LETTER • OPEN ACCESS
Floating microplastic loads in the nearshore
revealed through citizen science
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LETTER
Floating microplastic loads in the nearshore revealed through
citizen science
William P de Haan1,4, Oriol Uviedo1,4, Maria Ballesteros2, I´ngrid Canales1, Xavier Curto2,
Montse Guart1, Sara Higueras1,2, Alex Molina1, Anna Sanchez-Vidal1,∗
and The Surfing for Science Group3
1 GRC Geociències Marines, Departament de Dina`mica de la Terra i de l’Ocea`, Universitat de Barcelona, 08028 Barcelona, Spain
2 Surfrider Foundation Europe, Spanish Delegation, 20012 Donostia, Spain
3 Surfing for Science Group: Olívia A´lvarez, Guillem Arrufat, Pol Bassols, Juan Bautista, Olga Bantysh, Cintia Bavera, Elisabet Bonfill,
Andrea Comaposada, Augustina Dinovska, Oriol Domingo, Neus Estrada, Sergi Franch, Laura Galeano,Martí Llorente, EduardMarquès,
Patricia Martí, Ju´lia Ortega, Bernat Peralta, Joaquim Planells, Martí Ramírez, Aurora Requena, Laura Sa´nchez, Gisela Sol`a.
4 These authors contributed equally to this work.
∗ Author to whom any correspondence should be addressed.
E-mail: anna.sanchez@ub.edu
Keywords: citizen science, floating plastics, microplastics, nearshore, coastal water, Mediterranean Sea
Supplementary material for this article is available online
Abstract
Research on plastic pollution has rapidly expanded in recent years and has led to the discovery of
vast amounts of microplastics floating on the surface of subtropical oceanic gyres. However, the
distribution of floating plastic in the ocean is still poorly constrained, and there is a lack of
information from a few meters from the coastline where the largest plastic emissions take place.
Here, we provide a comprehensive study on the loads of plastic debris in the coastal surface waters
of the NWMediterranean Sea using data from 124 manta trawl deployments collected along
7 months by citizen scientists. Our results reveal that pollution by microplastics in the nearshore is
likely subject to seasonal variations associated to a combination of hydrodynamic and
anthropogenic pressures. The high proportions of microplastics found indicate that potential
breakdown of plastics in the nearshore may take place in line with previous works. We prove that
citizen science is a powerful tool in plastic research to monitor microplastics in the nearshore as it
provides scientifically meaningful results while stimulating citizen engagement. Future studies may
benefit from targeting specific scientific open questions by using the citizen science methodological
approach presented here.
1. Introduction
Investigations on plastic pollution began in the
remote open ocean far away from the coastal zone.
(Carpenter and Smith 1972) became the first to
document what 30 years later would be known as
‘microplastics’—plastic particles smaller than 5 mm
(Thompson et al 2004)—in the Sargasso Sea. In the
decade of the 90s, large accumulations of plastics
were found floating in the North Pacific Subtropical
Gyre (Moore et al 2001), and the North Atlantic Sub-
tropical Gyre (Law et al 2010). Since then, extended
research has focused on the role of ocean gyres and
the open ocean on plastic accumulation (Law et al
2010, van Sebille et al 2012, 2015, Cózar et al 2014,
Eriksen et al 2014, Lebreton et al 2018). However,
there appears to be a major fraction of all plastic
dumped into the ocean since 1950 that is missing as it
has not been found in surveys tracking floating plastic
debris in the open ocean by now. The current estim-
ates ranging from 14.4 to 269 Mt of plastic accumu-
lated at the open ocean surface layer (Cózar et al 2014,
Eriksen et al 2014, van Sebille et al 2015) are signific-
antly lower than the predicted tens ofmillions ofmet-
ric tons that should be floating in the ocean based on
accumulated global plastic emissions (Jambeck et al
2015), although Weiss et al (2021) recently demon-
strated 2–3 orders of magnitude lower microplastic
inputs by rivers relative to current estimates.
During the last decade, several studies have sug-
gested that plastic entering the ocean likely sinks to
the seafloor, either because of negatively buoyancy
© 2022 The Author(s). Published by IOP Publishing Ltd
Environ. Res. Lett. 17 (2022) 045018 W P de Haan et al
or because biofouling increases density of floating
plastic (Woodall et al 2014, Kaiser et al 2017, Kane
et al 2020). But there are recent evidences that a
natural sorting for plastic debris is occurring in
coastal environments, where a major part of enter-
ing plastics are stranded and captured, and only a
small fraction eventually escapes and accumulates in
the open ocean (Lebreton et al 2019, Onink et al
2021). Considering the predicted growth in plastic
waste (Borrelle et al 2020), knowledge of the occur-
rence and fate of microplastics in the coastal zone
and the mechanisms governing their temporal vari-
ability and capture and release is a key challenge
that needs to be urgently addressed. This is also
required to detect potential hot-spots, current trends
in floating plastic budgets (Galgani et al 2021), con-
strain and calibrate numerical models (Kaandorp
et al 2020) and eventually assess the implementation
of regulatory and management frameworks, such as
the Marine Strategy Framework Directive (Direct-
ive 2008/56/EC), or the Single-Use Plastics Directive
(Directive (EU) 2019/904).
