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Environmental Pollution xxx (2017) 1e12

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Environmental Pollution

journal homepage: www.elsevier .com/locate/envpol

Microplastics in sea coastal zone: Lessons learned from the Balticamber*

Irina Chubarenko a, *, Natalia Stepanova a, b

a P.P.Shirshov Institute of Oceanology of Russian Academy of Sciences, Atlantic Branch, Laboratory for Marine Physics, prospect Mira, 1, Kaliningrad, 236022,Russiab P.P.Shirshov Institute of Oceanology of Russian Academy of Sciences, Sea currents Laboratory, Nakhimovski prospect, 36, Moscow, 117997, Russia

a r t i c l e i n f o

Article history:Received 5 October 2016Received in revised form7 January 2017Accepted 27 January 2017Available online xxx

Keywords:MicroplasticsAmber washing-outCoastal zoneStormy conditions

* This paper has been recommended for acceptanc* Corresponding author.

E-mail address: irina_chubarenko@mail.ru (I. Chub

http://dx.doi.org/10.1016/j.envpol.2017.01.0850269-7491/© 2017 Elsevier Ltd. All rights reserved.

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a b s t r a c t

Baltic amber, adored for its beauty already in Homer's Odyssey (ca. 800 B.C.E), has its material densityclose to that of wide-spread plastics like polyamide, polystyrene, or acrylic. Migrations of amber stones inthe sea and their massive washing ashore have been monitored by Baltic citizens for ages. Based on thecollected information, we present the hypothesis on the behaviour of microplastic particles in sea coastalzone. Fresh-to-strong winds generate surface waves, currents and roll-structures, whose joint effectwashes ashore from the underwater slope both amber stones and plastics e and carries them back to thesea in a few days. Analysis of underlying hydrophysical processes suggests that sea coastal zone understormy winds plays a role of a mill for plastics, and negatively buoyant pieces seem to repeatedly migratebetween beaches and underwater slopes until they are broken into small enough fragments that can betransported by currents to deeper areas and deposited out of reach of stormy waves. Direct observationson microplastics migrations are urged to prove the hypothesis.

© 2017 Elsevier Ltd. All rights reserved.

1. Introduction

Pollution of seas and oceans by macro- and microplastic parti-cles rapidly increases and raises serious concern (Thompson, 2015;Hidalgo-Ruz et al., 2012). Microplastics (MPs) are found nowadaysin fish (Lusher et al., 2013) and birds (Trevail et al., 2015), in deep-ocean sediments (Thompson et al., 2004; van Cauwenberghe et al.,2013) and arctic water (Lusher et al., 2015) and ice (Obbard et al.,2014); its surface attracts toxins and chemicals from the environ-ment (Hidalgo-Ruz et al., 2012; Andrady, 2011; Engler, 2012; Matoet al., 2001) and may convey them up the food chain (Hidalgo-Ruzet al., 2012; Andrady, 2011; Ivar do Sul and Monica, 2014). At thesame time, various dimensions, shapes and densities of MPs(conventionally, particles smaller than 5 mm (Hidalgo-Ruz et al.,2012; Ivar do Sul and Monica, 2014; Arthur et al., 2008)) make itdifficult to predict its behaviour in highly turbulent marine envi-ronment. It is suggested (Hidalgo-Ruz et al., 2012) that energeticprocesses in sea coastal zones under stormy conditions play animportant role in mechanical destruction and re-distribution of

e by Eddy Y. Zeng.

arenko).

nko, I., Stepanova, N., Micropoi.org/10.1016/j.envpol.2017.0

plastic pieces. However, not any field observations of MPs behav-iour in the sea coastal zone have been available up to now, andmany basic questions remain still not answered: which physicalfactors are of primary importance for MPsmigrations, where all themarine plastics is and why it is so dispersed everywhere.

Surprisingly, the Baltic amber (succinite) and impressive storiesabout its washing out from the sea to the shores of the South-Eastern Baltic (Fig. 1) can shed some light onto these processes.Succinite material density of about 1.05e1.1 g cm�3 is close to thatof widely used plastics like polyamide (nylon, 1.02e1.05 g cm�3),polystyrene (1.04e1.1 g cm�3), or acrylic (1.09e1.20 g cm�3)(Chubarenko et al., 2016), and nowadays MPs are abundantly foundashore together with amber crumbs (Fig. 1).

Baltic citizens have monitored the behaviour of amber in the seacoastal zone for centuries, hunting for these drops of resin ofancient pine-trees called “tears of Heliades”, the daughters of He-lios, the Sun God. Along with great works of literature (like OdysseybyHomer (Heubeck et al., 1988)), myths (e.g., those by Ovid (1997)),and ancient trading documents, there are quite many physicallyrational observations and facts describing the process of ambermigration in the sea and, especially, addressing the phenomenon of“amber washing-out” after severe storms, when sometimes tons ofamber pieces are thrown by the Baltic from its bottom to the

lastics in sea coastal zone: Lessons learned from the Baltic amber,1.085

Fig. 1. Amber crumbs (highlighted by yellow arrows) and microplastics (in red ovals) are washed ashore together (beach of the Vistula Spit, south-eastern part of the Baltic Sea,photo by I. Chubarenko). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

I. Chubarenko, N. Stepanova / Environmental Pollution xxx (2017) 1e122

beaches. These extraordinary events are even especially termed bylocal citizens e “brosy” e after a Russian word «бросать» (pro-nounced as “ brosat’ ”), meaning “throw out”.

In this paper, wemake an attempt to analyse the collected set ofhistorical and observational facts from the point of view of physicaloceanography, trying to (i) deduce physical drivers of the washing-out processes and (ii) project the results onto the problem oftransport and fate of plastic particles in the sea coastal zone.

2. Materials, methods and sources of information

Any systematic monitoring or scientific investigation on theBaltic amber migrations and beaching is not yet available. The mainmethod of this study is cross-comparison of the information fromhistorical and local sources e with present-day knowledge on theBaltic Sea oceanography and hydrophysical processes in the seacoastal zone.

Comparisons of material and transport properties of ambercrumbs andmicroplastics are performed as the first step. They referto both seminal studies on loose-boundary hydraulics by Shields(1936), who by chance dealt also with amber and plastics, andrecent publications (e.g., Critchell and Lambrechts, 2016; Ballentet al., 2012, 2013), as well as to our own experience with micro-plastics particles (Chubarenko et al., 2016; Esiukova, 2017;Khatmullina and Isachenko, 2017; Zobkov and Esiukova, 2017).

