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Article

Chemical and Physical Characterisation of Microplastics Present on Beaches of the Cantabrian Coast, Bay of Biscay (Spain)

by
Uxue Uribe-Martinez
1,2,*,
Thomas Maupas
1,2,3,
Aritz Lapazaran
1,
Ruben Rodriguez
1,
Olivia Gómez-Laserna
1,
María Ángeles Olazabal
1,
Juan F. Ayala-Cabrera
1,2 and
Alberto de Diego
1,2,*
1
Department of Analytical Chemistry, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Barrio Sarriena s/n, 48940 Leioa, Spain
2
Research Centre for Experimental Marine Biology and Biotechnology (Plentzia Marine Station, PiE-UPV/EHU), University of the Basque Country (UPV/EHU), Areatza Hiribidea, 47, 48620 Plentzia, Spain
3
CNRS, IPREM, Université de Pau et des Pays de l’Adour, 2AV du Président Pierre Angot, 64000 Pau, France
*
Authors to whom correspondence should be addressed.
Hydrology 2025, 12(11), 298; https://doi.org/10.3390/hydrology12110298
Submission received: 9 October 2025 / Revised: 5 November 2025 / Accepted: 7 November 2025 / Published: 10 November 2025
(This article belongs to the Special Issue Recent Research Advances in Microplastics in Water and the Environment)
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Abstract

We investigated the presence, chemical/morphological characteristics, and distribution of microplastics (MPs, 1–5 mm) in three beaches located at the southeast of the Bay of Biscay, an area where this kind of study is scarce. Sampling was carried out in March 2022/2023 and October 2023/2024. The microplastics found were chemically characterised by Raman spectroscopy and morphologically described (size, shape, and colour) by visual observation. A total of 836 MPs were found, with Atxabiribil beach showing the highest mean concentrations (15 MPs kg−1), followed by Sonabia (10 MPs kg−1) and Gorliz (3 MPs kg−1). The highest concentrations were recorded in March 2023 and the lowest ones in March 2024, with no clear seasonal trend. Foam, fragments, and pellets were dominant, although filaments, films, and fibres were also found. White MPs were the most abundant, followed by blue and black items. Polyethylene, polypropylene, and polystyrene, in this order, were the most common polymers. In conclusion, we report here valuable information about the abundance and characteristics of MPs in beaches located in an area poorly investigated previously. The results obtained underline the importance of the implementation of regular monitoring campaigns to estimate the impact and consequences that plastic pollution has in our coastal environments.