In 2020 we initiated the citizen science project
‘Surfing for Science’ based on floating microplastics
collection by volunteers, so-called citizen scientists,
with enhanced spatial and temporal coverage. Dur-
ing 7 months, starting from October 2020, 14 social,
environmental and sports associations distributed
along 330 km in the north-western Mediterranean
coast organized citizen scientists to acquire scientific
samples in the nearshore from paddle surf boards
or other boats with a weekly/biweekly frequency
(figure 1). This project represented a paradigm shift
in microplastic research, allowing to fill the gap in
knowledge of this transition coastal area, and act-
ively involving citizens in the acquisition of scientific
samples and the generation of new scientific data.
2. Methods
2.1. Participatory process and sample acquisition
Camins et al (2020) provided the scientific com-
munity with the design of an affordable and easy
to use manta trawl to be attached to a paddle surf
board, a kayak, or any other paddling or rowing
boat to acquire scientific samples to study micro-
plastic pollution in the nearshore. The trawl com-
prised the net, with a mesh size of 0.33 mm and
a collector bag (or cod end) at the end. The net
dimensions used were 0.38 × 0.30 m in most
locations. Exceptionally, a net of 0.34 × 0.26 m
was used in one single location (i.e. Sant Sebastia`
beach; figure 1). The Wikiloc Outdoor, S.L. free
App (Wikiloc 2019) installed in a smartphone was
used for geolocation while trawling (including lat-
itude, longitude, time, and distance trawled). Flow-
meters could not be handed out to all participatory
associations. Instead, previous work was conducted
to determine the distance shift between the track
distance measured by the GPS systems and flow-
meter measurements (figure S1 available online at
stacks.iop.org/ERL/17/045018/mmedia).
For every trawl, the citizen scientist entered the
water with the trawl on the surf board or other boat,
to which it was attached, and paddled/rowed to the
beginning of the transect. Then the volunteer star-
ted paddling for about 1 h (between 1 and 2 km;
figure 1(C)) following a consistent parallel transect
to the coast when possible. Alternatively, concentric
transects were performed in locations with beaches
bounded by breakwaters or harbors to ensure repres-
entativeness of the area (e.g. Barcelona; figure 1(B)).
After each trawl, the net was carried onboard before
reaching back to shore andGPS recordingswere inter-
rupted. Once back to the beach, the paddle trawl was
rinsed thoroughly with freshwater to ensure that all
the plastic debris ended up into a replaceable collector
bag, which was then sent directly back to the laborat-
ory at the University of Barcelona.
2.2. Sample processing
Once in the laboratory the collector bag was inver-
ted, and all debris were washed out in a 0.33 mm
sieve. Plastics were carefully extracted using stand-
ard stainless-steel tweezers under a stereo-microscope
(10× to 50×) in a clean laboratory. For all collec-
ted samples, microplastics between 0.33 and 5.00mm
and mesoplastics (i.e. 5.00–25.00 mm) were captured
with multi-staged 0.33 and 5.00 mm sieves, respect-
ively. Macroplastics (i.e. >25.00 mm) were manu-
ally extracted with forceps and carefully washed to
assure that no smaller plastics remained attached
to its surface. Plastics were then dried at room
temperature and placed separately on a 90 mm
glass Petri dish to obtain plastic count, measure-
ments and properties. Each Petri dish containing
plastic particles was scanned twice with a modified
and color-calibrated HP G4050 flatbed scanner with
a charge-coupled device sensor at high resolution
(1200 dpi; 47.2 pixels mm−1). To enhance the back-
ground contrast with plastics, a dark background was
used in the first scan, whereas a diffused and illu-
minated background was used in the second scan.
Both scans were precisely aligned with custom scan
software and processed with ImageJ v1.53 software
(Schneider et al 2012). Images were uploaded to the
Instagram social network (Surfing For Science Lab
2019) accompanied by the code of the sample and the
map of the transect performed, and the associations
involved were tagged.
