The coastal waters of the south-east Bay of Biscay a dead-end for neustonic plastics.

Numerical models point to the south-east Bay of Biscay as a convergence area for floating particles, including plastics. The few existing studies on plastic abundance in the area mainly focus on open waters and yet information on the coastal area is limited. To fill this gap, neustonic samples were taken along the coastal waters of the south-east Bay of Biscay (2017-2020) to define the spatial distribution of plastic abundances and composition. Results show an average plastic abundance of 739,395 ± 2,625,271 items/km2 (998 ± 4338 g/km2). French waters were more affected, with five times higher plastic abundances than Spanish coasts. Microplastics represented 93 % of the total abundance of plastic items (28 % in weight), mesoplastics 7 % (26 %) and macroplastics 1 % (46 %). This study demonstrates that this area is a hotspot for plastic with levels in coastal waters similar to those in the Mediterranean Sea or other litter aggregation areas.

Introduction

Marine litter and plastic pollution have been tagged as a new threat to the oceans. This is supported by the European Marine Strategy Framework Directive, which considers marine litter as one of the Descriptors, i.e., Descriptor 10, to monitor the good environmental status of European seas and oceans. Some authors have argued that it is not by far one of the most urgent matters, pointing instead to climate change, habitats and biodiversity loss, overfishing, interactions of different pollutants (especially those contaminants of emerging concern such as pharmaceuticals and pesticides), and cumulative impacts of different human pressures (Borja and Elliott, 2019; Tiller et al., 2019). Nonetheless other authors state that marine litter influences all of the above due to their capacity to alter the carbon cycle (Shen et al., 2020) or to transport pollutants and different species (Capolupo et al., 2020; Kesy et al., 2019; Kooi et al., 2017). Regardless, the plastic phenomenon has boomed in a fast-dominated media society. This has rocketed the calls for actions not only to visualize this problem (everybody has seen and/or suffer it) but also to find preventive and mitigation solutions by engaging society, scientific community, and industry together. It is unquestionable that marine litter has become a pressing issue to the public, thereby reaching the top of the agenda for policy makers and governments at all levels (Maes et al., 2019).

The marine litter costs the tourism and recreation sector up to €630 million per year, €62 million for the fisheries sector, equivalent to a reduction of nearly 1 % of the total revenue generated by the EU fleet in 2010 (Van Acoleyen et al., 2013). As an answer to this and other pressing issues, different directives such as the Single-Use Plastic (Directive (EU) 2019/904) and the Port Reception Facilities (Directive (EU) 2019/883), or the Extended Producer Responsibility strategy have been promoted at European level. At local level, several authorities have also shown interest in adopting preventive and mitigation solutions, but they lack the understanding on the origin and source of the marine litter that ends washed up in their coastlines, who produces them, and how they accumulate or disperse (Ruiz et al., 2020). This is precisely what has happened is the south-east of the Bay of Biscay (SE BoB). This region has been highlighted by recent studies, based on Lagrangian computations from numerical models and observations, as a cul de sac for floating plastic (Declerck et al., 2019; Lebreton et al., 2012; Pereiro et al., 2019; Ruiz et al., 2022a; van Sebille et al., 2012). Some of the local authorities of this region, i.e., in Gipuzkoa (Spain) the Provincial Council of Gipuzkoa, and in the Pyrénées-Atlantiques (France) the Communauté d'Agglomération Pays Basque and the Ville de Biarritz, have shown interest in providing solutions, but they felt that they lacked the knowledge to face this problem and implement valid solutions (pers. comm. as part of LIFE LEMA project).

Despite the ubiquitous presence of floating plastics, there are relatively few reports of joint quantification of micro-, meso- and macro-plastics for the Bay of Biscay (BoB). The first observations carried out during micro and macrolitter monitoring actions revealed that micro (756,865 items/km2) and macrolitter abundances (3.13 items/km2) fell in the same range of the values observed in other European regions (Ruiz et al., 2022b). The BoB presents a moderate level of microplastic pollution (Mendoza et al., 2020), and modelling studies have recently revealed that microplastics in the BoB tend to be freely scattered oceanward while larger items tend to stay close to shore. Particularly the longest residence times and abundances for the BoB are observed in the SE region of the BoB both for sea-based related floating macro and microplastic (Pereiro et al., 2019; Rodríguez-Díaz et al., 2020; Ruiz et al., 2022a). The coastal waters of the SE BoB also present particular structures at the submesoscale domain (<10 km horizontally), called marine litter windrows, that tend to aggregate floating macrolitter, seafoam, seaweeds (Cózar et al., 2021; Ruiz et al., 2020). Such is the abundance of marine litter windrows in the BoB that local authorities are engaged in active fishing for litter initiatives that aim at retrieving the floating marine litter from the sea-surface of coastal waters along these convergence lines (Andrés et al., 2021).

