the same manner (vacuum filtration of water and collection of particles onto glass microfiber paper). Potential microplasticsterm·dom were identified and isolated based on a previously described method 49 . Briefly, individual filter papers were visually examined under a dissecting microscope (Olympus SZX10) equipped with a polariser and camera (Q Imaging Retiga 2000R) and potential microplasticsterm·dom were identified based on characteristic features 49 . Potential microplasticsterm·dom from each sample were photographed, described according to form (fibre, fragment, etc) and length measurements were taken prior to transferring to a clean filter paper. Potential microplasticsterm·dom were assigned to two broad categories (fibres, fragments) and to six length categories: 0.1–0.5 mm, 0.5–1.0 mm, 1.0–2.0 mm, 2.0–3.0 mm, 3.0–4.0 mm, 4.0–5.0 mm. Filter papers with potential microplasticsterm·dom from each sample were stored in clean, labelled petri dishes. All potential microplasticsterm·dom were analysed by Fourier transform infraredterm·mth (FT-IRterm·mth) spectroscopy on a Bruker Vertex 70 Infrared Spectrometer coupled to a Hyperion 1000 microscope according to a previously described method 49 . Briefly, each sample spectrum was compared with those of known standard polymers in the (i) Bruker Optics Attenuated Total Reflectance (ATR) Polymer and (ii) Synthetic Fibres ATR libraries. Samples which produced spectra with a match <60% were automatically rejected. All remaining spectra (>60%) were individually examined to ensure that there was clear evidence of peaks from the sample corresponding to known peaks of standard polymers. Overall, matches with >70% similarity were accepted while some between 60–70% similarity were accepted.

Method validation and contamination prevention Several measures were taken to minimise contamination of samples. In the field, (i) microplasticterm·dom sampling was conducted upwind of all other activities, (ii) nitrile gloves were used when handling ice coresterm·key, (iii) the manual pump used at the ice stations was flushed with water prior to pumping seawater, and (iv) the stainless steel sieve that was used at the ice stations had a wooden cover affixed to it during filtration. In the lab, (i) ice processing was conducted on a wooden surface, (ii) the wooden work area was washed down with Milli-Q water in between processing of individual ice coreterm·key subsections, (iii) all equipment (scraper, saw) was washed with MilliQ water, (iv) lab coats, cotton clothing and gloves were worn during sample processing, and (v) all containers used during sample processing were cleaned using Milli-Q water. Checks were conducted to determine whether there was any contamination during sample processing. Clean petri dishes with filter paper were left exposed to the air during ice coreterm·key and laboratory processing to determine if there was any airborne contamination. To determine whether there was any additional contamination during the processing of the melted sea iceterm·key, method blanks were conducted. For each method blank, 750 mL of Milli-Q water was placed into clean Ziploc bags and left for 24–48 hours. This water then underwent the exact processing as would have occurred for water from an ice coreterm·key sub-section.

Data analyses Source areas and backward drift trajectoriesterm·key for the sea iceterm·key cores were estimated using the AWI ICETrack application. The lagrangian approach traces sea iceterm·key backward in time using a combination of satellite-derived low-resolution drift products 28 . So far, ICETrack has been used in a number of publications to examine sea iceterm·key sources, pathways, thickness changes and atmospheric processes acting on the ice cover cover 2 , 29 , 30 . For each ice coreterm·key, input to the application included sampling date and location (latitude, longitude) while output included a plot of the estimated pathway as well as ancillary data associated with the pathway. Predicted sea iceterm·key thickness by a thermodynamic model and measured sea iceterm·key thickness was compared in order to assess the validity of the findings for the various sea iceterm·key cores. All maps were generated using Ocean Data View (ODV) Version 4.7.10 50 and all graphs were generated using R version 3.4.4 51 . Univariate analyses were conducted to determine whether (i) there were significant differences in microplasticterm·dom concentrations among the sea iceterm·key cores (Kruskall-Wallis) and, (ii) there were significant correlations between microplasticterm·dom concentration and sub-section depth of the ice coresterm·key (Spearman’s Rank Correlation). Multivariate analysis, i.e. Principal Components Analysis (PCA), was conducted to provide further discrimination between the ice coresterm·key.

Supplementary information Supplementary Information.

Acknowledgements The authors acknowledge the invaluable support of the staff of the Swedish Polar Research Secretariat (especially Jeanette Axelsson, Robert Holden, Lars Lehnert, Asa Lindgren, Axel Meiton and Per Salo) and the crew of icebreaker Oden with the Arctic Oceanterm·key 2016 expedition. The expert guidance of Mr. Andrew Tonkin (University of Plymouth) during FT-IRterm·mth analyses is also acknowledged. The first author also acknowledges the support of staff (Bengt Berglund, Ted Juliusen, Michelle Nerentorp, Sofi Jonsson, Nicke Juuso), fellow early career scientists (especially Hans-Martin Heyn, Luz Maria Ramirez) and other researchers (especially Asa Johannisson) during the expedition. Sampling in the Arctic Oceanterm·key was funded by the Swedish Polar Research Secretariat (SPRS) under the Early Career Scientist (ECS) Programme in which the first author was a participant. This work was also co-funded through a MARES Grant. MARES is a Joint Doctorate programme selected under Erasmus Mundus and coordinated by Ghent University (FPA 2011-0016). The funders had no role in study design, data collection, analysis and interpretation, decision to publish, or preparation of the manuscript.

Author contributions L.K., I.O. and K.G. designed the study. K.G. provided training and logistical support during the AO16 expedition. L.K. and K.G. sampled sea iceterm·key cores and surface waters beneath ice floesterm·key during the AO16 expedition. L.K. processed sea iceterm·key cores, conducted FT-IRterm·mth analyses, wrote the manuscript and analysed the results. T.K. estimated the backward trajectories and potential origin of the sea iceterm·key. I.O. provided feedback on the analyses. All co-authors reviewed the manuscript.

Data availability All data related to this study is available without restriction.

Competing interests The authors declare no competing interests.

Footnotes Supplementary information is available for this paper at 10.1038/s41598-020-61948-6.

References Associated Data Supplementary Materials Supplementary Information.

Data Availability Statement All data related to this study is available without restriction.