Plastics were counted and the maximum length
and surface area of each particle was recorded. Plastic
items were also manually classified according to their
nature and shape in rigid fragments, flexible/rigid
films or sheets, filaments, foam, pellets and spher-
uloid or granular microbeads. Synthetic or cellu-
lose fibers were not considered in this study, as
fibers could pass through the net and external fiber
2
Environ. Res. Lett. 17 (2022) 045018 W P de Haan et al
Figure 1. The study area, showing nearshore sampling locations conducted by citizen scientists over a 330 km stretch along
north-western Mediterranean coast (A), a zoom-in on five beaches sampled in Barcelona city (B) and a volunteer conducting a
survey with the paddle trawl and a stand-up paddle board (C). The associations that provided the structure for implementing the
citizen science project where SK Kayak (Llança`), Ocean Cats (Palamós), Base Na`utica (Arenys de Mar), Escola del Mar (Montgat),
Blue Salt School (Llevant—Barcelona), Anèl·lides, Oceanogami i Base Na`utica (Mar Bella—Barcelona), Ungravity Freestyle
Company (Nova Ica`ria—Barcelona), Manihi Surf School (Barceloneta—Barcelona), Anywhere Watersports (Sant
Sebasti`a—Barcelona), Escola Garbí (Castelldefels), Club Na`utic (Comarruga), and Pl`ancton Diving (l’Ametlla de Mar).
contamination was not prevented. The mean color of
each plastic wasmeasured and automatically assigned
to the colors in the reference color palette employed
by Martí et al (2020), which recently provided a sys-
tematic method for color measurements of plastics.
The color palette categorizes 120 colors into 13 main
colors and 9 hues, besides transparent and translu-
cent, and is based on standard Pantone© RGB scores.
The Euclidean Distance between the mean color val-
ues of each plastic and the reference color palette was
used to assign the color codes to each plastic.
After image processing, each sieved size fraction
was weighted to the nearest 0.1 mg. Consistency
between digital measurements and sieving of plastics
was assured by manually transferring and reweigh-
ing all macroplastics and mesoplastics present in the
sieved microplastics fraction, thus preventing overes-
timation of microplastics due to sieve fractionation
(Michida et al 2019).
Finally, a subset of 285 random-selected micro-
plastics (∼1.1% of the total) from two samples of
each location were chemically identified using a Per-
kin Elmer Frontier FT-IR Spectrometer with a dia-
mond crystal ATR accessory at the Scientific and
Technological Centres of the University of Barcelona
(CCiTUB). Microplastics were selected introducing
an observer bias following the calculated propor-
tions of colors and nature of microplastics found
(i.e. the most common colors and shapes found were
more often selected and analyzed). FT-IR spectro-
scopy allowed the identification of the polymer com-
position of each item based on IR absorption bands
3
Environ. Res. Lett. 17 (2022) 045018 W P de Haan et al
that represent the presence or absence of specific
functional groups in the material. The spectral range
analysed was between 4000 and 220 cm−1 with a
4 cm−1 resolution and 16 accumulations. The spec-
tra collected were analysed with Open Specy v0.9.3
software (Cowger et al 2021) and OMNICTM PictaTM
(Thermo Fisher 2021) to identify the compounds
by comparing them to spectra references in library
databases.
2.3. Quality control and quality assurance
Efforts weremade to assure quality and consistency of
the data during sampling acquisition and treatment.
The project enrolled volunteers with and without
scientific background, including college and gradu-
ate students, and devoted volunteers selected by
the participating associations. Either way, project
coordinators were reachable by volunteers in case of
any eventualities in the sampling process. A sampling
protocolwas handed-out to participating associations
with instructions for app usage and GPS recordings,
approximate tow lengths and duration, sample stor-
age and net washing after each use. After sample col-
lection the collector bag was sent to the laboratory
within zip-lock bags for scientifically reliable sorting.
This is very important, as a sample contamination
during the collected debris from the collector bag to
a bottle, or volunteer bias if plastic debris are sorted
in-situ would result in a bias in plastic abundances
or characteristics. For instance, in this study >2000
plastic items were found in a single trawl, with an
average size of 2.37 mm (range 0.39–49), so stringent
and controlled sample processing in the laboratory by
professionals is needed (Bonney et al 2014).
Back in the laboratory, selection of plastics and
microplastics was carried out by a small group of
observers associated to the University of Barcelona
and the Surfing for Science project, including stu-
dents, researchers and laboratory technicians. How-
ever, samples and selected particles were routinely
verified by a single trained observer so that observer
bias was reduced. Samples, sieves, and petri dishes
were covered wherever possible to minimize periods
of exposure, and a cleanworkspacewasmaintained by
keeping all surfaces and equipment clean using eth-
anol wipes.
2.4. Data analysis
All data management and analyses were carried out
using R v4.0.5 software (R Core Team 2021) using
packages dplyr, ggplot2, stats, agricolae, car and
rstatix and mapped and spatially analyzed using
Quantum GIS v3.20 software (QGIS Development
Team 2021). Tracks recorded by the volunteers using
a smart-phone built-in GPS were manually corrected
by removing inaccurate data points falling onshore
or far from the expected trawl path. Unrealistic fea-
tures, such as small deviations in the track were
corrected by smoothing the tracks with the QGIS
built-in algorithms.