1. Introduction

The risks posed by plastics to the marine ecosystem and human health are a consequence of decades of production without proper management and disposal of these materials [1,2]. It is estimated that more than 10 million tonnes of plastic waste are dumped into the oceans annually, and a large proportion of this is single-use plastics, which account for around 50% of total production [3,4,5].
When they are released into the natural environment, a degradation process initiates that can take hundreds to thousands of years due to their polymeric and hydrophobic nature. Macroplastics (<25 mm) break down into smaller fragments or particles, called microplastics (MPs, <5 mm) [6]. Although the major sources of MPs are those from the shredding of plastic waste, considered secondary MPs, plastic parts are also released that are manufactured directly in the micrometre size to meet their intended application (primary MPs) [7].
MPs are ubiquitous in different environmental compartments around the world, such as the atmosphere [8,9], soil [10,11], water [12], and sediments [13,14]. In addition, several studies have demonstrated the presence of MPs in different organisms, from small organisms such as copepods or mussels [15], fish [16,17] to birds, marine mammals [18], or even humans [19,20], which can cause hazardous effects. They have also been found in remote areas such as polar regions or deep waters [21,22,23]. In addition to the direct consequences that ingestion of these particles can have, MPs can act as vectors of contaminants, either by releasing compounds that have been added during polymerisation or by adsorbing contaminants found in the environment [24,25,26].
Coastal environments host a large number of marine and terrestrial species and are therefore of great economic and ecological importance. More than 30% of the planet’s coasts are sandy beaches that are used by the population due to their economic, ecological, recreational, and cultural value [27]. Beaches are highly affected by plastic waste. Much of the waste from anthropogenic activities arrives from rivers, estuaries, or land that, due to currents and waves, ends up accumulating on beaches, causing massive damage to these ecosystems. Therefore, these environments are major indicators of coastal plastic pollution. The concentration of MPs on beaches varies considerably between regions, largely due to the unequal contribution of continents to plastic debris reaching the sea. Several studies, such as those compiled by Simantiris & Vardaki (2025) [28], show widespread pollution on the coasts of Europe and other continents. Asia is the main emitter, responsible for 81% of the total, with extremely high levels on beaches in countries such as Taiwan (up to 53,200 MP/kg) and Guam (19,300 MP/kg). North and South America contribute 10%, with outstanding concentrations in Canada (25,000 MP/kg), Bermuda (18,800 MP/kg), and Mexico (13,392 MP/kg), although other areas such as Brazil and Alaska show low levels. On the other hand, Africa contributes 8% and shows high values in Morocco (34,200 MP/kg) and South Africa, but very low values in countries such as Ghana (0.03 MP/kg). In contrast, Oceania, with less than 0.4% global contribution, shows the lowest concentrations even in Australia (1.2–350 MP/kg) and New Zealand (<50 MP/kg). Europe contributes less than 1% of the total amount of poorly managed plastic waste to this environmental concern. Many investigations focus on the Mediterranean Sea, where levels range from 137 to 900 MP/kg depending on the region, with the Cabrera Archipelago National Park (900 MP/kg) and the Adriatic Sea (up to 392 MP/kg) standing out. However, the highest European concentration is found in the North Sea, with a record of 496,000 MP/kg in the East Frisian Islands (Germany). High levels have also been detected in the Azores (7500 MP/kg), while other areas such as Russia and the UK show more moderate figures [28,29].
In the case of the Bay of Biscay (BoB), a marine region located in the northeastern Atlantic Ocean that joins the coasts of northern Spain and western France, studies based on oceanographic models identified this region as an area of marine litter accumulation [30,31], due to its particular oceanographic dynamics, which, supplemented with field data, revealed that the BoB presents a moderate level of MP pollution. Even so, information on the distribution and total amounts is limited [32]. In the case of MPs studies on beaches, from 2013 to 2022, several beaches along the northern coast of Spain, south of the BoB, were monitored through CEDEX (Centre for Public Work Studies and Experimentation, Spanish Ministry of Development), including four BoB beaches: Covas (Galicia), Oyambre (Cantabria), Itzurun (Zumaia, Gipuzkoa) from 2019, and Frejulfe beach (Asturias) from 2021. The results revealed a high spatial and temporal variability, finding concentration ranges from 2016 to 2022 of 0.97–44.31 MP/kg, 1.70–24.35 MP/kg, and 2.99–94.77 MP/kg in Oyambre, Covas, and Itzurun, respectively [33]. Additional data from the northeastern BoB were obtained by Phuong et al. (2018) [34], who characterised MP in intertidal sediments from three beaches in the Pays de la Loire region (France): Pen-Bé, Coupelasse, and Aiguillon Bay. Two studies were conducted in October 2015 and March 2016 showing concentrations of 67 MP/kg with no significant differences between sites and stations [32].
The BoB is home to a dense coastal population of more than 18 million inhabitants, with important industrial, port, tourist, and agricultural activities, which generates a strong pressure on the marine environment. Based on its population, there is an estimated annual consumption of 1.76 million tons of plastics, which reinforces the interest in studying their environmental impact, especially pollution by MPs. Although studies on this area increased in recent years, especially in surface waters [32], coastal environments such as beaches, which are environments of great ecological, economic, and touristic interest, remain relatively understudied. Therefore, the aim of this study was to evaluate MP pollution on three beaches in the southeastern BoB. To this end, (i) the abundance of MPs on each beach was quantified, (ii) seasonal variability was assessed, and (iii) the types and morphologies of dominant polymers were identified. The three beaches are located near the Nerbioi-Ibaizabal estuary, an area where industrial, port, and fishing activities have been fundamental to the development of the region. However, this intense human activity has also caused significant pressure on the environment, especially on coastal waters and ecosystems, which have not been studied in depth in relation to microplastic pollution [35]. Four sampling campaigns were conducted from the fall of 2022 to the spring of 2024 to investigate the presence of MPs, as well as their chemical and morphological characteristics, in these coastal environments.