Information on historical aspects of an amber collecting processin the south-eastern part of the Baltic Sea, exceptionally massiveamber washing-out events, and the largest beached amber pieceswas collected from books (Beck and Shennan, 1991; Finlay, 2006)and web sites of museums (Physical and chemical qualities ofamber, 2016; Palanga Amber Museum, 2016), jewellery makers(About amber, 2016; About Baltic amber, 2016; Large beaching ofamber in Pionerskiy, 2016), hobby/spare time sites (Amber and itsgathering, 2015; Catching of amber in the Baltic Sea, 2015).

Observational facts, particular event descriptions, specific fea-tures of amber washing-outs were summarised from variousdispersed sources, including historical (Finlay, 2006; Beck andShennan, 1991) and present-day publications (Kireyeva, 1960;Amber and its gathering, 2015), web (Alekseev, 2015;Pushkarskaya, 2015; Nuyakshev, 2014; The storm threw tons ofamber and seaweed to the coast of the Baltic Sea, 2013) and TVsources (Treasure hunters attacked the Baltic coast scattered with

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amber, 2015; Privalov, 2016) as well as direct communicationsand discussions with local citizens and hobby-hunters. Analysis ofthose field situations was complemented by meteorological infor-mation from regular governmental weather stations (Reliableforecast, 2004).

Geological information on general amber deposit structure anddepths of openings of amber-containing layer to the sea bottomcame from the geological atlases (Atlas of geological andenvironmental geological maps of the Russian area of the BalticSea, 2010) and amber mining plant site (Geology, 2016).

Oceanography of the Baltic Sea has already been quite wellinvestigated. The referred magnitudes of typical stormy windsspeed, direction, and fetch, as well as significant wave height,period, and length of the developed surface waves are based onfundamental investigations published in books (State andEvolution of the Baltic Sea, 2008; Lepp€aranta and Myrberg, 2008).

Discussions on particular mechanisms of transport and mixingin the coastal zone refer to classical works on the formation of roll-structures in the upper layer mixed layer (Langmuir, 1938; Craikand Leibovich, 1976; Thorpe, 2005; Leibovich, 1983), and wave-induced sediment transport (Dean and Dalrymple, 2001; VanRijn, 2012).

Finally, the observed behaviour of amber pieces is projectedonto that of microplastics particles, and a scheme of migration ofplastic pieces in the sea coastal zone is suggested.

3. Results and discussion

3.1. Amber vs microplastics: physical and transport properties

Amber, plastics, and marine flotsam appear together on wrack-lines of the beaches of the south-eastern part of the Baltic Sea afterwindy weather (e.g., Esiukova, 2017). Before projecting theobserved behaviour of amber pieces onto that of plastics in generaland MPs in particular, let us compare their physical and dynamicalcharacteristics.

Material density. For both amber and plastics, only the range ofmaterial densities has a physical sense. Amber is naturally inho-mogeneous, and the stones may also have various inclusions.Amber does not change its density with time in marine waters, andeven does not get bio-fouled. On the other hand, plastics maycontain various additives, which initially influence their density,

lastics in sea coastal zone: Lessons learned from the Baltic amber,1.085

I. Chubarenko, N. Stepanova / Environmental Pollution xxx (2017) 1e12 3

and under environmental conditions plastics change their densitydue to weathering and bio-fouling, which, correspondingly, de-creases or increases their density. Overall, the density of ambermeasured in our laboratory varied between 1.05 and 1.1 g cm�3;some sources mention values up to 2.00 g cm�3, which is very rarecase (e.g., (About amber, 2016)). Many widely used plastics fit thisrange, like polyamide (nylon, 1.02e1.05 g cm�3), polystyrene(1.04e1.1 g cm�3), acrylic (1.09e1.20 g cm�3), and some are close,like polyethylene (0.97e1.28 g cm�3 with additives), polyester(1.01e1.46 g cm�3) (Chubarenko et al., 2016).

Shape. MPs are shown to have various shapes, most commonbeing 3 d regular or irregular beads, flattened wreckage, flakes,films, fibres. Amber pieces (Fig. 2) have rather irregular but defi-nitely volumetric (3 d) shapes; many of them break in parts instormy waves and have sharp fresh cleavages. Fig. 2 shows MPs andamber crumbs collected on the beach at the same time: it seemsthat there is no prevalent shape of MPs particles beached togetherwith amber. Note that all of them were beached with lots ofseaweed and marine flotsam (see Fig. 1 and photos in SupplementMaterial), so that plastics were entangled and could not float freely.

Particle principle shape (3 d-particle, or flat flake, or thin fibre) isshown to be very important for its motion (Khatmullina andIsachenko, 2017). However, smaller details seem to be not soimportant. Shields (1936) in his well-known laboratory investiga-tion of re-suspension threshold for different bed loads did not find“essential influences of the grain shape upon themagnitude of bed-load movement” Shields (1936, p. 28): he used 3 d sharp-edged(amber and granite) and rounded (brown-coal) particles for bed-load.

Size, age, and mechanical damages. In the context of this study,important is comparison of sizes and qualities of plastic and amberpieces, beached during the same environmental conditions. Undercalmweather and weak winds, no amber is present on the beaches.Weak to moderate winds typically bring (in case the “event” hap-pens) amber crumbs with small wooden pieces (Fig. 1), accompa-nied by almost exclusively MPs. The stronger winds e the largerplastics and amber pieces are beached, often with wood only(without seaweed), but MPs and small amber crumbs remain aswell. Fig. 3 shows such “moderate-winds” beached plastics: it hassevere mechanical damages and bio-fouling, and the dates printedon them indicate 3e5 years of residence in the sea. After storms,

Fig. 2. Plastic and amber pieces beached after 7e8 m s�1 wind episode: (a) various micropamber pieces found at the same time but rare for such wind speed (photo by I. Chubarenk

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together with large amber “brosy”, very heavy and large plasticobjects can be washed out: large boxes, entangled fishing nets andropes, etc. Again, amber crumbs and MPs are also present (e.g.,Esiukova, 2017).