The numerical and mass (dry weight) concentra-
tions of plastic debris (items m−2, mg m−2) were cal-
culated by multiplying the manta trawl mouth length
by the tow length (determined by the flowmeter
and the Wikiloc App). Plastic abundance and mass
concentrations were log-transformed and compared
among locations using one-way ANOVA’s followed
by Tukey’s HSD post-hoc pairwise comparisons test
on the means for significant responses. Any temporal
trends were formally tested using a non-parametric
Mann-Kendall’s trend test. Wind and current speed
and direction data for each sampling location was
sampled from daily gridded data available from the
Copernicus Marine Environment Monitoring Ser-
vice (Clementi et al 2021) and the Environmental
Research Division’s Data Access Program data-sets
(Remote Sensing Systems 2021), respectively. Fur-
ther, population density (hab. km−2) around a
10 km circular buffer at each location was calculated
using data from the Statistical Institute of Catalonia
(IDESCAT 2016). The fractal dimension as a measure
of rugosity of the coastline (i.e. Compa et al (2020))
was also calculated within this buffer area. Physical
and anthropogenic forcing data was compared with
plastic concentrations using Pearson correlations. A
rolling average was calculated to test the influence
of wind and current data on the previous 14 d of
sampling. The normality of the residuals of the fitted
values was checked using Shapiro–Wilk’s test for the
fixed factors and the homogeneity of variance using
Levene’s test where applicable. The data is presented
as the mean ± standard deviation (SD) and the level
of statistical significance level was set at 5%. 95% BCa
bootstrapped confidence intervals (CI) of the mean
concentrations are reported.
3. Results and discussion
3.1. Floating plastic debris in the nearshore
Early acknowledgment of the problem posed by
plastic debris to ocean health involved observations
by citizen scientists, such as local fishermen or beach
users (Cundell 1973, Holmström 1975, Gregory
1991). Since then, our common understanding of
the sources, pathways and sinks of plastics has been
mainly attributed to growing research on beaches and
in coastal and offshore areas (Cózar et al 2014, Suaria
et al 2016, van Sebille et al 2020, Simon-Sa´nchez et al
2022). Unfortunately, far less is known about the con-
centrations, properties and composition of plastics
in the nearshore, which seems to play an import-
ant role in beaching and trapping processes glob-
ally (Morales-Caselles et al 2021, Onink et al 2021,
Sanchez-Vidal et al 2021). Over the past years, only
some studies have shared insights on how plastics
behave in the nearshore by means of numerical,
4
Environ. Res. Lett. 17 (2022) 045018 W P de Haan et al
laboratory and field studies under controlled or isol-
ated boundary conditions (Hinata et al 2017, Ho and
Not 2019, Forsberg et al 2020, Kerpen et al 2020,
Alsina and van Sebille 2021). Furthermore, only few
studies have successfully monitored floating micro-
plastics in the nearshore at constant shallow depths
and close distances from shore. For instance, van der
Hal et al (2017), Vianello et al (2018) and Compa
et al (2020), have sampled microplastics at distances
as close as 5–10m from shore and up to∼5m depths,
Pedrotti et al (2016) and Ruiz-Orejón et al (2018)
have sampled microplastics beyond ∼100 m from
shore, whereas Zeri et al (2018) and Baini et al (2018)
targeted areas beyond ∼500 m. In this study, a top-
down citizen science approach was used to engage
volunteers to collect scientific samples. The enroll-
ment of volunteers and nature of sampling equip-
ment allowed the collection of a total 129 samples
of floating plastics at previously overlooked distances
from shore and shallow depths (table 1). Five samples
(∼4%) were discarded due to inaccurate or absent
GPS tracks and excluded from the study. The total
recorded area and filtered volume of the survey was
approximately 65 264 m2 and 19 469 m3, respect-
ively. Previous work showed that flowmeter measure-
ments significantly decreased the average trawl dis-
tance by 23.1% (range 10.4–31.9), which likely had an
impact on subsequent measurements of plastic mass
and abundances (figure S1). Hereby, we discuss our
results considering them only a minimum estimate
of plastic loads in the nearshore of the investigated
locations.