2. Materials and Methods

2.1. Study Area

In this work, the presence of MPs between 1 and 5 mm was investigated in three beaches (Sonabia, Atxabiribil, and Gorliz) located on the Cantabrian coast, southeast of the BoB (Figure 1).
The tourist season in this region, specifically the bathing season, begins on 1 June and ends on 30 September [36]. However, given that favourable weather conditions usually begin between April and May, it is common for some people to start frequenting the beaches before the official start of the season. For this reason, sampling campaigns were carried out in October 2022, March and October 2023, and March 2024, with the aim of adequately representing the conditions before and after the period of greatest recreational use. Thus, the March sampling campaigns corresponded to a time prior to the start of the official season and before the increase in visitors, while the October sampling campaigns were carried out after the end of the bathing season, when the number of beach users decreases significantly.
Sampling was not conducted on the beach of Gorliz in October 2022. These beaches were selected based on the amount and type of the anthropogenic activities carried out in their surroundings, such as industry, tourism, urban development, and harbour activities, among others. Table 1 summarises some characteristics of the beaches potentially linked to the presence of MPs on them. All of the beaches have a low to medium slope and fine sand. From west to east, Sonabia is a 125 m long beach oriented to the northwest, influenced by the general dynamics of the BoB, although partially protected by rock formations. It is characterised by low urban pressure and medium tourist pressure, mostly related to aquatic sports. The beach can be reached on foot by a steep path about 500 m long, not adapted for vehicles. Atxabiribil is an open beach oriented to the northwest where the level of exposure to the waves is high. It is a semi-urban beach that, except at high tides, is linked to Arrietara Beach (826 metres long in total), and is affected by considerable tourist pressure. It is accessible by road and has facilities for beach users (showers and parking area, etc.). As is the case with most urban beaches in the Bizkaia region of the Basque Country, Atxabiribil Beach is included in the beach cleaning and conditioning system. This means that it is cleaned every day during bath season. Due to the proximity to the mouth of the Nerbioi-Ibaizabal estuary and the predominant surface currents to the east, Atxabiribil Beach can be influenced by the waters from the estuary and the Port of Bilbao. Finally, Gorliz beach is 842 m long and is also oriented to the NW. It is located in a semi-enclosed bay with a low degree of exposure to waves. It borders the Plentzia beach at its western end and is relatively close to the mouth of the Butroe estuary. This semi-urban beach, with a high number of bathers, is also included in the cleaning and conditioning system for the beaches of Bizkaia. It also has road access and user facilities. This beach can be influenced by discharges from the Gorliz WWTP (Wastewater Treatment Plant, 17,288 population equivalent), which discharges into the coastal area via a submarine outfall approximately 1 km from Gorliz beach, by the waters of the Butroe estuary, and by the Plentzia marina, which is located at the outlet of the estuary [37,38,39].

