Marine Pollution Bulletin 97 (2015) 5–12

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Marine Pollution Bulletin

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

Review

Microplastics in the marine environment: Current trends and futureperspectives

http://dx.doi.org/10.1016/j.marpolbul.2015.06.0080025-326X/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: gabrielbarbboza@gmail.com (L.G.A. Barboza).

Luís Gabriel Antão Barboza a,⇑, Barbara Carolina Garcia Gimenez b

a CAPES Foundation, Ministry of Education of Brazil, 70040-020 Brasília, DF, Brazilb Federal University of Paraná (UFPR), Center for Marine Studies (CEM), Coastal and Oceanic Systems Graduate Program (PGSISCO), Av. Beira-Mar, s/n, 83255-976 Pontal doParaná, Pontal do Sul, Paraná, Brazil

a r t i c l e i n f o

Article history:Available online 10 June 2015

Keywords:MicroplasticPlastic pollutionMarine debrisEmerging pollutants

a b s t r a c t

Over the last decade, the presence of microplastics on marine environments has become an importantenvironmental concern and focus of interest of many researches. Thus, to provide a more integrated viewof the research trends regarding this topic, we use a scientometric approach to systematically assess andquantify advances in knowledge related to microplastics in the marine environment. The papers that weused for our assessment were obtained from the database Thomson Reuters (ISI Web of Science), between2004 and 2014. Our results reveal the overall research performance in the study area of microplasticspresent in the marine environment over the past decade as a newly developed research field. It has beenrecognized that there are several important issues that should be investigated. Toward that end, based onthe suggested directions on all papers reviewed, we point out areas/topics of interest that may guidefuture work in the coming years.

� 2015 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52. Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.1. Characteristics of publication outputs, distribution of subject categories, and journals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2. Geographic distribution of the authors and productivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.3. Distribution of educational/research institutes and collaborative networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4. Final considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Appendix A.Supplementary material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1. Introduction

The accumulation of plastic and its debris in marine and coastalenvironments is due to the intense and continuous release of thispollutant into the environment (Possatto et al., 2011; de Sá et al.,2015). The improper disposal of this waste combined with itsgrowing production and inefficiency management, leaves no doubtabout its potential impact and threat to these environments.

Recently, research on the environmental impact of plastic wastehas added a new dimension with the discovery and investigation ofmicroplastic particles (Law and Thompson, 2014). The presence ofsmall plastic fragments in the open ocean was pointed out for thefirst time in the 1970s (Carpenter and Smith, 1972). In the follow-ing decades, with accumulating data on ecological consequences ofsuch debris, the topic has received increasing research interest(Andrady, 2011). However, it was not until 2004 that R.C.Thompson and collaborators coined the term ‘‘microplastics.’’ Intheir paper, ‘‘Lost at sea: Where is all the plastic?,’’ Thompsonet al. (2004) were the first researchers to use that term in the

6 L.G.A. Barboza, B.C.G. Gimenez / Marine Pollution Bulletin 97 (2015) 5–12

scientific literature, to describe microscopic plastic particles in themarine environment. Since then, growing efforts have been madeto document the presence of microplastics and their impact on thisecosystem.

Although the defined size of these microparticles has variedbetween different studies over the past decade, the NationalOceanic and Atmospheric Administration (NOAA) now definesthe term ‘‘microplastic’’ as fragments smaller than 5 mm in diam-eter (Arthur et al., 2009). In the marine environment, microplasticscan be generated from primary sources and enter directly into theenvironment as plastic pellets that were used as raw material inthe plastic industry and/or in hygiene and personal care products(Fendall and Sewell, 2009; Cole et al., 2011). They may also enterindirectly from secondary sources, such as fragments and fibersobtained from the fragmentation of larger plastic debris, resultingfrom photothermal degradation, oxidation, and/or mechanicalabrasion (Andrady, 2011).

Regardless of their origin, plastics have been manufactured tobe durable, a fact that allows them to remain for years in the envi-ronment. This is particularly true in the marine environment,where the plastic degradation may take decades (Hidalgo-Ruzet al., 2012). In the marine environment, the existence ofmicroplastic debris, including primarily classes of polyethylene(PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride(PVC) (Andrady, 2011), proliferate, migrate, and accumulate in nat-ural habitats from pole to pole and from the ocean surface to thebottom of the sea (Ivair do Sul and Costa, 2014).