3.1.1. Plastic loads and variability in the nearshore
We found a total 24 970 plastics in all samples col-
lected across locations. Among all plastics found,
93.9% were microplastics, 5.7% mesoplastics and
0.4% macroplastics (table S1). This resulted in an
overall averagemicroplastic abundance of 0.41± 0.81
items m−2 (CI: 0.31–0.61), which was close to the
total overall abundance (table 1). Average micro-
plastic mass was 0.07± 0.19 mg m−2 (CI: 0.05–0.12)
and was closely related to microplastic abundances
(r = 0.76, p < 0.001). Coastal population has been
among the main factors explaining (>60%) plastic
loads in coastal waters of the Mediterranean Sea, fol-
lowed by rivers (32%) and fisheries (6%) (Kaandorp
et al 2020). Accordingly, we found rather bell-curved
trend from north to south of the study area, and
closely followed the year-round coastal population
density profile (r = 0.45, p < 0.001 for abundance
and r = 0.30, p < 0.001 for mass; figure 2). Des-
pite statistical differences were found across loca-
tions regarding abundances (ANOVA, F11 = 3.47,
p < 0.01) and mass (ANOVA, F11 = 2.46, p < 0.01),
significant differences were only constrained to the
low microplastic concentrations found in nearshore
waters of Llança`—i.e. hosting >4.7 thousand inhab-
itants and a density of >170 hab km−2 (IDESCAT
2016)—compared to highly loaded samples collec-
ted in nearshore waters of Barcelona city—i.e. host-
ing >1.6 million inhabitants and a density of >16.4
thousand hab km−2 (IDESCAT 2016)—(figure 2).
Overall, the estimated plastic loads were around the
same levels found offshore in the central gyres of
the world’s oceans (e.g. Lebreton et al 2019, Wilcox
et al 2020), and higher than most studies conducted
within the Mediterranean Sea (Simon-Sa´nchez et al
2022), including samples taken at only ∼4 km from
shore in the study area (de Haan et al 2019). Our res-
ults depict generally higher microplastic mass loads
nearshore (r = 0.18, p = 0.04), as found by Ped-
rotti et al (2016) and Compa et al (2019), and sug-
gests that it may harbor part of the missing floating
plastic stocks globally (Thompson et al 2004, Onink
et al 2021), together with the deep-sea floor, water
column, beaches or animal lattices—i.e. see review
by van Sebille et al (2020). However, we found lower
average loads compared to previous work conducted
nearshore around the Balearic Islands (Ruiz-Orejón
et al 2018, Compa et al 2020), which seems to fit
with both influence of seasonal population increase
in summer leading to higher amounts of mesoplastics
and macroplastics found there, and the seasonal vari-
ations of hydrodynamic structures occurring in sum-
mer that may potentially retain plastics nearshore
(Macias et al 2019, Mansui 2020). Indeed, between
October 2020 and June 2021, a trend towards lower
microplastic loads was generally found towards Feb-
ruary and March—e.g. see clear trends at Llança` and
Castelldefels—, especially observed north and south
of the study area, whereas higher concentrations were
found before and after the summer months (figures 3
and S2). A decrease in microplastic loads nearshore
was associated to higher nearshore wind and current
speeds in February and March, suggesting that dilu-
tion by beaching, mixing in the water column of hori-
zontal spreading of plastics may play an important
role (figure S3). Furthermore, an increasing mono-
tonic trend in abundance was only observed for Mar
Bella (figure 3; Kendall’s tau= 0.51, p < 0.001), where
most of the sampling effort was concentrated (table 1
and figure 2). Meanwhile, plastic size also decreased
over time (Kendall’s tau = −0.44, p < 0.001), sug-
gesting effective plastic breakdown in a period of only
weeks to months and little alongshore and cross-
shore transport during the survey. We suggest that
artificial or naturally semi-enclosed beaches in the
nearshore—e.g. limited by breakwaters or harbors,
such as Mar Bella—may trap plastics more effect-
ively leading to the formation of local temporary hot-
spots. We further support these findings as higher
microplastic loads positively associated to the coastal
rugosity (r = 0.21, p = 0.01), as also previously
found by Compa et al (2020) in the nearshore of
the Balearic Islands. Furthermore, the high loadings
of microplastics commonly found in some locations,
for instance up to 5.62, 4.07 and 2.74 items m−2 in
5
Environ. Res. Lett. 17 (2022) 045018 W P de Haan et al
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6
Environ. Res. Lett. 17 (2022) 045018 W P de Haan et al
Figure 2. Abundance and mass concentrations of microplastics across locations in the north-western Mediterranean Sea (A) and
(B). Dashed lines indicate survey averages (in black), minimum (in green) and maximum (in red) microplastic concentrations.
Shared letters above each bar in (A) and (B) mean no significant difference according to Tukey’s HSD pairwise comparison test
with a 95% confidence interval.
Figure 3. Summary of microplastic abundance (A) and mass (B) concentrations of the investigated locations over the sampled
period. Note that all y-axes are log10 transformed.