This contribution, jointly with the work presented in Ruiz et al. (2022a; 2020; 2022b) for micro and macro litter monitoring provide new insights on the background knowledge on marine litter in the BoB, and particularly the coastal areas of the SE BoB; information that can assist the local authorities in being more prepared to address plastic pollution in the coastlines they manage. The questions to be answered by the present contribution are how much plastic is in the coastal waters of Gipuzkoa and Pyrénées-Atlantiques (SE BoB), where do plastics accumulate, and when. It also aims to provide a first description on the temporal and spatial differences on the plastic abundances in the study areas, and to explore the possible correlations between plastic abundances and distance to the coastline and depth, and between plastic particles of different size. This contribution is part of LIFE LEMA project (https://www.lifelema.eu/en/), which was devoted to developing smart tools to monitor, forecast and collect floating marine litter from coastal waters of the SE BoB.

Material and methods

The study area

The SE BoB (Fig. 1) is the section of the BoB (a semi-enclosed gulf located in the north-east of the Atlantic Ocean) that encompasses the area between the south-western coast of France and the north-eastern coast of Spain. The continental shelf along the northern Iberian Peninsula is narrow (30 to 40 km width on average) (Solabarrieta et al., 2014), while the shelf increases in width progressively with latitude along the French coast (known as the Armorican and the Aquitanian shelves) (EMODnet, 2019). The coastline along the northern Iberian Peninsula is also steep, with a pronounced continental slope (10 to 12 %) and numerous canyons (Borja et al., 2019). It includes a multitude of cliffs between beaches and small bays, while the French coast (from the south to the Loire River mouth) is straight, flat and sandy (OSPAR Commission, 2000). River basins in the peninsula are small, with short rivers that pass through highly industrialised areas; opposite, in the south-western coast of France rivers are bigger and longer. Water mass balance in this region is mainly influenced by rivers outflow, being the Adour (France) the most important river in the SE BoB. With mean annual discharge flows of 300 m3/s (Morichon et al., 2008), the Adour presents peak flows exceeding 1000 m3/s in spring (Laiz et al., 2014), and runoff under 500 m3/s in summer (Declerck et al., 2019). The population density is 18 million inhabitants (299.6 inhabitants per km2) (Borja et al., 2019). The main human activities in this maritime region are fishing, maritime transport, and tourism (OSPAR Commission, 2017).

Fig. 1

Study site located in the SE Bay of Biscay (A), with the Neustonic sampling stations positions (B). The blue points are related to the sampling stations (1–7) of the Pyrénées-Atlantiques coast (France) and the yellow points to the sampling stations (8–40) of Gipuzkoa coast (Spain). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The water circulation in the SE BoB is complex and modelled by diverse factors. In winter, the circulation is mainly governed by the Iberian Poleward Current (IPC). The IPC is a seasonal slope current flowing eastwards over the slope of the Spanish coast and northwards over the slope of the French coast (Solabarrieta et al., 2014), which origin lays on the thermohaline circulation (Huthnance, 1986) and it is seasonal (Solabarrieta et al., 2014). In summer, a completely different tendency is observed. The water circulation over the slope is reversed and has intensities three times weaker than those observed in winter, with predominant (westerly) currents over the Spanish slope (Charria et al., 2013; Solabarrieta et al., 2014). During the transition periods (spring, early autumn) no clear pattern is observed, with weak and high variable currents (Charria et al., 2013; Solabarrieta et al., 2014). On the shelf, riverine input, wind intensity, tides, waves and local winds introduce variability in the surface circulation (Charria et al., 2013). The seasonal variability of the main circulation patterns and those resulting from forcings acting locally determine the retention patterns in the area. Findings show a significant spatial and temporal variability on the residence time values, characterized by a strong seasonality with higher retention conditions observed in spring and summer (Declerck et al., 2019; Rubio et al., 2020). In this area, in autumn both the wind and slope current regime favor the rapid evacuation of the particles towards the north, although interannual variability is also observed (Declerck et al., 2019; Rubio et al., 2020).