2.2. Sample Collection and Processing

The sampling was carried out following recommendations proposed by the Guidance on Monitoring of Marine Litter in European Seas guidance document [40]. At each beach and campaign, 5 samples (replicates ‘R1’ to ‘R5’) were taken at 20 m separated from each other in a 100 m transect at the high tide line, using a 50 × 50 cm quadrant (2500 cm2) (Figure 2). The surface layer of sand (2 cm thick) from each quadrant was collected (about 5000 cm3 = 5 L) using a stainless-steel spatula and sieved through a 1–5 mm mesh, using pre-filtered seawater (200 µm) to facilitate the procedure. After sieving, the material retained on the 1 mm sieve was quantitatively collected using a metal brush and a metal spoon and stored in a glass bottle to transport them to the laboratory and store them in freezer at 4 °C until analysis. In order to estimate possible contamination due to atmospheric inputs and clothes of the operators, procedural blanks were made by passing a similar amount of pre-filtered seawater through the sieves and collecting the remaining material in the 1 mm sieve. In addition, an additional portion of approximately 200 mL of sand was collected from each beach and campaign in order to estimate its density. To do this, the sand was transported to the laboratory and dried at 50 °C in an oven. A 50 mL portion of dry sand was measured using a volumetric flask, and its mass was measured on a balance. The concentration of MPs throughout the text is expressed in items/kg of dry sand. The conversion between the volume of wet sand (in litres) and the mass of dry sand (in kilograms) was performed taking into account the estimated density as described above and assuming that the water content of the sand sieved in situ was negligible.
Once in the laboratory, the material collected in each sampling campaign/site (that retained on the 1 mm sieve) was subjected to a density separation process to remove dense, non-plastic material. For this purpose, an all-glass phase separator, detailed in Maupas et al. (2024) [14], was used with a saturated NaCl solution (density approximately 1.2 kg L−1), as most of the MPs found in beaches are floating material of marine origin, less dense than seawater. This is the case of polyethylene (PE), polypropylene (PP), and polystyrene (PS). After separation, the floating material was filtered through a 1 mm mesh stainless steel filter, thoroughly washed with Milli-Q water, and quantitatively transferred to a glass Petri dish. The samples were dried in a desiccator, and clearly non-plastic materials such as pieces of wood, shells, algae, and vegetation debris were visually identified and removed using metallic tweezers. The remaining material was considered to be candidate MPs. All material used during sampling and sample treatment was made of glass or metal, previously washed with Milli-Q water and acetone. Finally, all candidate MPs were chemically characterised by Raman and/or FTIR spectroscopy as described below, confirmed or not as plastic material, and confirmed MPs (836 particles were finally identified as plastic items) were counted and classified in terms of shape, colour, and polymer type. The chemical structure characterisation was carried out using an InnoRaman ultra-mobile Raman spectrometer (B&WTEKINC, Newark, NJ, USA). The spectra were measured with a laser wavelength of 785 nm, in a range from 100 to 3000 cm−1, with acquisition conditions of 5 s, 5 scans, and 60% laser power, although these conditions were modified according to the need of the matrix. However, in some cases and due to the colour of the particle, the pigment masked the polymer signal, so the identification by Raman spectroscopy was not possible. In these cases, the chemical characterisation was carried out using Fourier-Transform InfraRed (FTIR) Spectroscopy combined with Attenuated Total Reflectance (ATR) sampling mode (Jasco 6300 FTIR, Tokyo, Japan). The spectra were measured in a wavelength range from 600 to 4000 cm−1, with a resolution of 4.0 cm−1 and 100 scans for the acquisition conditions. In both cases, MPs were identified by comparing their Raman or FTIR-ATR spectra with those of a polymer reference. Figure S1 of the Supplementary Material shows the spectra obtained by Raman spectroscopy and FTIR-ATR of the most common polymers found on beaches, used as a reference for sample identification. Sampling blanks were processed in the same way as the samples, in order to estimate possible contamination due to atmospheric inputs and the operators’ clothing during the sample treatment and analysis steps. No MP was detected in these blanks.

3. Results

3.1. Presence of Microplastics on Beaches and Its Evolution over Time

The results showed significant amounts of MPs on all beaches and in all sampling campaigns. The number of MPs found in each of the five replicates (R1–R5) did not follow a systematic behaviour on any of the beaches and campaigns (i.e., any systematic increasing or decreasing trend along the beach, such as from east to west or vice versa, were found, as it was initially expected). Consequently, MP data from each beach and campaign are given as the total sum of MPs recovered from the five replicates. The values for the individual replicates are detailed in Table S1 of the Supplementary Materials.
A total of 836 particles between 1 mm and 5 mm were identified as MPs in the three beaches (309 in Sonabia, 466 in Atxabiribil, and 79 in Gorliz). Considering all three beaches at the same time, in October 2022 (2.9 ± 2.8 MPs kg−1) and March 2023 (2.8 ± 2.6 MPs kg−1), a higher average concentration of MPs was found than in October 2023 (1.9 ± 2.4 MPs kg−1) and March 2024 (0.5 ± 0.3 MPs kg−1). Similarly, considering all the four campaigns at the same time, the results showed that Atxabiribil was the beach with the highest mean concentration of MPs (2.9 ± 2.8 MPs kg−1), followed by Sonabia (1.9 ± 1.9 MPs kg−1) and Gorliz (0.7 ± 0.5 MPs kg−1). However, due to the high variability found among replicates, beaches, and campaigns, as reflected in the high plus/minus values associated with the average concentrations, the difference among them is not statistically significant (as verified by performing a 2-factor (beaches and campaigns) ANOVA with 5 replicates per beach/campaign, p-value = 0.05, Fcampaign = 0.65 (Fcrit = 4.76), Fbeach = 1.14 (Fcrit = 5.14)). In Table S1, the concentrations in MPs kg−1 obtained for each site and campaign are also reported.
The evolution over time of the amount of MPs found on each of the beaches (Figure 3) revealed different patterns. In Sonabia, the highest concentration occurred in October 2023, with a continuous increase from October 2022 onwards and a sharp decrease in March 2024. Atxabiribil systematically showed higher amounts of MPs in March than in October, but the presence of MPs was dramatically lower in October 2023–March 2024 than in October 2022–March 2023. Concerning Gorliz (no sampling in October 2022), the number of MPs remained low and constant in 2023, and only one MP was recovered in March 2024.