With a quick and remarkable trajectory of interest shown by theacademic community, and by non-governmental and regional mar-itime organizations – that have developed programs and guideli-nes for assessing marine litter, including microplastics – theresearch in this area has been accompanied by an almost exponen-tial increase in the number of publications. Despite this, large gapsstill need to be elucidated, particularly those referring to the origin,transport, interactions, and fate of microplastics in the marineenvironment (Cole et al., 2011; Oliveira et al., 2013).

Therefore, we present herein a scientometric analysis that isaimed at systematically evaluating and quantifying advances inknowledge of microplastics in the marine environment over thepast ten years of investigations (2004–2014), thus providing amore integrated view of the research trends regarding this topic.Our aim was to answer the following questions: (i) To what extentdid the scientific literature on microplastics in the marine environ-ment grow over the past decade? (ii) In which journals were thesestudies published? (iii) What have been the questions addressed inthe studies published so far? (iv) From which part of the world arethe researchers working in this area? (v) What is the proportion ofindependent and collaborative studies in the past ten years? (vi)How many educational/research institutes are involved in researchon microplastics in the marine environment? (vii) How is the inter-national collaborative network organized between countries andeducational/research institutes? (viii) What are the gaps and/orfuture research perspectives in this area? Answering these ques-tions will enable us to construct a research profile related to thepresence of microplastics in the marine environment, which mayserve as a potential guide for future investigations.

Fig. 1. Number of papers on microplastics published between 2004 and 2014 andexpected growth.

2. Materials and methods

The study was conducted in November 2014 through a biblio-graphic survey using the Thomson Reuters database (ISI Web ofScience, apps.webofknowledge.com), and it examined papers pub-lished between 2004 and 2014. The keyword ‘‘microplastic⁄’’ wasused as search criteria to retrieve valid data records in the data-base. The asterisk at the end of the word ensured the inclusion of

the term in both singular and plural forms. We considered publica-tions on marine environments in journals covered by the ISI Webof Science, in which the aforementioned term appeared in the titleand/or keywords of the manuscripts. Therefore, in this review wenot included technical reports and academic theses.

Conventional analysis of the scientific results, subject cate-gories, journals, authors, countries, and educational/research insti-tutes was performed by using descriptive statistics, and it wasprocessed with Microsoft Excel software. The geographical world-wide distribution of the authors was plotted using QGIS softwareversion 2.6.1 (Sherman et al., 2014).

In order to observe the temporal trends in relation to the focusof researches, the variable ‘‘keywords’’ was ordered over time bythe Principal Components Analysis (PCA). To achieve this goal, inaddition to the keywords of the authors, also was used as unit ofanalysis ‘‘Keywords Plus’’ obtained through the ISI Web ofScience platform. In total 365 keywords were related. Of these,we selected 24 for analysis that had a frequency equal or greaterthan five, considering all years (2006 to 2014). The years 2004and 2005 were removed, because at the first there was no mini-mum frequency of keywords and at the second were unpublishedpapers related to the topic. To remove the influence of the numberof studies in different periods (current periods have more pub-lished papers), we organized the matrix as follows: proportion ofnumber of papers with a keyword in a given period by total num-ber of papers in the same period. The PCA was based on the corre-lation matrix (Legendre and Legendre, 1998), and was performedusing the PC-Ord 5.01 (McCune and Mefford, 1999).

The type of collaboration was determined based upon the affil-iation of the authors. The term ‘‘national collaborative articles’’ wasassigned when researcher affiliations belonged to the same coun-try/institute, whereas the term ‘‘international collaborative arti-cles’’ was assigned when articles were co-written by researchersfrom multiple countries/institutes. As visualization tools for net-work analysis, UCINET software version 6.5 (Borgatti et al., 2013)and NetDraw version 2.1 (Borgatti, 2002) were used to assess thecorrelation with regard to the international collaboration betweencountries and educational/research institutes.

3. Results and discussion

3.1. Characteristics of publication outputs, distribution of subjectcategories, and journals

From 2004 to 2014, the annual production of publicationsshowed an increasing trend for studies on microplastics in themarine environment (Fig. 1). This trend was observed mainly in2011 and 2013, when the annual growth rate of publications

Fig. 2. Journals used to publish studies on microplastics in the marine environment over the past decade (2004–2014).