Sant Sebastia`, Nova Ica`ria and l’Ametlla de Mar, sug-
gest that litter windrows (i.e. aggregation of float-
ing litter at the submesoscale domain) may also play
a role in nearshore waters by concentrating plastics
on the order of ⩾10 relative to the background
concentrations (Cózar et al 2021). For instance, this
7
Environ. Res. Lett. 17 (2022) 045018 W P de Haan et al
Figure 4. Normalized plastic size distributions across studies (A) and plastic size moving averages of 200 plastics for each plastic
shape found (B). In figure (A), data from the global ocean (red line; n= 7359) and the Mediterranean Sea (green line; n= 3901)
is obtained with a 0.2 mm-mesh Manta Trawl. Blue lines include data from the Mediterranean Sea offshore at >10 km (light blue;
n= 4469) and 1–10 km (medium blue; n= 1830) and nearshore at <1 km (dark blue; n= 8053) sampled with a 0.3 mm-mesh
Manta Trawl. Purple line includes data from nearshore waters at <1 km (n= 3145) obtained with a 0.3 mm-mesh Manta Trawl
around the Balearic Islands (Mediterranean Sea). In this study we counted a total 24 970 plastics within nearshore waters between
0.01 and 1 km (average 0.15± 0.12) from shore. Data from Pedrotti et al (2016) includes measurements from Cózar et al (2015).
Size classes in the x-axis follow a 0.1-log series of 28 size classes starting from 0.2 mm. Note that the y-axis is log10 transformed.
may be particularly important during moderate and
steady winds or near river plume fronts (Cózar et al
2021).
3.1.2. Size distributions and plastic properties in the
nearshore
Microplastics were almost exclusively found within
nearshore waters (93.9%; see above and table S1).
Plastics <1 mm represented 29.5% whereas most
plastics were within the 1–5 mm size range (table S1).
In figure 4(A) we show the particle size distributions
(PSDs) of plastics compared to other studies con-
ducted in the Mediterranean sea nearshore and off-
shore, and global ocean, which displays a power law
regime up to 1 to 0.5 mm in length before decreas-
ing towards smaller sizes. Our findings show that
the nearshore may highly contribute to the floating
microplastic oceanic stocks, as anticipated by (Onink
et al 2021). The apparent higher relative amounts
of microplastics found compared to other studies
offshore (Cózar et al 2014, 2015) may result from
the combination of effective fragmentation nearshore
(Pedrotti et al 2016) and long residence times due
to slow degradation of floating plastics in the coastal
zone, which has been suggested to drive the cur-
rent floating stocks of plastics, and likely respond
to plastic inputs from years to decades back from
now (Lebreton et al 2019, Weiss et al 2021). In any
case, comparison of PSDs shows that our results are
fairly comparable to those found by Pedrotti et al
(2016) and Ruiz-Orejón et al (2018) in nearshore
waters (<1 km), but progressively differ with offshore
waters as plastic size becomes larger. This agrees with
findings fromRyan (2015), suggesting that higher rel-
ative surface area to volume ratios of smaller plastics
may lead to increased epiphytic growth and bal-
lasting in coastal areas. We find little evidence for
expected coastal trapping of larger plastics due to
onshore Stokes drift (Isobe et al 2014). As pointed
out above, seasonality in physical conditions during
sampling may be responsible, for instance seasonal
differences in onshore Stokes drift transport leading
to increased beaching (Olivelli et al 2020), or wind-
age (Ryan 2015), potentially reducing larger floating
plastics in the nearshore.
Information about the sources can be inferred
from the types of plastics found and chemical ana-
lysis. We only found a few pellets and spherules
and microbeads (<1%), although the latter may have
not been effectively captured by the 0.33 mm-mesh
net due to their small size (figures 4 and 5). Foams—
e.g. sourced from food containers and fishing
activity—and filaments—e.g. sourced from the fish-
ing activity—constituted almost 11% of the plastics
found (figure 5). However, the vast majority of
plastics found were composed of fragments and films
and sheets (>80%), which potentially originated from
the breakdown of larger plastic pieces. We find a
strong predominance of thin films and sheets com-
pared to offshore areas, where fragments are usually
found in much higher proportions (e.g. Suaria et al
2016). This observation fits with the higher amounts
ofwrappers (39%), bags (22%) and beverage contain-
ers (11%) selectively occurring in nearshore waters
(Morales-Caselles et al 2021). Furthermore, rigid
sheets may ultimately resemble fragments as their
8
Environ. Res. Lett. 17 (2022) 045018 W P de Haan et al
Figure 5. Percentages of different properties of floating plastics (n= 24 970) sampled by volunteers in the northwestern
Mediterranean Sea. Plastics categorized by shape or category of all locations together (A) and across locations (B). A random
subset (n= 285) of microplastics chemically identified with ATR-FTIR spectroscopy (C). Finally, all colors and color hues found
associated to floating plastics including TRANS or transparent and translucent plastics without any associated color (D).
area-to-thickness ratios becomemore equal over time
due to fragmentation. Small-sized plastics following
breakdown can more easily escape nearshore dynam-
ics (e.g. Isobe et al 2014), which may partly explain
why more fragment-like plastics are identified in off-
shore waters. This can be observed in figure 5(B),
where films and sheets loose predominance towards
smaller sizes, as opposed to fragments. Differences in
local sources may also potentially explain the differ-
ences of types of plastics found across locations—e.g.
foams predominated towards the north, where also
more fishing activity takes place—(Chi squared test,
p < 0.001; figure 5(B) and table S1).