3.2. Chemical and Morphological Characterisation of Microplastics

The classification of MPs according to their shape and colour was carried out by visual inspection of each individual item. Most of the MPs detected were fragments (51%), but pellets (26%) and foams (23%) were also identified. 2% of the MPs were classified as ‘other’, which includes filaments, films, or fibres. Pellets are considered primary MPs (emitted to the environment in the MP form), while all other forms are secondary MPs (resulting from the fragmentation of larger plastics) [7]. Hence, about 25% of the MPs identified in this work were primary MPs and the remaining 75% were secondary MPs. The distribution by shape obtained in each beach and sampling campaign is shown in Figure 4.
As it can be observed, fragments were the only category detected at all sites and all sampling campaigns, being the most abundant, except in October 2022 and March 2024 on Atxabiribil, and in October 2023 on Gorliz and October 2023 on Sonabia, where pellets and foams, respectively, were predominant. In Sonabia, fragments were clearly dominant in the March campaigns, while their presence was significantly reduced in favour of foams in the October campaigns. A similar trend is observed in Gorliz, but, as data from October 2022 is missing and in March 2024 only one MP was found, it would be necessary to confirm this behaviour with the results of new sampling campaigns in the future. Distribution by shape in Atxabiribil changed from one season to another. While pellets were dominant in October 2022 and March 2024 (something that has only been observed on this beach), and fragments in March 2023, a similar amount of fragments and foams were detected in October 2023, with an unexpectedly high percentage of MPs catalogued as “others” in this campaign.
Regarding classification of microplastics by colour, white MPs were in the majority, followed by blue and black MPs (Figure 5).
Other colours that were found occasionally or represent less than 5% of the total (such as green, red, pink, or grey) were grouped under the category “others” (Table S2). This category was more abundant than black or blue ones in most cases. An increase in the percentage of black MPs was observed in the October campaigns in Atxabiribil and Gorliz. However, as no data are available for October 2022 in Gorliz, a systematic trend cannot be confirmed. In March 2024, only white MPs were found in Atxabiribil and Gorliz, while in Sonabia fewer white MPs than in previous campaigns were identified, although this colour still represented more than half of the particles recorded. All the foams found were white, while there were fragments of all colours, and the pellets were mainly black and white.
As expected, Raman and FTIR analysis identified the following three types of polymers: polyethylene (PE), polypropylene (PP), and polystyrene (PS). All these polymers were present on all beaches and sampling campaigns, in different proportions, except on Gorliz in March 2024, where only one PP white fragment was found (Figure 6). PE was always dominant, except in Gorliz, where PS was the most abundant polymer in October 2023. In October 2023, a significant increase in the proportion of PS was observed on all three beaches, becoming the majority polymer in Gorliz (60%) and increasing the percentages to 38% and 36% in Sonabia and Atxabiribil, respectively.
A simultaneous consideration of polymer type and shape (Figure 7) revealed that all foams were PS. Most of the fragments and pellets found were PE and, to a lesser extent, PP. A similar distribution was found in the category “others”, which includes filaments, films, and fibres.