Sink or float? Amber (density 1.05e1.1 g cm�3) cannot float inBaltic water (1.006e1.014 g cm�3); its density does not changewithtime in the sea, and it does not get bio-fouled. Concerning plastics,beached together with it, the answer is not so obvious: plasticdensity may change due to weathering or bio-fouling, relativelylight PE films may catch sand, while heavier PET bottles maycontain air bubbles, etc. The direct checking of plastic objects fromthe wrackline shows (see Supplement Video “Sink”), however, thatmost of them are sinking almost immediately. Measured density ofbeached wet wooden sticks is about 1.02 g cm�3, i.e., they also sinkinwater. In accordwith this are observations by (Ballent et al., 2012,2013): they report that their collection of high-density MPs is madefrom objects stranded after a high tide on sandy beaches of theNorth Sea. High-density pellets, used for laboratory experiments in(Ballent et al., 2012, 2013), were collected on sandy beaches inCalifornia.

Thus, an essential part of the beached plastics is heavier thanwater. This is true for both tidal (like the North Sea) and non-tidal(like the Baltic Sea) beaches; at least for the latter ones - presence ofweakening surface waves is important.

Critical shear stress for amber and MPs. Some information ondirect measurements of re-suspension threshold (initiation ofmotion) in water current can be found from both amber and MPsside, as well as from investigations of other bed-load materials ofthe “plastic” density range (Table 1).

In seminal work by Shields (1936) on sediment re-suspensionand transport by water currents, along with barite, granite andbrown coal, amber crumbs were occasionally used as one of theinvestigated bed loads. Amber fragments with grain size from 0.3 to3 mm (mean diameter d ¼ 1.56 mm) and density 1.06 g cm�3,resulting from the cutting of larger amber pieces, are incorporatedin sediment research as “easily movable … sharp edged” bed-loadparticles. Critical shear stress for such particles, re-calculated fromthe amber-points on the Shields (1936) diagram (Fig. 4), is about t*~0.03e0.04 N m�2, and critical velocity u* ¼ 4e6 mm s�1.

From MPs side, only very limited information is still available.Ballent and co-authors (2012; 2013) reported measured in

lastics: fragments, folded and stretched films, foam; (b) amber typical-size crumbs; (c)o).

lastics in sea coastal zone: Lessons learned from the Baltic amber,1.085

Fig. 3. Beached wreckage (aed) has severe mechanical damages and bio-fouling, suggesting its long residence at the stony bottom. Note a colony of Balanus barnacles on the bottleneck (b). The dates printed on the pieces (c: 01.06.2011; d: exp.05.06.2013), collected on 3 April 2016 on the beach of the Vistula Spit, indicate that they spent in the sea at least 3e5years (photos by I. Chubarenko).

Table 1Laboratory measurements of re-suspension threshold in fluid flow for particles similar to MPs in size and density. Coarse sand is shown for comparison.

Material Density,r, g cm�3

Mean diameter,d, mm

Shape Critical shear stress, t*,N m�2

Critical velocity,u*, mm s�1

Source

Amber cuttings 1.06 1.56 sharp edged 0.03e0.04 4e6 Shields, 1936Brown coal 1.27 1.8e2.5 angular, with rounded corners 0.13e0.35 5e20 Shields, 1936Plastic pellets (black) 1.06

(1.03*)3.5e4.7 rounded 0.025 4.9 Ballent et al., 2012

Plastic pellets (opaque) 1.07(1.04*)

3.3e4.4 rounded >0.18 >13 Ballent et al., 2012

Plastic pellets (transparent) 1.13(1.09*)

3.8e5.1 rounded >0.18 >13 Ballent et al., 2012

Coarse sand 2.65 0.5e1 rounded 0.27e0.47 16e22 Berenbrock and Tranmer, 2008

* “effective” density in fresh-water is calculated here, since (Ballent et al., 2012) experiments were conducted in saltwater with density 1.03 g cm�3.

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laboratory dynamical characteristics for three kinds of industrialpre-production high-density plastic pellets collected on sandybeaches of Los Angeles County (California), see Table 1. They wereof rounded shape and somewhat larger than the Shields amberparticles (3.3e5.1 mm), with densities from 1.06 to 1.13 g cm�3.

In a view of inter-comparison, one should have in mind thatthere are three differences in the (Shields, 1936) and (Ballent et al.,2012) experiments. Firstly, the working fluid in (Shields, 1936) wasfresh water, whilst (Ballent et al., 2012) worked with saltwater withdensity 1.026 g cm�3; for our analysis, in Table 1 both real and“effective” densities are shown for (Ballent et al., 2012) pellets.Secondly, different experimental techniques were used: in (Shields,1936), bed-load material was re-suspended by unidirectional flowin an open channel, whilst in (Ballent et al., 2012) a rotating re-suspension chamber was used. While comparing these methods(Tolhurst et al., 2000), found the calculated threshold values“relatively comparable”, but affected by the “device size”; thus, inour case, some deviations are anticipated. And the third differencelies in the principal difficulty to define what exactly (formally) is“the beginning of motion”: Shields (1936) defines the threshold by

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“extrapolating the bed-load transport curve to the time bed-loadmovement ceases” (Shields, 1936, p. 10), and (Ballent et al., 2012)report the bed-load shear/velocity “at which 50% of the particlesrolled, slid or saltated on the chamber floor” (Ballent et al., 2012, p.18761). Nonetheless, the experimental points for both ambercrumbs and plastic pellets fit quite well in the classical “Shieldscurve” (Fig. 4), which expresses in dimensionless form the borderbetween regimes of “motion” and “no motion” of the bed-loadmaterial under action of fluid flow.

One more example can be incorporated in the present analysis.Brown coal (lignite) particles from Shields (1936) experiments,with their density of 1.27 g cm�3, can be considered as a model forMPs like that from Plexiglas (density 1.24 g cm�3), polyurethane(1.17e1.28 g cm�3), or polyvinylchloride (1.16e1.58 g cm�3). Theparticles used in three runs in Shields (1936) were characterized bythe mean diameters 1.77, 1.88 and 2.53 mm, and described as“angular, with rounded corners”. Again, using the correspondingpoints from the Shields (1936) diagram, one obtains the values forcritical shear stress to be about t* ~ 0.13e0.35 N m�2, and criticalvelocity 5e20 mm s�1.

lastics in sea coastal zone: Lessons learned from the Baltic amber,1.085

Fig. 4. The classical Shields (1936) diagram, showing the dependence of dimensionless shear stress from the particle Reynolds number, is used to describe the initiation of motion ofnatural bed-load material of different grain size in the fluid flow. Points for amber and lignite from (Shields, 1936), and for high-density preproduction pellets from (Ballent et al.,2013) are shown. Note that (Ballent et al., 2012) used another experimental technique.