L.G.A. Barboza, B.C.G. Gimenez / Marine Pollution Bulletin 97 (2015) 5–12 7

reached 6.5% and 18%, respectively. From these publications, it isclear that this increase was mainly due to the increase in the num-ber of studies carried out in microcosms. The aim of those studieswas to understand the correlation between the ecological conse-quences of microplastics exposure in the marine environment, aswell as the relationship between these particles and the adsorptionof different pollutants and/or environmental contaminants.

Studies on microplastics in the marine environment have beenpublished in a wide range of journals, many of which are consid-ered high-impact journals (Fig. 2). Over the past decade, a largenumber of publications were found in four main journals, whichtogether were responsible for 62.5% (68 articles) of all publishedarticles. These journals were as follows: Marine Pollution Bulletin(30.3%), Environmental Science & Technology (17.5%),Environmental Pollution (9.2%), and Marine Environmental Research(5.5%).

The articles published in these journals sparked interest inapproaching different questions over the past ten years of research.Briefly, the subjects that were mainly investigated by researcherswere related to the impacts of microplastics on the marine envi-ronment, their ability to interact (sorption/desorption) with otherenvironmental contaminants, the quantification and characteriza-tion of these plastic microparticles in marine and coastal environ-ments, transport routes of microplastic debris and possible placesfor their deposition, ingestion of microplastics by marine organ-isms and their potential transfer between organisms of differenttrophic levels, and the development of analytical techniques anddetection methods for the presence of microplastics in theseenvironments.

We observed a temporal trend involving the keywords associ-ated with the analyzed papers (Fig. 3). The interpretation ofFig. 3 is made from the overlap between Fig. 3a and b. As describedbelow, each number refers to a keyword. Fig. 3 shows a segregationof keywords into two distinct groups along the first axis. In the firstgroup (I and IV quadrants), related to the years 2006, 2007, 2009,2010 and 2012, the keywords are scarce and in general with broadsense (e.g., marine environment). In the second group (II and IIIquadrants), related to the years 2008, 2011, 2013 and 2014, the

keywords are more diverse and connected to more specific topics(e.g., marine debris and marine litter). This indicates that the morerecently developed research involves different areas, with a ten-dency of multi and interdisciplinary studies. Clearly, this is becauseof the growing number of publications involving the theme ‘‘mi-croplastics’’, especially in the last two years (2013 and 2014).

3.2. Geographic distribution of the authors and productivity

Regarding the research focused on microplastics in the marineenvironment, researchers from different parts of the world havecontributed over the past decade to an increase in scientific knowl-edge in the area. During this period, articles from 337 authors werepublished. The main spatial clusters of authors working in this fieldare spread over 33 countries and 135 different cities, with clearlydistinguishable clusters in the United States, Western Europe,Oceania, and East Asia, in addition to several other smaller groupsin other parts of the world (Fig. 4).

In absolute terms, the United States alone has contributed themost researchers; they can be found in 19 different Americancities. In relative terms, Western Europe houses most researchersin the field (50.8%), spread in 66 different cities. The groups ofauthors from Western European are mainly located in the UnitedKingdom, France, Germany, Spain, and Italy. Japan and SouthKorea represent the main areas where authors are located in EastAsia.

3.3. Distribution of educational/research institutes and collaborativenetworks

It is clear that collaborative networks between the differentinstitutes play an increasingly important role in scientific research;this is also the case in the research field of microplastics in themarine environment. Of the studies published in the past decade,35.8% (39) were independent articles, i.e., published by a singleinstitution, while 64.2% (70) were inter-institutional collaborativeinvestigations, including national (44% or 62.9%) and international(26% or 37.1%) collaborations (see Fig. 5).

I 1 Mediterranean sea III 11 Ingestion2 Marine environment 12 Plastic debris

II 3 North Pacific 13 Debris4 Persistent organic pollutants (POPs) 14 Transport5 Sea 15 Mytilus edulis6 Marine debris 16 Marine Strategy Framework Directive7 Particles 17 Accumulation8 Environment 18 Chemicals9 Pollution 19 Sediment

10 Zooplankton 20 Marine litter21 Coastal waters

IV 22 Microplastics23 Infrared spectroscopy24 Plastic(s)

(a)

(b)

Fig. 3. Principal Components Analysis (PCA) with Pearson correlation coefficients(a) and scores (b), showing the temporal occurrence of keywords. The first and thesecond axis of PCA accumulated 24.79% and 20.32% of the total variance,respectively.

