Edited by: Juliana Assunção Ivar Do Sul, Leibniz Institute for Baltic Sea Research (LG), Germany
Reviewed by: Odei Garcia-Garin, University of Barcelona, Spain; Ana Luzia De Figueiredo Lacerda, Federal University of Rio Grande, Brazil
This article was submitted to Toxicology, Pollution and the Environment, a section of the journal Frontiers in Environmental Science
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
Measuring local levels of marine pollution by microplastics (MP) and identifying potential sources in coastal areas is essential to evaluate the associated impacts to environment and biota. The accumulation of floating MP at the sea surface is of great concern as the neustonic habitat consists of a feeding ground for primary consumers (including filter-feeders) and active predators, which makes these organisms a relevant via of MP input into the marine trophic chain. Here, a baseline evaluation of MP accumulation at the sea surface was conducted with a neuston net (335 μm mesh) at the Arrábida coastal area, in Portugal. The study site encompasses a marine protected area and an estuary, both under strong anthropogenic pressures due to multiple activities taking place. A short-term investigation on local spatiotemporal distribution, concentration and composition of MP was performed for the first time, through the monthly collection (summer 2018 to winter 2019) of samples at 6 stations. All the neuston samples contained MP and their mean concentration was 0.45 ± 0.52 items m−3 (mean ± SD). Both the averaged MP:neuston and MP:ichthyoplankton ratios were higher in December, when concentrations of organisms decreased. Temporal distribution patterns followed expected trends, as MP concentration was clearly higher in winter months due to precipitation and runoff. Although mean MP concentrations did not vary significantly between sampling stations, there was a spatial distribution of MP in relation to particle shape and size. Fragments were the most abundant shape and MP belonging to 1–2 mm size class were dominant. Amongst a diversity of 10 polymers identified by FTIR analysis, polyethylene (PE), polypropylene (PP) and copolymer PP/PE were the most abundant. Potential links between local sources/activities and the different polymers were suggested. Altogether, the information provided in this study aims to raise awareness among the identified sectors and consequently to act toward the prevention of MP inputs in the region.
Tackling marine plastic pollution became a major planetary challenge of the 21st century. Besides the worldwide scientific contribution to the topic for more than one decade and the increasing public awareness, governments have proven their commitment by implementing more sustainable measures and encouraging both initiatives and changes (European Commission,
Pioneer studies focused on plastic debris abundance and distribution in the marine environment inevitably verified that plastic pollution could act at a wide size range (from macro to nanoplastics), at a broad spectrum of impacts, as skin injuries or smothering from entanglement, gastrointestinal tract lesions or blockage from ingestion, and event act as vectors of pathogens and chemicals (Laist,
Regarding the origin of MP, it was considered to be either primary, if manufactured in microscopic size ranges (as industrial pellets and abrasives or microbeads from personal care products); or secondary, if resulting from fragmentation of larger objects (fishing gear, packaging, fibers from synthetic textile washing, paint flakes from nautical coating and dust from vehicle tire) (Cole et al.,
Regarding the impacts on marine biota upon MP ingestion, besides physical harm [e.g., damage in the gastrointestinal tract with inflammatory responses (von Moos et al.,
Understanding the exposure of primary consumers to MP became essential to evaluate the consequent implications in the marine trophic chain (including eventual detrimental impacts on human health due to seafood contamination). This triggered an increase in research aiming at calculating encounter rates between MP and primary consumers, based on their concentrations and ratio (Collignon et al.,
Both the Sado estuary and Professor Luiz Saldanha Marine Park, located at the Portuguese west coast, are important nursery areas for fish larvae (Borges et al.,
The study area, located on the west coast of Portugal, encompasses the south-facing coastal area between the city of Setúbal and the village of Sesimbra (
Map of the study area with the location of sampling stations at the coastal area between Setúbal city and Sesimbra village, at the Portuguese west coast. Urban tissue includes industry and tourist facilities. The isobath lines were provided by Instituto Hidrográfico and the submarine outfalls location was given by Águas do Sado (Setúbal WWTP) and SIMARSUL (Sesimbra WWTP). Map creation was based on 2 information layers: (1) land use and occupation of 2018
Established westward from the estuary, is Professor Luiz Saldanha Marine Park (hereafter marine park), a sheltered coastline from the prevailing north and north-west winds by the Arrábida mountain chain (Henriques et al.,
Six sampling campaigns were conducted from August 2018 to February 2019 (summer to winter), at 6 stations (
Name, distance from the estuary (km) and GPS (datum WGS-84) coordinates of each sampling station.