I. Chubarenko, N. Stepanova / Environmental Pollution xxx (2017) 1e12 5

Fig. 4 illustrates conditions of “initiation of motion” of particlesin the fluid flow (Shields, 1936): the dependence of the dimen-sionless Shields critical shear, q ¼ t*

ðrp�rwÞ,g,d, from the dimension-

less particle Reynolds number, Re* ¼ u*,dv . Here, t* is dimensional

critical shear stress, rpeparticle density, rwe water density, g e

acceleration due to gravity, u* - critical velocity, d e diameter of theparticle and n e water kinematic viscosity. We highlighted theShields (1936) data points for amber and lignite (density1.27 g cm�3), and added points for three kinds of plastic pelletsfrom (Ballent et al., 2012). Points for coarse sand (Casey, 1935; citedafter Shields, 1936) are also shown for comparison. Thus, smallparticles of similar to plastics density are used in sedimentologyresearch, and both amber crumbs and “MPs” (with both sharp-edged and rounded particles) obey the initiation-of-motion de-pendencies obtained for natural loose bed-load materials.

Physically, amber, sands and (used in the above experimentsquite large and 3 d-shaped) MP particles have similar criticalShields dimensionless shear of about q ~ 0.03e0.04, which meansthe analogous re-suspension threshold in unidirectional current forthe particles with the same Re*. It could be expected that films,flakes and fibres, in contrast to 3 d-fragments and rounded pellets,hardly obey this dependency. At the same time the observationsshow that, in the case of “brosy”, MPs of various shapes are beachedtogether, see Fig. 2 for example. This indicates contribution of othertransport mechanisms for amber/plastics migrations and beaching;most probably these are wave-induced motions.

It is important to note for future experiments on MPs transportthat the re-suspension threshold measurements in sedimentologyusually follow a unified (Shields, 1936) technique: bed-load (loosesand, amber or MPs) is exposed to the flowmoving above it. Hence,such experiments characterize the initiation of motion of MPs overthe bed, entirely covered by the analogous (MP) particles. Underreal-sea conditions, MPs seem generally to be re-suspended fromnatural bottom materials, which means at least influence ofanother bed roughness (i.e., another boundary layer thickness) anduse of the particle Re* number based on dimensions of both MPparticle and bottommaterial. Additional laboratory experiments onMPs re-suspension from bottoms covered by silt, sand, cobbles, etc.Are very desirable to obtain re-suspension threshold more

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applicable to in-situ conditions for MPs. In addition, along with there-suspension by water flow, the oscillatory wave motions seem tobe valuable for easy-moveable plastic particles.

3.2. Baltic amber: appearance and migrations in sea coastal zone

Appearance of amber on the beach is quite a typical phenome-non for the south-eastern part of the Baltic Sea, from Poland toLithuania, and is very attractive for both local citizens and tourists.As a consequence, the conditions when it happens are well knownand rather characteristic of this region: after wind episodes of morethan 7e10 m s�1, the wrackline of the beaches e here or there e isoften delineated with amber crumbs. Just windy episodes bringnumerous but small crumbs of amber (2e5 mm), like those shownin Fig.1: the photowas taken on 25 September 2015 on the beach ofthe Vistula spit during a very calm weather period - but right afterthe transient 4-h episode of gentle breeze up to 6 m s�1 from thenorth-west (frontally to this shore). Heavier winds and especiallysevere winter storms may present amber samples up to 12 kg - thelargest known piece, found on the Baltic shore of Prussia in the 19thcentury (Baltic Countries Mineral Industry Handbook, 2016). Theother two giant beached amber samples, reported in historicaltexts, were about 9.7 and 7 kg. Such washed out stones areextremely rare: all the history knows less than 10 stones heavierthan 5 kg (Baltic Countries Mineral Industry Handbook, 2016). Thelargest amber piece in the present-day collections, exhibitednowadays in the Berlin Natural Science Museum (Germany), is47 cm long and weighs 9.8 kg (About amber, 2016). Concerning themaximum total volume of the beached amber, local legendsmention the autumnal event in 1862, when almost 2 tons of amberwere beached near Yantarny (former Palmniken, Fig. 5) during onestorm (Finlay, 2006; Beck and Shennan,1991). After a storm of 1914,people collected about 870 kg of amber on the northern beaches ofthe Sambian peninsula (Beck and Shennan, 1991). Along withamber, the wrackline after storms contains seaweed, shells, andwet wooden pieces; and nowadays they are always mixed withsome plastic garbage and microplastics particles.

Baltic amber is fossil resin produced by pine trees which grew inNorthern Europe about 50 million years ago. In the course of time,

lastics in sea coastal zone: Lessons learned from the Baltic amber,1.085

Fig. 5. Map of the south-eastern part of the Baltic Sea, showing the location of open amber mining near the city of Yantarny. The isobath 20 m (in blue) is the offshore limit ofprobable openings of amber-containing blue clay to the underwater slope (Geology, 2016). The moraine outcrops (thick black lines) are a potential trap of amber (Atlas of geologicaland environmental geological maps of the Russian area of the Baltic Sea, 2010). An effective wind fetch for the given region (State and Evolution of the Baltic Sea, 2008) and typicalsites of amber ‘brosy’ (Atlas of geological and environmental geological maps of the Russian area of the Baltic Sea, 2010, the scheme initially compiled by S. Isachenko after enquiryof miners) are shown by red and yellow points, with the colour corresponding to wind direction. Map of Europe: http://www.freeworldmaps.net; the map in background wasdrafted in open software Quantum GIS (ver. 2.10) www.qgis.org/ru/site/forusers/download.html, with final production in CorelDraw X4. (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

I. Chubarenko, N. Stepanova / Environmental Pollution xxx (2017) 1e126

by polymerization and oxidation, the resin was transformed intoamber (About amber, 2016). The world largest deposit (about 90%)of amber (Baltic Countries Mineral Industry Handbook, 2016) islocated in Kaliningrad region (Russia) near the city of Yantarny(Fig. 5), with open mining on land. Meanwhile, the undergrounddistribution of layers of the amber-containing “blue clay” and theiropenings to underwater slope in the sea are poorly known. Withinthe open mining site, the layer of the blue clay is located approxi-mately between 10 and 20 m below the sea level (Geology, 2016),and has a slight inclination towards the land, suggesting that itsopening to the underwater slope might be within the depth rangeof 8e15 m.