8 L.G.A. Barboza, B.C.G. Gimenez / Marine Pollution Bulletin 97 (2015) 5–12

Over the past decade, 132 educational/research institutes pub-lished studies on microplastics in the marine environment. Thecollaborative network of active institutes established by theNetDraw program showed a complex collaborative relationship.The node size represents the degree of specific importance of agiven institute within a collaborative network (Fig. 6a). In thisway, the results show that the University of Plymouth – UK (A1)was, in the past decade, the flagship within the network, havinga strong correlation with 30 other research institutes (Fig. 6b).The University of Exeter – UK (A46), IFREMER– FRA (A25), theUniversity of Edinburgh – UK (A13), ALGALITA – USA (A4), NorthCarolina State University – USA (A45), the University of Western

Australia – AUS (A12), and AWI – Alfred Wegener Institute –DEU (A59) were the main institutes that also exhibited the ten-dency to collaborate with other research centers.

The publication outputs showed that 81.3% (26) of the countriesestablished collaborative networks among themselves. China,South Korea, Romania, Finland, Singapore, and the Netherlandswere the only countries in which the authors were not part of acollaboration. As shown in Fig. 7, the UK was the main cooperationpartner with other countries, collaborating with 17 countries. Thiswas followed by the United States, which cooperated with 13countries.

In the last decade, mainly in the past five years, the term ‘‘mi-croplastics’’ entered in the popular lexicon (GESAMP, 2015). Evenso, we recognize that the history of these small plastic particlespresent in ocean, dates back to early 1970s [see, e.g., Carpenteret al. (1972) and Wong et al. (1974)], and that other term deriva-tions may be found in the literature, such as ‘‘neustonic plastic par-ticles’’ (used by the researchers cited above and others).

Although we consider in our analysis, only journal papers cov-ered by the ISI Web of Science, worth pointing out that the consid-eration of microplastics by international agreements (e.g., RIO+20and MSFD) as a specific indicator of litter pollution, and more gen-erally the consideration of pollution by marine litter as a real con-cern (e.g., Honolulu strategy, UNEP regional actions plans, MSFDand GESAMP working group) has supported a number of studiesin the recent years (on distribution, methods, implementation ofmonitoring, etc.). This clearly may explain the increase of publica-tions. Therefore, it is important to highlight the role of politicaldecisions as a major factor in increased efforts of research andactivities, which aimed at minimizing the real problem of the pres-ence of microplastics in marine environment. Such decisionsand/or initiatives must act to understand the dynamics andimpacts of the presence of this type of marine litter, and preventingfurther inputs or reducing total amounts in the environment(Eerkes-Medrano et al., 2015).

As demonstrated in this study, there is an increased interest inresearch on microplastics in the marine environment. The concen-trations of these microparticles in the environment are beingreported worldwide (Claessens et al., 2013), including in areas thatare naturally protected [see, e.g., Batzan et al. (2014)], as well asmore remote ones (Obbard et al., 2014). Although scientific evi-dence has quickly been reported in the literature regarding the fateof microplastics and their impact on these environmental systems,many critical issues are still poorly understood (Ivair do Sul andCosta, 2014). Therefore, it is implicit that important questionsregarding production, transport, fate, and the physical and chemi-cal effects of microplastics present in the marine environmentshould be investigated (Zarfl et al., 2011).

Due to their small size and presence in both pelagic and benthicecosystems, a growing number of studies show that microplasticsare potentially bioavailable for ingestion by a wide range of organ-isms (Browne et al., 2008; Cole et al., 2011). Several studies reportthat these particles may be ingested by invertebrates, e.g., poly-chaetes, crustaceans, echinoderms, bryozoans, and bivalves, aswell as vertebrates such as fishes and birds, in addition to planktonand zooplankton organisms (Von Moos et al., 2012; Cole et al.,2013; Ivair do Sul and Costa, 2014). Thus, it has been suggestedthat the intake of these microparticles can cause direct physicalinjuries and also facilitate the transfer of chemicals to organisms,including those intentionally incorporated during the manufactur-ing process, as well as environmental contaminants that could beabsorbed on the plastic surface (Oliveira et al., 2013).

Therefore, microplastics can absorb persistent, bioaccumula-tive, and toxic substances (PBT), including persistent organic pollu-tants (POPs) and metals (Rios et al., 2007; Gouin et al., 2011). Onceingested, the absorbed pollutants may be transferred to marine

Fig. 4. Spatial distribution of the authors of papers published in the past decade (2004–2014).