Figure 5(C) shows that almost 70% of micro-
plastics were identified as low and high density poly-
ethylene (PE), followed by polypropylene (PP) and
polystyrene (PS). Polyolefins (PE and PP) are the
most produced polymers in Europe with over 49%
demand, and used mainly in single-use food pack-
aging and automotive parts (Plastics Europe 2020),
which reasonably fits with our findings. Togetherwith
PS (in expanded form), which is mainly used in food
packaging, fishing or insulation, these polymers are
positively buoyant in seawater (ρ < 1.02 g cm−3) and
consistently represent a great fraction of the micro-
plastics found floating in nearshore waters in the
Mediterranean Sea (Zeri et al 2018, Compa et al
2020), and elsewhere globally (Suaria et al 2016).
Interestingly, a few polymers were found with higher
density than seawater (ρ > 1.02 g cm−3), which may
be explained by a combination of intensive mix-
ing by waves and wind causing them to surge to
the surface, and closeness to coastal sources. These
added to 6.1% of the total, including polyethyl-
ene terephthalate—e.g. mainly used in plastic bottles
and cosmetic packaging—, acronitrile butadiene
styrene—e.g. required in multiple shock-absorbing
or heat-resistant applications—, nylon—e.g. used in
fishing gear—and others (<1%), including ethylene-
vinyl acetate and PE/PP co-polymers.
Finally, over half out of the 120 colors of the refer-
ence palette were used for classification. Transparent
and translucent plastics were most frequently found
(65.3%), as they were associated to films and sheets,
spherules or microbeads and pellets (see above).
Plastics of different tones of gray, including black
and white followed (23.9%). Other colors, grouped
into bluish-green (i.e. sky, blue, cyan, turquoise and
green), amber (i.e. yellow, brown and orange) and
reddish (i.e. red, pink, violet and magenta) were less
frequent, accounting only 6.8%, 2.8% and 1.2% of
colors, respectively (table S1). Overall, the preval-
ence of transparent and translucent plastics is similar
to findings of Martí et al (2020) globally (i.e. 47%).
Conversely, we found less variety of colors in the
nearshore than they found floating in enclosed seas
at ∼5 km from shore. However, the low preval-
ence of amber-like plastics associated to yellowing
or oxidative stress aligns with views that plastics in
the nearshore may be relatively short-lived before
9
Environ. Res. Lett. 17 (2022) 045018 W P de Haan et al
reaching remote areas (Andrady 2011, Martí et al
2020). In any case, it is unclear how different colors
can impact the ingestion rates of plastics by marine
organisms, especially in coastal areas where marine
biodiversity peaks. Recent findings show transpar-
ent and translucent and black plastics prevail in fish
(∼40% and 30%), whereas white and blue plastics
prevail in turtles (∼67% and 40%) and cetaceans
(∼38% and 30%), which may respond to differ-
ent feeding strategies or environmental exposure of
organisms (see review by López-Martínez et al 2021).
3.2. The success of citizen science in plastic
pollution studies
Citizen science in plastic pollution studies is not new,
successful programs have incorporated volunteers to
provide repeated sampling for time series as well as
synoptic collections over wide geographic regions,
and providing access to unexplored sites (van der
Velde et al 2017, Zettler et al 2017). These programs
have provided key contributions to the understanding
of the impact of plastic pollution on themarine envir-
onment, and even thoughmost projects have focused
onmacroplastics—as they are more easily sampled—
microplastics have been recently in the spotlight.
Beach surveys and clean-up initiatives such as
‘International Coastal Cleanup’, ‘World CleanupDay’,
‘Marine Litter Watch’, or ‘International Pellet Watch’,
have increased our understanding on the nature and
extent of plastic pollution in beaches at regional
and global scales (Takada 2006, Storrier et al 2007,
Hidalgo-Ruz and Thiel 2015, Bergmann et al 2017,
Bosker et al 2017, 2017, Lots et al 2017, Nelms et al
2017, Haarr et al 2020, Roman et al 2020). The
shallow seafloor has been investigated by volunteer
divers of the ‘Dive Against Debris’ initiative that
have provided scientific information on the abund-
ances and characteristics of underwater litter (Consoli
et al 2020). Furthermore, integrated large data-sets of
marine litter collected by citizen science programs in
rivers, beaches and shallow seafloors have provided
a first overview of the origin, transport and ultimate
fate of litter items as small as 0.5 mm up to 1 m in the
global ocean (Hidalgo-Ruz and Thiel 2013, Rech et al
2015, Morales-Caselles et al 2021), and entanglement
and ingestion of plastic debris by wildlife (Wilcox et al
2016) including fish destined for human consump-
tion (Liboiron et al 2016) has also been documented
through citizen science initiatives.