Until the 13th century, the seacoast inhabitants collected amberdirectly from the seashore, and later they learned how to obtainamber by combing the seabed from small boats with nets and longsticks (Finlay, 2006). With the appearance of the diving suit, thedirect collection from the bottom became also popular. Amberpieces can often be found on the bottom of slight depressions at thedepths of 5e15m, or stuck in-between boulders in areas with stony

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bottom.At the bottom of the sea, amber is usually found around the

Sambian peninsula at the depths from 3 to 12 m, maximum 15 m.One reason for this could be that the layer of the ancient LitorinaSea “blue clay” is located at the depths of 4e15 m below the sealevel. It stretches from the Vistula Spit to the Curonean Spit andfurther north (Atlas of geological and environmental geologicalmaps of the Russian area of the Baltic Sea, 2010), but opens to thesea bottom only off the Yantarny deposit (Fig. 5). At the same time,amber is never washed ashore without plenty of seaweed, woodensticks, and natural marine litter. This suggests that the beachedamber pieces are not directly washed out from the bottom sedi-ments, but are caught by seaweed clouds during their underwatermigrations, just like other slightly negatively buoyant wooden (orplastic) pieces. An important observation is that e after stormyweather - amber is typicallywashed ashore together with red algaeFurcellaria lumbricalis, which are abundant in the eastern part of theBaltic Proper and compose a belt along the shore, dominating onsubmerged rocks at 6e15 m depth (Kireyeva, 1960). Furcellaria can

lastics in sea coastal zone: Lessons learned from the Baltic amber,1.085

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also grow in large floating mats (Kireyeva, 1960; Furcellaria, 2015),and such mats are also mentioned quite often as washed ashoretogether with amber during the “brosy” events. One way oranother, it is quite obvious that amber pieces, which are heavierthan water, are washed out of the sea during storms from thedepths down to 6e15 m.

3.2.1. The events of “brosy”As observations show, amber pieces are able to move up-slope

and small pieces appear at the shoreline even at moderate winds(see Fig. 1 and photos in the Supplement Material); we are notaware of any systematic monitoring of such cases. The major part ofobservational facts describes the cases after severe storms, whenamber “brosy” are large and impressive indeed. Amber hunting inthe sea has a long historical trace, and is at present quite a popularspare time activity in Kaliningrad region. It is regarded under theRussian law as hobby-fishing or collecting berries in nature, andmany hunters enjoy extreme fishing for amber stones in the stormysurf. As a consequence, general signs indicating favourable condi-tions for the “brosy” are well known - but the exact place and thevery moment are still almost unpredictable. These signs and char-acteristic features of the “brosy” events, the specific weather con-ditions are repeatedly mentioned in various sources includingbooks (Baltic Countries Mineral Industry Handbook, 2016; Finlay,2006; Beck and Shennan, 1991; Atlas of geological andenvironmental geological maps of the Russian area of the BalticSea, 2010), web-sites (About amber, 2016; About Baltic amber,2016; Amber and its gathering, 2015), news (Alekseev, 2015;Pushkarskaya, 2015; Nuyakshev, 2014; Large beaching of amber inPionerskiy, 2016; The storm threw tons of amber and seaweed tothe coast of the Baltic Sea, 2013; Treasure hunters attacked theBaltic coast scattered with amber, 2015; Privalov, 2016) and per-sonal communications. All this information can be summarised asfollows.

(I) The events of extraordinarily massive “brosy” of large amberpieces to the shore typically take place only several timesper year, after the strongest storms, which are usuallyobserved during “windy seasons” - autumn and especiallywinter; spring and summer storms bring amber as well,however exceptionally strong winds are rare during warmseasons.

(II) Favourable are the situations when strong northerly orwesterly winds raise especially high waves.

(III) Duration of a storm is important: at least two-three stormydays are necessary.

(IV) The last stage of development of the storm is the mostfavourable: amber is washed up from the bottom of the BalticSea only immediately after the storm, not during or a few dayslater. This happens several hours after the beginning ofwindweakening, whenwinds are ceasing already e but the stormis still vivid, and when waves are still high - but the seaalready begins to calm down.

(V) The event of “brosy” takes place not along the entire shore-line, but only at certain separate (and quite random) lo-cations (Fig. 5; photos in Supplement Material), where windis perpendicular to the shoreline. This way, most frequently ithappens at some place along the concave 20-km longwestern fragment of the Kaliningrad shoreline from theVistula Spit to Yantarny (Fig. 5): closer to the spit undernorthern or north-western winds, and closer to Yantarnyunder western winds and winds with a south-westerncomponent. The “brosy” may happen along the northernKaliningrad shore as well (from cape Taran to Zelenogradsk)- whenwinds are from the north or north-west. The location

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of amber “brosy” is not permanent even within one stormyevent.

(VI) Patches: amber stones arrive to the shore together with acloud of seaweed (especially e with red algae Furcellarialumbricalis), wooden sticks, and other suspended material.This - quite compact - cloud suddenly rises from the bottomin a separate spot a few kilometres (not specified in thesources; approximately, 1e3 km, personal communication)away from the shore, gets distributed throughout the entirewater depth, from the bottom to the surface, and drifts to-wards the shore, where all the material is for some time keptmixed-and-suspended in the swash zone or beached as apatch of some 30e70 m long (e.g., see photos in theSupplement Material). Sometimes, the main (the largest)patch may be accompanied by 1e2 smaller ones, 400e500mapart, which containmainly seaweed, almost without amber.

(VII) “The Baltic withdraws its treasures”: usual/every-dayexperience shows that no amber is present on/in the beachsands. A stormy seaweed patch in the swash zone may begindrifting off-shore already in an hour or two; the shoaledseaweed cloud may stay on the beach up to several days;small (3e4 mm) amber crumbs washed out after moderate-wind events can be found on the beach for about one day. Inaccord with these time scales are investigations of agar-containing Furcellaria lumbricalis: its beached mats areshown to be carried back to the sea in 1e2 days (Kireyeva,1960).

Here are selected only those specific features of meteorological,hydrophysical, and topographical conditions corresponding to thecases of “brosy” that are mentioned in several independent sources.Below, we examine them from the point of view of physicaloceanography, aiming at prediction of the behaviour of plastic litterand microplastic particles in the sea coastal zone.