4. Discussion

4.1. Presence of Microplastics on Beaches and Its Evolution over Time

All these results suggest that, contrary to what might be expected, there is no general pattern to all beaches in terms of MP accumulation on them. It seems more likely that the accumulation of MPs is very specific to each beach, and depends on the specific effect of general factors such as geographic (orientation, slope, size, protection from direct ocean influence, and proximity to sources of MPs), climatological (wind direction and speed, rainfall, and storms), or hydrodynamic (main currents, waves, and sea state). In this sense, the absence of significant seasonal variations in the abundance of microplastics observed in this study coincides with that described for the Bay of Biscay, where studies indicate that seasonality does not fully explain the variability in the distribution of floating debris, with short-term processes and local inputs being more decisive factors [30,31].
The concentrations of microplastics (average of all campaigns) found in this study were compared with those found on other European beaches [28]. The results obtained in Sonabia, Atxabiribil, and Gorliz showed low concentrations compared to, for example, the average values recorded on Mediterranean beaches, although there were exceptions where concentrations were similar to or even higher than those obtained on beaches on the Catalan coast [41]. On the other hand, the average concentrations found in Sonabia (10 MP kg−1), Atxabiribil (15 MP kg−1), and Gorliz (3 MP kg−1) were comparable to the averages of most of the BoB beaches considered in CEDEX, based on all the results available for the period 2016–2022 [33]. The methodologies used in both studies were similar, although in this work, the sieving was carried out directly on the beach, while at CEDEX, the samples were processed in the laboratory. The concentration reported for Itzurun Beach (39 items kg−1), only about 70 Km to the east of Gorliz, was higher than the concentration reported in this study. Frexulfe Beach (14 items kg−1), about 300 Km to the west of Sonabia; Oyambre (11 items kg−1), about 100 Km to the west of Sonabia; and Covas (11 items kg−1), about 400 Km to the west of Sonabia, were sensibly lower than in Atxabiribil but slightly higher than in Sonabia. The beach of Gorliz showed lower concentrations than those recorded on the beaches from BoB. In conclusion, the concentrations of MPs found on the beaches of Sonabia, Atxabiribil, and Gorliz are of the same order as those found on other beaches located in the BoB and other European coasts (see Table S3) [28,33,34,42].

4.2. Chemical and Morphological Characterisation of Microplastics

The dominance of fragments over other shapes, which is consistent with the results of surface water studies in the BoB, suggests that one of the main sources of MPs is related to the fragmentation of larger plastics that end up reaching the coast [32].
As the BoB has been identified as an area where floating plastics accumulate, the continuous degradation and mechanical abrasion of these plastics would promote their fragmentation, leading to a greater abundance of microplastic fragments that would eventually be transported to the coast [30,31].
Broadly speaking, the results of this study followed a similar distribution to that found on other beaches of the Iberian Peninsula [33] and on beaches in the southeast of the BoB [32], where fragments, foams, and pellets were the three most abundant categories. At Itzurun beach (Zumaia, Spain), a seasonal variation between spring and autumn was observed: during spring, the percentage of foams increased, while in autumn, fragments dominated. This trend contrasts with the results of the present study, in which an increase in foams was observed during autumn. Concerning the distribution by shape of MPs in beaches located in the southeast [42] and northeast of the BoB [34], in La Antilla, Zurriola, and Hendaye, higher percentages of foams and pellets in comparison to fragments were found, while on other beaches of the Pays de la Loire (France), as in this study, fragments were dominant.
Colour additives play an important role in the degradation of plastics, as, in addition to modifying their appearance, they can act as stabilisers and increase resistance to degradation [43]. Studies such as the one by Key et al., 2024 [43], showed that black and white pigments prolonged environmental stability, while colours such as red, green, or blue offered less protection, leading to faster degradation, loss of colouring, and formation of microplastics. The number of coloured MPs was relatively small compared to white MPs. As in many studies conducted on different sandy beaches around the world, white MPs dominate due to: (i) the high production of white items that end up in the sea and (ii) the discolouration of all coloured plastics [33,44,45,46,47].
PE was also predominant on beaches studied in other coastal environments [28,32]. This data agrees with the fact that PE, PP, and PS are the most widely produced polymers for single-use products and packaging, accounting for approximately 90% of the 400.3 million tonnes of plastic produced annually [4]. Expanded polystyrene is a material widely used in the packaging, construction, and fishing industries, as its chemical nature makes it low in density and an excellent insulator [48]. It fragments easily, and its residues can be efficiently transported in the marine environment by wind and water, due to its low density and minimal weight. This may be a determining factor for its significant presence on beaches. PE and PP are the raw materials for many commonly used plastic products, especially single-use products such as packaging and bottles, which are one of the main sources of pollution in rivers and oceans [3]. Like PS, they have a lower density than seawater, which allows them to be transported by surface currents, waves, and wind-induced drift, accumulating on coasts and beaches. The type/shape distribution of MPs found in this study is similar to those found in floating plastic and sediments of the BoB [31].
Although it is difficult to identify the sources of MPs on beaches due to their high dispersion [30] and the absence of significant differences in number, polymer type, and shape between them, Atxabiribil beach is influenced by its proximity to the mouth of the Nerbioi-Ibaizabal estuary, whose waters receive discharges from WWTPs, as well as from industrial activities related to plastics and ports [37,49]. Gorliz beach could also be affected by these waters, although to a lesser extent due to its more enclosed and protected nature. However, this beach is located in the same bay where the Butroe estuary, a WWTP, and a small recreational port flow into, which could constitute additional local sources of MPs [38]. In contrast, Sonabia Beach has no direct sources of pollution nearby, but its location exposed to the Bay of Biscay makes it susceptible to the arrival of MPs carried by sea currents and prevailing winds, which can carry floating material from different coastal areas or even from the open sea [30,39].