St1 | Setúbal | 0 | 38.51970 | −8.89348 |
St2 | Figueirinha beach | 5 | 38.48294 | −8.94286 |
St3 | Portinho da Arrábida | 10 | 38.46124 | −8.99428 |
St4 | Fully Protected area | 15 | 38.44652 | −9.04146 |
St5 | Sesimbra | 20 | 38.43987 | −9.09325 |
St6 | Mijona beach | 25 | 38.42905 | −9.14605 |
Following each tow, the content in the cod end container was thoroughly poured into a 250 μm stainless steel mesh sieve (where larger pieces of biological material as sticks, seagrass leaves and algae, were rinsed with filtered seawater before being discarded) and then stored in glass jars. A small aliquot (ca 50 ml) per sample was collected and preserved separately, in 100 ml of 70% ethanol, to allow the identification of neustonic organisms and the calculation of the MP:neuston and MP:ichthyoplankton ratios. The neuston samples (
Due to the considerable volume of biological material present, samples were processed according to Gago et al. (
Particle shape definition.
Fragment | Hard or soft irregular particle |
Film | Thin and malleable, flimsy particle |
Foam | Lightweight, sponge-like particle |
Fiber | Thin line, equally thick throughout its entire length, frequently curled |
Filament | Thicker and straighter than fiber |
Bead | Spherical particle |
Selection of particles for polymer identification, from all shapes (
Total of particles and number of MP selected for FTIR per shape.
Fragment | 1,480 | 220 |
Film | 557 | 26 |
Foam | 638 | 6 |
Fiber | 109 | 12 |
Filament | 61 | 27 |
Bead | 75 | 18 |
2,920 | 309 |
Polymer identification was achieved by Fourier Transformed Infrared Spectroscopy (FTIR). The majority of the particles (mainly between 1 and 5 mm) were analyzed in attenuated total reflectance (ATR) mode. Spectra were acquired using an Agilent Handheld 4300 FTIR Spectrometer with a DTGS detector, with controlled temperature and a diamond ATR sample interface; the analyses were performed at the sample surface. Spectra were acquired with a resolution of 4 cm−1 and 32 scans. For fibers and smaller particles (mainly at the 0.335-1 mm size range), analyses were carried out in a Nicolet Nexus spectrophotometer coupled to a Continuμm microscope (15x objective) with an MCT detector. Spectra were collected in transmission mode, with a resolution of 8 cm−1 and 128 scans. The spectra are shown here as acquired, without corrections or any further manipulations, except for the occasional removal of the CO2 absorption at ca. 2,300–2,400 cm−1. The identification of polymers was first made by searching in the extensive polymer spectral database of the Department of Conservation and Restoration (FCT NOVA) and the assignments were confirmed by analysis of the polymers characteristic bands (Hummel,
The airborne contamination was analyzed by exposing wet filters to the air (procedural controls; blanks), both during field (inside a hanging open glass jar, at the boat deck, one per sampling campaign) and lab work (inside Petri dishes, one at the left and one at the right of the working area, per group of 3 samples). All the fibers extracted from a sample which were similar to those found in the respective blanks (from field and lab work) were excluded from results. Sources of contamination were also minimized both during field and lab work by using glass, stainless steel and aluminum materials. Samples were kept covered at all times, both cotton lab coat and nitrile gloves were always worn, and benches and equipment were rinsed before use with Milli-Q filtered water and ethanol.
The biovolume of neuston aliquots was registered after 1 h of sedimentation in the graduated cylinders and then homogenized (manual stirring). Three subsamples of 2 ml each were analyzed under a stereomicroscope using a Bogorov counting chamber. Apart from insects, neuston organisms mainly consisted of zooplankton. Dominant groups (fish larvae and eggs, Mysidacea, Polychaeta, Chaetognata, Apendiculata, Bivalvia larvae, zoea and megalopa of Brachyura, Cladocera, naupli of Cirripedia, Copepoda, Echinodermata larvae, Amphipoda, Isopoda and Insecta), rather than individual species or genera (Di Mauro et al.,
To evaluate how the MP:neuston ratio varied temporally (along 6 months) and spatially (between the 6 stations), a Kruskal-Wallis test was performed. This non-parametric test, conducted after the invalidation of parametric assumptions, was followed by
A two-way ANOVA without replication was performed to assess whether temporal (6 campaigns) and spatial (6 stations) variation occurred in MP concentration (dependent variable). This parametric test was used after Box-Cox transformation of original data to meet normality (Shapiro-Wilk test) and homogeneity of variances (Levene test) assumptions.