Fig. 5. Independent and collaborative production of articles per year (2004–2014).

L.G.A. Barboza, B.C.G. Gimenez / Marine Pollution Bulletin 97 (2015) 5–12 9

organisms. The interactions between these compounds inside thebodies of these marine organisms may alter the distribution, bio-transformation, and/or toxicity of environmental contaminants.This may lead to an increase in the concentration of contaminantsand the potential risk for these to be incorporated into superiortrophic chains, thus threatening the health of animals includinghumans (Teuten et al., 2009; Hidalgo-Ruz et al., 2012; Oliveiraet al., 2013).

However, many of these effects and processes involving thepresence and accumulation of microplastics in the marine environ-ment still remain to be elucidated, and the long-term conse-quences are still unknown (Moore, 2008; Wright et al., 2013).Therefore, the challenge of understanding all these issues is leftto future research. Based on the suggested directions in the discus-sions of the all papers reviewed, in particular on the papers of Coleet al. (2011), Ballent et al. (2013), Depledge et al. (2013), Wrightet al. (2013) and Lusher et al. (2014), we made a synthesis of

areas/topics of interest for future research on microplastics in themarine environment, as bellows:

(1) Understand which are the marine organisms that are mostaffected by the presence of microplastics.

(2) Evaluate the presence and effects of microplastics in themarine environment through organisms used as sentinelspecies, and apply new integrated monitoring tools.

(3) Determine the impact (mortality, morbidity, and/or repro-duction) caused by microplastic ingestion by the marinebiota and evaluate what is the influence and impact of thedifferent forms and types of microplastics in marineorganisms.

(4) Understand the capacity and transport mechanisms ofmicroplastics and their contaminants through the marinefood chain via trophic interactions, as well as to estimatethe associated impact of these processes at the level of thepopulation and the ecosystem.

(5) Optimize and implement methodologies of high throughputsampling of microplastics to better compare the results ofdifferent studies and to develop methods to detectmicroplastics present in the water and sediments.

(6) Evaluate what are the ecological consequences of exposureto microplastics in marine environments, especially in criti-cal areas such as biodiversity hotspots.

(7) Evaluate the consequences of microplastics for humanhealth.

(8) Increase the knowledge about the origin, path, fate, andmicroplastic behavior in the water, including the effects offragmentation and bio-incrustation.

(9) Increase knowledge of the effects resulting from the concen-tration of additives in microplastics over time, their bioavail-ability, and the associated toxicological impact and thepersistence time of microplastics in the environment (Fig. 8).

4. Final considerations

The presence and accumulation of these microparticles in themarine environment is a current and growing concern. In recent

Fig. 6a. Collaborative network between the educational/research institutes in studying microplastics in the marine environment (2004–2014). ⁄The complete list ofeducational/research institutes can be found in Supplementary Material.

Fig. 6b. Collaborative network of the institute more active in studying microplastics in the marine environment over the past decade (2004–2014).

10 L.G.A. Barboza, B.C.G. Gimenez / Marine Pollution Bulletin 97 (2015) 5–12

years, plastic pollution in the ocean, including microplastics, hasbecome a relevant environmental concern for scientists,non-governmental organizations, rulers, and even for the lay peo-ple. Scientometric analysis showed the overall research perfor-mance in the study area of microplastics in the marineenvironment over the past decade (2004–2014), as a newly

developed research field. This shows that this field has had a con-tinuous growth of publications. Nevertheless, despite the great sci-entific advances that were made over this period, more advancedand in-depth studies are required to understand the majority ofthe questions and processes that remain unknown. Therefore, thecontribution of the scientific community by way of additional

Fig. 7. International collaborative network between countries in the past decade (2004–2014).

Fig. 8. Research aspects of interest future with regard to marine environment microplastics.

L.G.A. Barboza, B.C.G. Gimenez / Marine Pollution Bulletin 97 (2015) 5–12 11

investigations to be carried out in the coming years will be funda-mental in trying to understand the real impact of these emergingmicro-contaminants that are present in coastal and oceanicsystems.

Acknowledgments

The first author acknowledges financial support fromCAPES/Scholarship No. Bex 13568/13-2, under the Science without

Borders programme. The authors would like to thank the anony-mous reviewers for their valuable comments and suggestions toimprove the quality of the paper.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.marpolbul.2015.06.008.

12 L.G.A. Barboza, B.C.G. Gimenez / Marine Pollution Bulletin 97 (2015) 5–12

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