3.3. Oceanographic analysis

Behaviour of particles in the sea coastal zone is one of the mostchallenging problems in marine sciences, being both highlycomplicated and utterly needed for a great variety of applications.This way, analysis of the “natural case study” of amber “brosy” isilluminating and informative. The authors are not aware of anysystematic investigation or interpretation of physical conditionsdriving the amber migrations in the coastal zone. At the same time,the collected information allows for suggestion of some answers onwhy, how and from which depths the negatively-buoyant amberstones are washed ashore.

3.3.1. Surface wave fieldAs follows from the items (I)-(III) listed above, the main physical

driver for massive “brosy” of large amber stones is a long-lastingstormy wind of proper direction e perpendicular to the shore-line: westerly/northerly winds for western/northern Sambianshore, correspondingly (see Fig. 5). Characteristics of winds andwave field in the Baltic Sea are well investigated (State andEvolution of the Baltic Sea, 2008), and show that exactly these di-rections offer the longest wind&wave fetch in the given area e aslong as 240e250 km (State and Evolution of the Baltic Sea, 2008)(Fig. 5). So it is not surprising that especially high waves (II) aredeveloped. After several days of storm (III), surface waves in opensea can be assumed already well-developed, and their length andheight can be estimated from their fetch. For the Baltic Sea, thestrongest stormy winds (wind speed more than 30 m s�1) generatethe highest (significant wave height H up to 7e8m) and the longest(wave length L up to 100-120-140m) surfacewaves possible for this

lastics in sea coastal zone: Lessons learned from the Baltic amber,1.085

I. Chubarenko, N. Stepanova / Environmental Pollution xxx (2017) 1e128

region (State and Evolution of the Baltic Sea, 2008). Let us assumethese values as approximation of the “open-sea boundary condi-tion” for the coastal zone. Conventionally, surfacewaves begin theirdeformation due to shoaling when the depth, d, is about a half ofthe wave length, L: d < 0.5 L. Thus, the “coastal zone” for the givenstormy situations begins from the depths of about 50e70 m.Further shore-wards, water motions under propagating wavesbecome ellipsoidal, with longer and longer horizontal axis, untilthey are transformed into reciprocating motion on the fore-beach(Fig. 6). In particular, they suspend bottom material and cut thebranches of Furcellaria off the bottom boulders.

While approaching the shore, surfacewaves become shorter andthen break in the surf zone. The surf zone (Fig. 6) begins at thedepth of about d ~ 2H, which is about 15 m for the longest waves inthe considered case. Other stormy waves are breaking over depthsfrom 2H to (1e1.5) H, or from 15 to 2e3 m for various H (State andEvolution of the Baltic Sea, 2008).

Thus, from the shore and to the 15 m-isobath, water is stirred bybreaking stormywaves, so that bottom sediments, growing at thesevery depths tangles of Furcellaria lumbricalis, and marine flotsamexperience these energetic motions. The 15 m-isobath is only3e5 km off-shore, while the amber-containing patches are said toarise “a few kilometers” off-shore (VI). Overall, several independentfeatures e the breaking depth of the highest stormy waves, theareal of growth of Furcellaria lumbricalis, the estimative distance tothe arising flotsam patches - point at the same depth range of10e15 m from which the largest pieces of amber can be washedashore by exceptionally strong storms of proper direction.

3.3.2. PatchinessPatchiness is quite a usual feature of hydrophysical, sedimen-

tological, biological and many other natural fields, and its spatialand temporal characteristics may shed some light onto the un-derlying processes. In the case of amber “brosy”, an obviouscompact flotsam patch is invariably described as the only source ofamber: amber is never washed ashore without seaweed or wetwood. Large “brosy” (after severe storms) bring red Furcellaria; inintermediate-scale events, the patch contains green algae (i.e., itcomes from shallower areas), and sometimes the Fucus vesiculosus(which is nowadays absent in the area and is observed in “brosy”patches only (Volodina and Gerb, 2013)). In the smallest cases ofwashing-outs (like the one shown in Fig. 1) amber crumbs areaccompanied only by small wet wooden sticks. This does notdirectly suggest, however, that larger amber stones are washedfrom deeper areas. This rather shows that stronger storms bring thealgae from deeper parts - while amber could, in principle, beentrained on the way, from any part of the underwater slope. With

Fig. 6. Surf zone of developed stormy waves in the region, depth range of growth

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weaker winds (and waves), there are no currents strong enough totransport larger pieces, even if they are not far from the shore.

In order to disclose the physical background, it is important tograsp spatial scale of patches and their evolution with time. Ob-servations show that sometimes during one storm several patchescan arrive to the shore (here or there, now or later). Howeverusually there is one patch at a time on the shore, which stays in theswash zone for an hour or so e and then drifts back off-shore. Thelocation of amber “brosy” is not permanent evenwithin one stormyevent, and is thus not related to bathymetry field or peculiarities ofthe coastline. The length of the shore face, where seaweed andamber are left on the beach, has a scale up to several hundreds ofmetres, however the very patch in water is much smaller indiametere a few dozens of metres only, see, e.g., photos on thewebsites news (Alekseev, 2015; Catching of amber in the Baltic Sea,2015) and in Supplement material. Video “The Patch” (in Supple-ment material) and photo in Fig. 7, taken at the shore of the VistulaSpit on 1 October 2016, show a quite typical episode after freshwesternwinds of about 7e10m s�1 and give an impression of thesescales. The very patch in water is ca. 15e20 m in diameter only,which can be identified by the behaviour of birds: they are huntingfor fish, attracted by the cloud of seaweed, wood, and other bottommaterial. At the same time, suspended algae of much lesser, but stilla considerable amount are present in the swash zone on both sidesof the patch (over about 200 m along the shore in the given case),what can be seen on the video: the foam on the wave crests inswash zone over these 200 m has a greenish shade, being whitefarther to the right and farther to the left. This is a proxy of the scaleof the beached flotsam patch, which will be left on the shore afterthe event and may often contain many small amber stones andcrumbs. Such a scale of the patchiness says in favour of two things:(i) since the rest of the beach line is (relatively) clean, this is not justan overall wind-wave mixing which is responsible for the flotsamwashing-out, and (ii) there is some mesoscale (not a large-scale)process at work, which lifts the material from the bottom andpushes it on-shore.