The effect of campaigns and stations (fixed factors; with 6 levels each) in MP concentration of each particle shape (multivariate data) was tested by a permutational multivariate analysis of variance (PERMANOVA), with 999 permutations. Data were square-root transformed and the resemblance matrix between samples was calculated based on Bray-Curtis similarities. When differences were statistically significant, pair-wise comparisons among levels were analyzed. Then, to determine which particle shape most contributed to explain the dissimilarity amongst each pair of samples, the similarity percentages routine (SIMPER; with a cut-off percentage of 90% for low contributions) was conducted. These statistical procedures, which were conducted in the Primer 6 software with the Permanova+ add-on (Clarke and Gorley,
From the total of particles (3,317) extracted from the 36 neuston samples, 353 (11%) were discarded for being considered airborne contamination fibers and 44 (1%) were excluded after being identified as non-plastic particles by Fourier Transformed Infrared Spectroscopy. Therefore, the assessment of the temporal and spatial distribution of MP (size range 0.335 to 5 mm) was based on a total of 2,920 particles. All samples contained MP, with a mean concentration of 0.45 ± 0.52 items m−3 (mean ± SD) and 40,822.58 ± 43,578.63 items km−2. While the highest concentration per cubic meter was found in February at Figueirinha beach (St2; 2.06 items m−3), the highest concentration per square kilometer was verified at Setúbal (St1; 203558.50 items km−2). Conversely, the lowest concentration (0.04 items m−3 or 2,068.85 items km−2) was observed at Mijona beach (St6) in October (
MP concentration (items m−3) in each sample (
Among 265 particles confirmed as microplastics by FTIR analysis, a total of 10 polymers were identified (
MP number and relative abundance (%) assigned to each polymer.
176 | 66.42% | Polyethylene (PE) |
48 | 18.11% | Polypropylene (PP) |
25 | 9.43% | Copolymer PP/PE |
5 | 1.89% | Polystyrene (PS) |
3 | 1.13% | Polyvinyl alcohol (PVA) |
3 | 1.13% | Rayon |
2 | 0.75% | Polyester |
1 | 0.38% | Polyurethane (PUR) |
1 | 0.38% | Poly(acrylic acid) (PAA) |
1 | 0.38% | Polyamide (PA) |
Representative infrared spectra of the identified polymers, analyzed in transmission (left column) and ATR mode (right column); ♦ identifies the presence of kaolin. The image assigned to each spectrum corresponds to the particle analyzed by FTIR.
Considering all samples, the MP:neuston ratio was 0.0009 ± 0.0013, with the highest ratio 0.0059 (or 1:168.398) occurring in December (
MP concentration (items m−3) (red bar), neuston concentration (items m−3) (gray bar) and MP:neuston ratio (dark gray dots), in each sample (
Variation of the MP to primary consumers ratio in the sampling period.
MP concentration in February was significantly higher than those found in all other campaigns (
Variation of MP concentration (items m−3; mean ± SE) per campaign
The relative abundance of six MP shapes (
Selected microplastics from each particle shape, in neuston samples from the Sado estuary and Professor Luiz Saldanha Marine Park.