Sampling effort and laboratory work

A total of 190 samples were collected during four sets of cruises onboard Miren Argia F/V (in 2017) and Itsas Belhara F/V (in 2018–2020), and from 40 sampling stations, covering an area of 85 km (870 km2). Sampling stations were strategically located at 2, 4, and 6 nautical miles off the coast, and in front of/or in between river-mouths. One neustonic sample was taken at each sampling station (Fig. 1) and month. In 2017, due to vessels' availability, the sampling focused on the Spanish coastline waters during Autumn months (September to November) (Fig. 1, yellow circles), totaling 94 samples (in October 28 out of 33 stations could only be sampled). Following the same protocol, and for the period 2018–2020, seven sampling stations were sampled during Spring and Summer months (May to September), and the sampling focused on French coastline waters (Fig. 1, blue circles); a total of 96 samples were collected. The sampling effort is summarised in Table S1 of the Supplementary Material. All samplings were undertaken in areas free of marine litter windrow structures (‘windrow-free areas’).

All samples were collected during daylight using a neuston net (100 cm × 50 cm, 500 μm mesh size), and equipped with a mechanical flowmeter to measure the volume of filtered seawater. The net was towed superficially (approximately the first 35 cm of the sea-surface were sampled) at a speed range of 2–3 knots for 15–20 min. After the tow, the net was rinsed onboard with prefiltered seawater to accumulate the entire sample in the cod-end, and then passed to a 335 μm sieve to ensure the retention of the microplastic fraction (>500 μm). Each sample was transferred to a coded plastic bag, which was stored and frozen at −18 °C until analysis.

In the laboratory, and once defrosted, the samples were rinsed with distilled water using a 335 μm sieve. Subsequently, the microplastic items were extracted manually from the samples with the aid of a stereoscopic microscope (SMZ-2T from Nikon). For those samples containing a significant amount of organic matter (i.e., plankton and microalgae), a pre-treatment was applied to reduce the organic load of the samples, which consisted of a peroxide oxidation (30 % hydrogen peroxide) in the presence of a Fe(II) catalyst (0.05 M) (following Masura et al. (2015)). Afterwards, the mixture was subjected to density separation in NaCl to isolate the plastic items through flotation. When the sample was complex to work under the stereoscopic microscope, due to it was too time and effort-consuming because of the type and amount of organic matter included, NaOH 6 mol/l was added to the sample and then it was digested with a Branson UltrasonicsTM Sonifier S-250A (200-Watt, 60 Hz) set at 25 % output control for 0.7 s duration of pulsation. Depending on the quantity and type of organic matter (algae being harder to dissolve than plankton), the sonication varied from 1 to 5 h. To avoid an excessive increase of temperature, the solution was kept in a cold-water bath for the duration of the process. Afterwards the sample was rinsed with distilled water with a 335 μm sieve.

The plastic particles of each sample were extracted under a stereoscopic microscope, classified, and stored according to their size in 3 petri dishes: micro (<5 mm), meso (5 mm–2.5 cm) and macro (>2.5 cm) plastics. The extraction, the size measurement and storage in petri dishes were done manually. The petri dishes were left to be dried for one day, covered with the petri cup, at ambient temperature before being weighted. Blank controls were routinely performed to determine if any contamination occurred at the lab. The blanks returned uncontaminated; thus, no blank correction procedure was applied to microlitter results. The number of items was also noted by each of the size class. Plastic abundance and mass concentration per water surface area (as items/km2, items/m3, g/km2, and g/m3) for plastic items in general, and by micro, meso and microplastic categories were calculated by dividing the total number or dry weight of plastics collected in each tow by the estimated area sampled, as the product of the trawling distance (derived from the starting and ending coordinates registered with a GPS) and the width of the net opening, and water volume filtered.

Vertical mixing derived from the wind on the sea surface shapes the distribution of plastic in the sea-surface. To represent this phenomenon, some authors adjust the abundances by a method proposed by Kukulka et al. (2012). Despite this, all numerical data present in this contribution refer to data without correction. Only the abundances represented in distribution maps (Fig. 2, Fig. 3) were adjusted following Kukulka et al. (2012). In those cases only, and for tows that presented an average friction velocity in water (U*) > 0.6 cm/s (100 % of the tows), their corresponding abundances, in terms of item/km2, were adjusted following Kukulka. For the Spanish coastal data, the wave data were extrapolated from the records of the closest buoy, the Bilbao-Vizcaya buoy (Puertos del Estado – http://www.puertos.es/en-us), whereas the wind data were obtained from the meteorological agency of Galicia (MeteoGalicia). This model, with a native resolution of 12 km, reproduces the offshore wind fields of the SE BoB with reasonable accuracy (Manso-Narvarte et al., 2018). Both wave and wind data were provided with an hourly average frequency. In contrast the wave and wind data for the French sampling were provided by the vessel Itsas Belhara F/V, as she measured these data onboard.