Supplementary video related to this article can be found athttp://dx.doi.org/10.1016/j.envpol.2017.01.085

3.3.3. Roll structuresThe next set of observational facts (IV-VI) says that the material

is washed ashore not during the storme but right after it, and is notjust uniformly distributed along the shore, but gathered in quitecompact patches (see photos in Supplement Material, Fig. 7 andvideo “The Patch” for example). What a mechanism could compact(at 10e15 m depth) and lift from the bottom a cloud of (usuallysinking) amber stones, wooden sticks and algae, and then carry it

of Furcellaria lumbricalis and trajectories of particles in wave-induced motions.

lastics in sea coastal zone: Lessons learned from the Baltic amber,1.085

Fig. 7. Patch of marine flotsam in coastal zone, marked by birds hunting for fish, and people waiting for amber washing-out from this patch. Note flattened sea surface stretchingoff-shore from the patch location (slick), and two more slicks in deeper sea area (photo by I. Chubarenko; 01.10.2016).

I. Chubarenko, N. Stepanova / Environmental Pollution xxx (2017) 1e12 9

onshore while stormy wind is weakening, but waves are still welldeveloped? Noteworthy details, not mentioned by observers orpublications, but grasped by the photo in Fig. 7 and in the video“The Patch”, are also the flattened rows on the sea surface (theso-called slicks), stretching from the area of the patch shoalingtowards the sea, and in deeper sea area. Near the shore, thisdamping of small-scale surface waves is obviously due to thefloating algae. Farther in the sea, the slick rows may also indicatejust a water divergence at the surface, not necessarily containingany floating material. (No oil spills, which potentially could lookalike, were detected in this area.)

Summarizing these two featurese stage of windweakening (IV)and slick rows (Fig. 7) - we suggest that the most probable physicalmechanism could be the Langmuir circulation (LC): the coherentsystem of large longitudinal rolls with their axes directed down-wind, which arise due to instability of wind-induced current inpresence of the Stokes wave drift (Langmuir, 1938; Craik andLeibovich, 1976; Thorpe, 2005) (Fig. 8). Other considered possiblealternative mechanisms, such as coastal fronts (for the Baltic Sea -Lepp€aranta andMyrberg, 2008), rip currents (State and Evolution ofthe Baltic Sea, 2008), convective exchange flows (Chubarenko,2010; Chubarenko and Demchenko, 2010), do not provide properconditions for the observed features (I)-(VII). Indeed, commonlyformed longshore fronts are surface convergence/downwellingzones, thus they cannot lift the material from the bottom; stormyrip currents carry the material offshore, i.e., in the opposite direc-tion; topography-originated currents would be associated withcertain permanent locations, but this is not observed.

The LC is a quite common phenomenon observed in large lakes,seas, and oceans during windy weather. It is manifested by long“windrows” of foam and algae at the water surface, associated withthe surface convergence zones, and slick lines in-between them,indicating the surface water divergence (Langmuir, 1938; Craik andLeibovich, 1976; Thorpe, 2005; Chubarenko et al., 2010; Dethleffet al., 2009). It is known that rolls of the LC arise readily at windspeeds of more than 5m s�1 and persist at higher and higher winds- until stormy mixing destroys their coherent structure (at morethan ca. 25 m s�1) (Thorpe, 2005; Leibovich, 1983). With the“brosy”, we have an opposite case: stormy wind ceases (IV) e sothat still strong down-wind currents and complicated wave-induced motions are now able to join in the 3D-roll-shaped circu-lation, embracing the entire mixed-layer depth. Along with thesurface windrows and slick rows, this circulation implies

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convergence/upwelling zones in the lower portion of the mixedlayer (see Fig. 8), where the flotsam, suspended by waves from theunderwater coastal slope, can be gathered and up-welled (V-VI).This is just a mirror-like image to the surface windrows (Dethleffet al., 2009).

Are the LC-associated upwellings physically able to lift amberstones or microplastic particles from the bottom? The settling ve-locity of microplastics (0.5e5 mm long) particles with close-to-amber density of 1.05 g cm�3 varies between 1 and 5 cm s�1

(Chubarenko et al., 2016), while the maximum upwelling velocityin LC under moderate winds is reported (Leibovich, 1983) to be1e1.5 cm s�1. In a highly turbulent sea after the storm the currentsare obviously much stronger. Large-scale horizontal water trans-port within the LC-upwelling zones is directed down-wind, so thatthe lifted patch is to be beached at the shore directly exposed to it(V). Thus, the LC is physically able to compact a seaweed cloud withamber and plastics, lift it from the bottom and transport to theshore line. Assuming the LC-nature of the observed flotsampatchiness and knowing the separation between the upwellingzones in-between Langmuir rolls for the given situation, one canestimate the distance between the slick rows. Taking the depth-to-width ratio of an individual cell (Leibovich, 1983) as about D/W~1.3÷3, noting from the above analysis that D should be at least15e20 m in order to lift the seaweed/amber from the bottom, wearrive at the minimum spacing between upwelling zones of orderof 100 m. Since (i) these zones, resulting from instability of wind-induced current in the presence of surface wave transport, areinherently different in intensity and (ii) regions of stony bottomsfavourable for the Furcellaria grows and amber settling aredispersed e the patchiness of the beached clouds (VI) looks justnatural. By geometrical reasons, the re-suspension of the bottommaterial by the LC-mechanism should take place at some distancefrom the shore, which agrees well with field evidence (V-VI).Shoaling of the LC may modify the row spacing; however, thisprocess has not been yet investigated.

In general, either instability of currents due to wind-waveinteraction or other generating mechanism is at work, the rollstructures in the upper layer of the sea seem to be responsible forthe observed phenomenon.

4. Conclusions: Projection to microplastics behaviour

Annoying presence of microplastics everywhere in nature

lastics in sea coastal zone: Lessons learned from the Baltic amber,1.085

Fig. 8. Probable mechanism of formation and on-shore transport of subsurface patches of slightly negatively buoyant seaweed, amber and marine litter: compaction and lift offloating near-bottom material and Furcellaria, cut off from sparse rocks by stormy wave motions, is due to upwellings in-between the cells of the Langmuir circulation.

I. Chubarenko, N. Stepanova / Environmental Pollution xxx (2017) 1e1210

causes much anxiety, which is multiplied due to severe lack ofknowledge on its behaviour, especially in marine environment.Unexpectedly, an impressive natural “case study” e the phenom-enon of “brosy” of Baltic amber after severe storms e provides aunique source of valuable information to this end: it allows to pointout the most important physical factors and to suggest the mech-anisms driving migrations of plastics in the sea coastal zone.