In contrast, there are less citizen science contri-
butions to monitor plastic floating in the sea surface.
The Trawlshare program developed by the 5-Gyres
Institute engages sailors to collect scientific samples
across the five subtropical gyres. Samples are sorted
onboard using simplified protocols (Trawl For Plastic
2021). The same team of researchers went on sev-
eral expeditions through all 5 subtropical gyres and
published the first global estimate of all plastic of all
sizes floating in the world’s oceans, totaling 270 000
metric tons from 5.25 trillion particles (Eriksen et al
2014).
There is an ongoing need to provide scientific-
ally robust data sets on floating plastic abundances
and characteristics making use of the citizen science,
as it provides clear advantages over conventional sci-
ence by operating at greater geographical and tem-
poral scales, reducing project costs and raising aware-
ness (Paradinas et al 2021). Citizen science not only
empowers volunteers as they participate in the col-
lection of valuable scientific data, but it also provides
health benefits and well-being by spending time and
interactingwithmarine and coastal ecosystems (Borja
et al 2020, Britton et al 2020).
4. Conclusions and final remarks
In the present work, a top-down citizen science
approach was used to determine microplastic loads
in the nearshore, an area previously undersampled
by the scientific community. We demonstrate along
with previous works around the globe that citizen sci-
ence is a reliable and powerful tool that can provide
invaluable scientific data at long and wide spatiotem-
poral scales. Accordingly, our findings show con-
siderable microplastic pollution in nearshore waters,
and a high spatial and temporal variability. A trend
with decreasing loads towards February and March
could be observed and was likely conditioned by local
hydrodynamic conditions, such as surface currents
and winds lowering floating microplastic loads. Fur-
thermore,microplastic concentrates in the nearshore,
either because of effective breakdown, coastal rugos-
ity or seasonal changes in hydrodynamic conditions.
The variety and closeness of sources in the nearshore
and population density further contributed to the
high variability of concentrations found. Our data,
together with data on estimated plastic inputs from
land and rivers, and from beach and offshore surveys
can been used to constrain numerical models, which
ultimately contribute to develop comprehensive risk
assessments and our understanding of the cross-shore
and vertical transport pathways, sinks, beaching and
fragmentation processes of plastics and microplastics
in the marine environment (van Sebille et al 2020).
Samplings did not target specific sub-domains within
the nearshore, such as the outer or inner nearshore
(i.e. swash and surf zone), which is often affected by
different turbulent motions affecting plastic abund-
ance and mass (Ho and Not 2019). However, in the
future, more targeted surveys demanding particular
needs to answer specific scientific questions around
plastic occurrences, distribution and dynamics in the
nearshore may also benefit from the citizen science
approach presented here.
10
Environ. Res. Lett. 17 (2022) 045018 W P de Haan et al
Data availability statement
The data that support the findings of this study are
openly available at the followingURL/DOI: 10.34810/
data149.
Acknowledgments
We sincerely thank all volunteers involved in sample
collection. The success of the Surfing for Science pro-
ject is driven by their efforts andmotivation to paddle
for about 1 h even in winter conditions. We thank the
Oceans and Human Health Chair and Rycsa.cat for
their support with the trawls. We would also like to
extend our gratitude to Guillermo Asensio and Vidal
Martínez from Asensio Comunicació Visual for their
amazing creative support, and Nu´ria Ferrer and Pilar
Hermo from the Scientific and Technical Centres of
the University of Barcelona for technical assistance
with the FT-IR spectrometer. This research has been
supported by the Fundación Española para la Cien-
cia y la Tecnología (FECYT) Project FCT-19-14747,
the Ajuntament de Barcelona Grant 20S02426-00,
the Facultat de Ciències de la Terra, and a Catalan
Government Grups de Recerca Consolidats grant to
GRC Geociències Marines (ref. 2017 SGR 315). The
authors declare nofinancial and non-financial com-
peting interests.
ORCID iDs
William P de Haan https://orcid.org/0000-0002-
4192-191X
Oriol Uviedo https://orcid.org/0000-0001-7362-
3629
Maria Ballesteros https://orcid.org/0000-0003-
0627-6537
Xavier Curto https://orcid.org/0000-0003-4193-
6190
Montse Guart https://orcid.org/0000-0002-1313-
6068
Alex Molina https://orcid.org/0000-0002-7155-
353X
Anna Sanchez-Vidal https://orcid.org/0000-0002-
8209-1959
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