5. Conclusions

This study contributes to the current state of knowledge on the distribution of microplastics in the Cantabrian Sea, BoB, by providing an initial assessment of pollution levels and characterisation of microplastics on the beaches of Sonabia, Atxabiribil, and Gorliz. The results confirm the widespread presence of microplastics along the study area of the Cantabrian coast. The distribution of microplastics in this study shows no spatial or seasonal variations, suggesting that the accumulation of MP on each beach depends on general factors such as the geographical or hydrodynamic characteristics of each beach. The dynamics of surface currents in the BoB can affect the arrival of MPs on this coast, in addition to the potential inputs to which each beach is subject, such as wastewater discharges, the proximity to the mouth of the Nerbioi-Ibaizabal estuary, or port activities. The concentrations found ranged from 0 to 30 MP kg−1, with Atxabiribil being the beach with the highest concentration of MPs (2.9 ± 2.8 MPs kg−1). These concentrations were comparable to the averages of most of the BoB beaches considered in different studies. PE, PP, and PS were the types of polymers found that were mainly found as secondary MPs. Although the use of NaCl for density separation could limit the recovery of denser polymers, these three coincide with the most frequently reported in numerous studies on MPs on beaches. In October 2023, there was an increase in foams, corresponding to PS. The predominance of these polymers, together with the high proportion of fragments compared to other forms, is in line with what has been observed in studies carried out in other coastal areas [13,28,32]. The results of this study highlight the importance of improving waste management, implementing strategies to reduce plastic use, and promoting actions that contribute to the protection of coastal ecosystems. In this context, it is therefore necessary to develop regular monitoring programmes to assess the extent of microplastic pollution and conduct more comprehensive studies.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/hydrology12110298/s1, Figure S1: (a) Raman and (b) FTIR-ATR spectra of the main polymers most commonly found on beaches; Table S1: Microplastics abundance and percentage characterisation by polymer type, shape and colour per replica, season and beach; Table S2: Microplastics percentage characterisation by colour per season and beach; Table S3: Results of the studies compared in this work about the presence of MPs in beach sediments.

Author Contributions

Conceptualization, U.U.-M., T.M., O.G.-L., J.F.A.-C. and A.d.D.; methodology, U.U.-M., T.M., A.L., M.Á.O. and R.R.; formal analysis, U.U.-M.; investigation, U.U.-M., T.M., A.L., R.R., J.F.A.-C., O.G.-L. and A.d.D.; writing—original draft preparation, U.U.-M.; writing—review and editing, J.F.A.-C. and A.d.D.; visualisation, U.U.-M., T.M., O.G.-L., J.F.A.-C. and A.d.D.; supervision, J.F.A.-C. and A.d.D.; project administration, A.d.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data will be made available on request.

Acknowledgments

This work has been financially supported by the Spanish Ministry of Science and Innovation (MCIN) through the PLASTEMER project (PID2020-118685RB-I00) and the ‘One Health Observatory Lighthouse (HOBE)’ project (TED2021-132109B-C21), and the Basque Government through the ‘Consolidated Research Group 2022-2025′ project (IT1446-22). U. Uribe-Martinez thanks the Consolidate Group (IT1446-22) project from the Basque Government for her predoctoral contract. T. Maupas also acknowledges his predoctoral grant contract from the University of the Basque Country (UPV/EHU).