Fig. 2

Plastic abundances (items/km2), in Spanish waters (data corrected by wind following Kukulka method).

Fig. 3

Plastic abundances (items/km2), in French waters (data corrected by wind following Kukulka method).

Statistical analysis

All statistical analyses were performed using MATLAB software (version R2021a). Two set of analyses were performed: the first considering all the data available (i.e., regardless the administrative region), and the other, treating French and Spanish data as independent data sets, as they were collected over different months and years, and spatial resolution. Data were tested for normality following Shapiro-Wilk test (<50 samples) and Kolmogorov Smirnov test (>50 samples). Since data were not normally distributed, the nonparametric Kruskal-Wallis test (K-W) was performed to identify the annual and monthly differences between plastic abundances and between sampling stations. Statistically relevant differences were considered when p-value <0.01. Spearman's rank correlation (SP) was used to test the correlation between abundances, the bathymetry and the distance to coastline. Likewise, Spearman's rank correlation was applied to get a better insight into the possible connections between particle abundance of macro-mesoplastics, meso-microplastics and macro- microplastics. The level of strong correlation was set up at p ≥ ±0.9, following Schober et al. (2018).

Results

Spatial and temporal abundance

Plastic fragments were found in 100 % of the samples, with a total of 195,330 plastic items found at the 40 stations. The highest abundance was observed in September 2019 along the French coast, with one sample presenting 23,560,179 items/km2 and 23.0 kg/km2. The abundances in the studied region were generally medium-high (Table 1), ranging from 33,614 to 23,560,179 items/km2, with average ± standard deviation (SD) values of 739,395 ± 2,625,271 items/km2 (median = 232,227 items/km2). The mass concentration of plastic fragments varied from 0.7 to 50,012 g/km2 (median = 74 g/km2, mean ± SD = 998 ± 4338 g/km2). All months presented at least one sample with >1,000,000 items/km2, excluding October 2017, July 2019, June 2020, July 2020, and September 2020. Abundances are mapped in Fig. 3, Fig. 4, and monthly abundances and mass concentrations are shown in Fig. 5. Detailed results, expressed by months, years and sampling stations are listed in Tables S2 and S3 of Supplementary material.

Table 1

Neustonic plastic results for coastal waters in the SE BoB by year. Values correspond to the mean, standard deviation (SD), median, min and max values of the samples obtained in the 2017-2020 survey. The results include macro, meso and microplastic items. Observed data, no correction was applied to the data.

		2017	2018	2019	2020
Plastic items/km2	Mean	255,601	1,395,467	1,382,354	905,779
	(±SD)	347,919	3,547,757	4,370,379	3,087,516
	Median	169,453	323,219	444,196	256,081
	Min	33,614	54,244	103,194	82,753
	Max	2,526,181	20,580,976	23,560,179	18,369,193
	Micro (Mean ± SD)	213,271±324,973	811,268±899,046	1,296,551±4,078,083	839,276±2,847,696
	Meso(Mean ± SD)	12,021±31,989	70,422±121,738	94,791±282,470	61,928±232,622
	Macro(Aver. ± SD)	2300±6272	13,307±34,856	8239±10,687	4574±8370
Plastic g(dw)/km2	Mean	312	1499	1238	2157
	(±SD)	934	3376	4325	8552
	Median	40	420	186	150
	Min	1	17	3	4
	Max	7047	15,329	22,982	50,012
	Micro (Mean ± SD)	62±304	447±987	539±2436	436±1335
	Meso (Mean ± SD)	60±187	300±682	468±1799	421±1248
	Macro (Mean ± SD)	173±838	753±1961	232±507	1300±6164

Fig. 4

Comparison between years of plastic abundance for Spanish (blue, 2017) and French (green, 2018–2020) coastal waters in the SE BoB. Sample points are shown and solid lines into boxes indicate median values and broken lines indicate mean values, boxes indicate first and third quartiles, and whiskers indicate the minimum and maximum values. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

Correlation between micro and mesoplastics abundances and mass concentrations calculated from data of French and Spanish samples. Observed data, no correction was applied to the data.