Variation of MP concentration (mean items m−3,
By decreasing order, the relative abundance of each size class (mm) was: 1-2 (36%) > 2-3 (24%) > 3-4 (16%) > 0.335-1 (15%) > 4-5 (9%). According to PERMANOVA results, MP concentration varied according to size class between campaigns (Pseudo-F = 7.69, P(perm) = 0.001) and stations (Pseudo-F = 2.55, P(perm) = 0.005). MP belonging to the 1-2 and 2-3 mm size classes explained (with more than 46% of cumulative contribution) the dissimilarities found between February and all the other campaigns and also between January and both November and October months (
Variation of MP concentration (mean items m−3,
The presence of MP in all coastal samples collected in this study is in accordance to reported MP pollution levels close to shore and to estuaries, either at Portuguese (Frias et al.,
In addition, if compared with surface waters of estuaries and contiguous coastal areas from other countries, our study area presents higher MP concentrations than those quantified by Lima et al. (
The accumulation of floating MP at the seawater surface layer leads to concerns about the exposure of neustonic organisms, such as zooplankton (including ichthyoplankton), to these synthetic particles and, consequently, of their active predators and filter-feeding biota (Collignon et al.,
As expected, the increasing tendency of MP concentration observed in winter months and the simultaneous decline of zooplankton and larval fish abundance (Cunha,
The average MP:neuston ratio verified in this work (0.0009) was low when compared to other studies: 0.002 at the Bay of Calvi (Collignon et al.,
Both temporal and spatial distribution variations were verified for MP concentration in our study site. As expected, MP concentrations increased significantly in winter months, achieving a maximum in February. This is in agreement with the reported increase of MP concentrations in marine coastal waters after storms and heavy rainfall, typically frequent in winter season for Mediterranean-type climatic conditions (Santos et al.,
Further explanations could rely on the hydrodynamics at the Arrábida rocky reef which may potentially enhance fragmentation of both MP or even larger items, by mechanical action against rocks (Eriksson and Burton,
Lastly, fragmentation enhanced during retention at the Arrábida nearshore may also contribute to export MP in the coastal drift, explaining the unexpected high concentration of MP reported further south by Frias et al. (
Bead and foam shapes presented distinct patterns in their distribution at the study area, unlike the other MP shapes. Both were predominantly collected in station 1 (Setúbal), contrasting with station 5 and 6 (Sesimbra and Mijona beach), with concentrations being higher in the January and February campaigns. The preponderance of foam shape (expanded polystyrene) in the estuary is potentially related to fisheries activities, consisting of secondary MP from the breakdown of buoys and cooler boxes for bait and catches, which despite the decrease of fishery activities during winter (DGRM,
Distribution patterns of MP according to their size were noticed both in time and space. In fact, the predominance of bigger sized MP (3-4 and 4-5 mm size) inside the estuary, the abundance increase of MP in December (beginning of winter), particularly MP belonging to the 3-4 mm size class and the high concentration of MP from intermediate size classes (1-2 and 2-3 mm) at january and mostly in February, suggest that MP inputs in this Portuguese region occur mostly close to Setúbal and mainly consists of larger particles which undergo fragmentation over time.
The preponderance of 1-2 mm sized particles among the 5 size classes, instead of the expected smallest size class (0.335-1 mm), according to Norén (
As several studies have already highlighted (Song et al.,
Polymer identification of particles in plastic pollution studies is essential to confirm visual identification processes (Löder and Gerdts,
As expected, MP pollution in this study was higher during the winter months, co-occurring with the usual decrease of primary consumers abundance in this season. The consequent increase of both MP:neuston and MP:ichthyoplankton ratios suggests therefore a critical time period for marine biota feeding in the neustonic habitat. Regarding MP spatial distribution, instead of a clear decreasing gradient from the estuary (area with higher human impact) to further coastal stations, a slight decline in concentrations was observed, suggesting a retention effect close to the Arrábida shore. Although fragments were the dominant shape, only foam and beads presented distinct variation in space, according to the location of their potential sources (fishing harbor and WWTP submarine outfall). The predominance of particles at the 1-2 mm size range instead of the smaller size range (0.335-1 mm), is suggested to be related with the sampling method used, although further studies would be required to clarify this hypothesis. The diversity of polymers reflects the multiple activities occurring in the estuary and in the marine park, highlighting the urgent need to disseminate findings locally, namely on fishing communities and in tourism, industrial and marine traffic sectors. Sharing scientific findings with society aims to increase public awareness about MP pollution and to inspire actions toward the prevention and reduction of plastic entering the marine environment.
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
DR conducted fieldwork sampling, laboratory procedures (MP extraction and characterization, neuston identification), statistical analysis, and wrote the manuscript. JA performed FTIR analysis, collaborated in the discussion and selection of the best method for MP extraction from neuston samples, provided assistance with laboratory procedures and at reporting FTIR analysis, and results. VO performed micro - FTIR analysis, assisted in the interpretation of spectra, and at reporting FTIR analysis and results. PS co-ordinated the study, discussed results, gave important contributions to the writing and to the English review of the text. MC reviewed and made important contributions to the text. All authors contributed to the article and approved the submitted version.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The authors thank ICNF collaboration, in particular to M. Henriques, for providing their patrol vessels as research platforms during sampling campaigns. We also thank park rangers C. Silva and A. Silveira, and several volunteers, especially M. D'Ambrosio, for all the assistance during fieldwork. We are also grateful to CCMAR Scientific Dive Centre for lending their boat for the August sampling campaign. We also acknowledge Instituto Hidrográfico for providing the isobaths data presented in the map of
1
2