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
ATRAttenuated Total Reflectance
BoBBay of Biscay
CEDEXCentre for Public Work Studies and Experimentation
FTIRFourier-Transform InfraRed (FTIR) Spectroscopy
FTIR-ATRFourier-Transform InfraRed Spectroscopy combined with Attenuated Total Reflectance
PEPolyethylene
PPPolypropylene
PSPolystyrene
WWTPWastewater Treatment Plant

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Figure 1. Map of sampling locations: (1) Sonabia (43°24′43″ N 3°20′05″ W), (2) Atxabiribil (43°24′43″ N 3°20′05″ W), and (3) Gorliz (43°24′57″ N 2°56′40″ W).
Figure 1. Map of sampling locations: (1) Sonabia (43°24′43″ N 3°20′05″ W), (2) Atxabiribil (43°24′43″ N 3°20′05″ W), and (3) Gorliz (43°24′57″ N 2°56′40″ W).
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Figure 2. Location of the quadrants (50 × 50 cm) parallel to the coastline and detail of the sampling method. Replicates ‘R1’ to ‘R5’ are represented by red dots.
Figure 2. Location of the quadrants (50 × 50 cm) parallel to the coastline and detail of the sampling method. Replicates ‘R1’ to ‘R5’ are represented by red dots.
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Figure 3. Total amount of MPs found on each beach and sampling campaign.
Figure 3. Total amount of MPs found on each beach and sampling campaign.
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Figure 4. Distribution of MPs in each beach and sampling campaign. ‘Others’ category includes filaments, films, or fibres.
Figure 4. Distribution of MPs in each beach and sampling campaign. ‘Others’ category includes filaments, films, or fibres.
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Figure 5. Colour distribution of MPs in each beach and sampling campaign. ‘Others’ category includes colours that were found occasionally or represent less than 5% of the total (green, red, pink, purple, brown, yellow, and grey).
Figure 5. Colour distribution of MPs in each beach and sampling campaign. ‘Others’ category includes colours that were found occasionally or represent less than 5% of the total (green, red, pink, purple, brown, yellow, and grey).
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Figure 6. Distribution by polymer type in each beach and sampling campaign.
Figure 6. Distribution by polymer type in each beach and sampling campaign.
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Figure 7. Distribution by polymer type of the different MP shapes found in this study. ‘Others’ category includes filaments, films, or fibres.
Figure 7. Distribution by polymer type of the different MP shapes found in this study. ‘Others’ category includes filaments, films, or fibres.
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Table 1. Beach characteristics and potential sources of MPs.
Table 1. Beach characteristics and potential sources of MPs.
BeachGeographical CoordinatesSurrounding
Area		Tourist FacilitiesRiver
Influence		Exposed to Wastewater DischargeHarbour
Activities		Regular Cleaning
Sonabia43°24′43″ N 3°20′05″ WNaturalLowNoNoNoNo
Atxabiribil43°24′43″ N 3°20′05″ WSemi urbanMediumNoNoNoYes
Gorliz43°24′57″ N 2°56′40″ WUrbanHighYesYesYesYes
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MDPI and ACS Style

Uribe-Martinez, U.; Maupas, T.; Lapazaran, A.; Rodriguez, R.; Gómez-Laserna, O.; Olazabal, M.Á.; Ayala-Cabrera, J.F.; de Diego, A. Chemical and Physical Characterisation of Microplastics Present on Beaches of the Cantabrian Coast, Bay of Biscay (Spain). Hydrology 2025, 12, 298. https://doi.org/10.3390/hydrology12110298

AMA Style

Uribe-Martinez U, Maupas T, Lapazaran A, Rodriguez R, Gómez-Laserna O, Olazabal MÁ, Ayala-Cabrera JF, de Diego A. Chemical and Physical Characterisation of Microplastics Present on Beaches of the Cantabrian Coast, Bay of Biscay (Spain). Hydrology. 2025; 12(11):298. https://doi.org/10.3390/hydrology12110298

Chicago/Turabian Style

Uribe-Martinez, Uxue, Thomas Maupas, Aritz Lapazaran, Ruben Rodriguez, Olivia Gómez-Laserna, María Ángeles Olazabal, Juan F. Ayala-Cabrera, and Alberto de Diego. 2025. "Chemical and Physical Characterisation of Microplastics Present on Beaches of the Cantabrian Coast, Bay of Biscay (Spain)" Hydrology 12, no. 11: 298. https://doi.org/10.3390/hydrology12110298

APA Style

Uribe-Martinez, U., Maupas, T., Lapazaran, A., Rodriguez, R., Gómez-Laserna, O., Olazabal, M. Á., Ayala-Cabrera, J. F., & de Diego, A. (2025). Chemical and Physical Characterisation of Microplastics Present on Beaches of the Cantabrian Coast, Bay of Biscay (Spain). Hydrology, 12(11), 298. https://doi.org/10.3390/hydrology12110298

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