Even though the sources of amber and plastics are different -amber enters water body from the “blue clay layer” of bottomsediments, and plastics mainly arrive from the coasts e they havesimilar densities and analogous transport properties. With present-day limited experience on MPs behaviour in the sea (Thompson,2015; Hidalgo-Ruz et al., 2012; Andrady, 2011; Ivar do Sul andMonica, 2014; Chubarenko et al., 2016; Ivar do Sul et al., 2009;Woodall et al., 2014), we, nevertheless, have already found somecommon features with those of amber. Being slightly heavier thanwater, both amber and plastics are able to migrate quite easily inmarine environment, and in both cases the location of initial sourceis proved to be not too important. Both kinds of particles arewashed ashore: the largest pieces e only after most vivid storms,whilst MPs and few-mm-long amber crumbs begin beaching afterjust fresh gales (Fig. 1).

Onemore similarity follows from the examination of state of thebeached plastic wreckage: plastic pieces on (soft) sandy beacheshave severe mechanical damages and bio-fouling that suggeststheir long residence among stones in deep sea (i.e., relatively farfrom these beaches, at depths more than 5e7 m), see Fig. 3.

Another physically based argument may have some value here.The main sources of plastics seem to be located at the coastlines(Thompson, 2015; Hidalgo-Ruz et al., 2012; Andrady, 2011). Indeep-sea bottom silts only tiny fibres - of all the kinds of plasticpollution -, are commonly reported to be in abundance (Thompson,2015; Hidalgo-Ruz et al., 2012; van Cauwenberghe et al., 2013;Lusher et al., 2015; Obbard et al., 2014; Andrady, 2011; Woodallet al., 2014). This is again in line with the behaviour of amber: wedid not meet any information that amber pieces were ever foundbelow 50e60 m depth in the Baltic Sea. When integrated with theanalysis of the features (I-VII) above, this means that amber andnon-buoyant plastics migrate between the coastline and certaindepth on underwater slope, driven by surface waves action and

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stronger currents in coastal zone. During calm weather periods,they are deposited there sincee as long as they are still large - thereare no bottom currents to carry them farther off-shore. This sug-gests an existence of a “plastic rim” around the deeper sea area.

Considering the behaviour of amber pieces as a physicallyequivalent example for negatively buoyant plastic fragments andpellets, wemay conclude that the MPs have a long and complicated“afterlife” within quite a limited depths range in-between coastsand the deep sea, being retained in the area of surface wavetransformation and coastal currents which are able to transport andre-deposit them (Fig. 9). Thrown on the beach, non-buoyant plas-tics are soon captured by waves and carried seaward (and along thecoast) as far as stormy currents can retain and carry them. Sincenon-buoyant macro-pieces tend to sink faster than smaller ones,and wind-wave currents weaken with the distance from the shoree they settle at some (hydrodynamically prescribed) depth, still inthe area of deformation of surface waves. Later, with some otherstorm, they seem to be further damaged and re-deposited withinthe sea coastal zone. They are also able to come back upslope (to theswash zone and the shore line) with some ceasing storm -e to befurther mechanically destroyed and UV-degraded, and washed tothe sea again and again during the next wind event e untilmicroplastic pieces will be small and light enough to leave thecoastal zone with stormy currents and be deposited in the deepersea, out of reach of stormy waves. Synthetic fibres, found every-where in deep sea sediments, seem to be an example of such easily-transported MPs. Fig. 9 illustrates the described cycle in moredetail, showing also bottom deformations in coastal zone due to thestorm action: equilibrium sandy bottom profiles (Dean andDalrymple, 2001; Van Rijn, 2012; Chard�on-Maldonado et al.,2015) during fair-weather conditions, under developing and un-der weakening stormy waves. This way, negatively buoyant plasticobjects should repeatedly migrate up- and down-slope within thecoastal zone, composing the “plastic rim” along the shore andexporting only very small particles (from our experience - fibresand small film flakes (Zobkov and Esiukova, 2017)) towards thedeep-sea area.

The overall conclusion is that the main, critical factors forplastics migrations in the sea coastal zone are the wind speed anddirection (fetch), wave heights, the exposure of the shore to the

lastics in sea coastal zone: Lessons learned from the Baltic amber,1.085

Fig. 9. Scheme of migrations of beach plastics in the sea coastal zone, driven by windy/stormy events: fragmentation, suspension and off-shore transport under developing stormyconditions versus deposition and on-shore move when wind weakens.

I. Chubarenko, N. Stepanova / Environmental Pollution xxx (2017) 1e12 11

wind, and the phase of wind development/attenuation. It isimportant to note that the presented analysis points on physicalfactors which are applicable to any kind of the shore in any sea/ocean, while amber stones and sandy sediments of the Baltic “casestudy” only illuminate the results of wind&wave forcing.

An ecological point is also worth noting here. The red algaeFurcellaria lumbricalis, which seems to be directly involved in theplastics migration process in the Baltic Sea, is also an importanthabitat-forming seaweed: its underwater “belts” provide spawninghabitat for many fish species, and for this reason some govern-ments even place regulations on the harvesting of this seaweed.This danger of presence in marine environment of plastics in gen-eral and microplastics in particular had not been disclosed beforethe presented analysis. May “amber tears of Heliades” help us tocope with “plastic tears” of our civilization.

Author contributions

NS collected historical and web information, prepared illustra-tions and videos. ICh performed the analysis and wrote the paper.The authors discussed and agreed on the results.

Author information

The authors declare no competing financial interests.

Please cite this article in press as: Chubarenko, I., Stepanova, N., MicropEnvironmental Pollution (2017), http://dx.doi.org/10.1016/j.envpol.2017.0

Acknowledgment

Investigations of physical and dynamical features of marinemicroplastics particles are supported by the Russian ScienceFoundation (project MARBLE, grant number 15-17-10020). Theauthors express sincere thanks to Andrey Krylov, Boris Chubarenko,Denis Smeshko, Sergey Isachenko for sharing their experience withamber washing-outs. Margarita Bagaeva kindly provided Englishlanguage assistance. We would like to thank the anonymous re-viewers for their helpful and constructive comments that greatlycontributed to improving the presentation of the material.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.envpol.2017.01.085.

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lastics in sea coastal zone: Lessons learned from the Baltic amber,1.085