Significant abundance differences were found between Spanish and French data sets (Mann-Whitney, p < 0.01); French waters presenting almost five times more plastic than the one observed in Spanish coastal waters. In Spanish waters, the largest abundances were observed in the eastern-most regions, close to the French waters. However, in November the largest abundances were well spread throughout the sampling area, despite presenting the lowest abundances in the most westerly sampling stations, close to Deba River (Fig. 2). In French waters the highest abundances were observed in August (1,928,036 ± 3,886,985 items/km2) and September (2,432,697 ± 6,549,578 items/km2). Spatially, the area of Biarritz resulted the most predominant to accumulate plastic, followed by the area close to the Adour River (Fig. 3). Nonetheless, no further analysis could be performed to investigate the statistical differences between sampling stations, and thus, determine a potential local hotspot due to the population size for each sampling stations was too small.

Regarding item size, microplastics were the most abundant for both Spanish (meanmicro = 213,271 ± 324,973 items/km2) and French waters (meanmicro = 963,020 ± 2,819,154items/km2). Overall, mesoplastics (meanmeso = 46,259 ± 164,956 items/km2) were one order of magnitude higher than macroplastics (meanmacro ± SD = 5781 ± 17,029 items/km2) and the abundance of both were much lower than that of microplastics (meanmicro = 624,562 ± 2,127,574 items/km2).

The only strong significant correlation found was the one between French meso and microplastics abundances (Spearman test, pitems/km2 = 0.9496, pitems/m3 = 0.9196, pg/km2 = 0.9157, pg/m3 = 0.8467) (Table 2, Fig. 5). Significant differences were detected between abundances of September, November and December for Spanish waters (K–W test, pitems/km2 = 0.004, pitems/m3 = 0.0059, pg/km2 = 1.998e−07, pg/m3 = 1.225e−06). It should also be noted that, only when the whole data set was used in the analysis (e.g., including French and Spanish data sets together), significant differences were observed between analysed years (2017–2020) (Kruskal-Wallis test p < 0.01). None of the other analysis showed any correlation or significant differences, including the analysis between abundance and bathymetry or the distance to the coastline.

Table 2

Summary of statistical analysis performed for neustonic plastics at the coastal sampled areas. Temporal, spatial and size comparisons were made on particle abundance (items/km2, items/m3) and mass concentration (g/km2 and g/km3). In bold, statistically significant results (p-value <0.01 for K–W test, limit >0.9 for SP test, as suggested by Schober et al. (2018).

Plastic abundance comparison	Area	Period	Normality test	Statistical test	P items/km2	P items/m3	P g/km2	P g/m3
By year	France	2018–2020	Shapiro-Wilk	K-W	0.0942	0.167	0.0554	0.0798
By month	France	2018–2020	Shapiro-Wilk	K-W	0.0585	0.0045	0.1394	0.0387
	Spain	2017	Shapiro-Wilk	K-W	0.004	0.0059	1.998e−7	1.225e−6
By sampling stations	France	2018–2020	Shapiro-Wilk	K-W	0.1235	0.1381	0.0259	0.0362
	Spain	2017	Shapiro-Wilk	–	Samples sizes were too small and the selection not representative
By depth	France	2018–2020	Kolmogorov	SP	−0.0649	−0.0503	−0.1297	−0.1313
	Spain	2017	Kolmogorov	SP	0.0294	0.0324	0.0725	0.0783
By distance to coastline	France	2018–2020	Kolmogorov	SP	−0.1809	−0.1761	−0.2408	−0.2440
	Spain	2017	Kolmogorov	SP	−0.0725	−0.0754	0.0347	0.0462
Macro and microplastic abundance correlation	Spain	2017	Kolmogorov	SP	0.4548	0.2922	0.0477	0.0594
	France	2018–2020	Kolmogorov	SP	0.3523	0.3473	0.5376	0.5315
Macro and mesoplastic abundance correlation	Spain	2017	Kolmogorov	SP	0.8636	0.8882	0.1185	0.1619
	France	2018–2020	Kolmogorov	SP	0.5996	0.6694	0.5440	0.4694
Micro and mesoplastic abundance correlation	Spain	2017	Kolmogorov	SP	0.6972	0.5258	0.8697	0.9026
	France	2018–2020	Kolmogorov	SP	0.9496	0.9196	0.9157	0.8467

Composition of plastic items: type, size and colour

In terms of the type of items, fragments were the most common items found in the microplastic fraction, followed by fibres; in the case of mesoplastics, fragments, fishing lines and films represented almost 98 % of the items; fishing lines were the most common macroplastics (Table 3). As for colour, almost half the items found in the study area were transparent, followed by white, black-grey, and green (the latter, only for meso- and macroplastics) (Table 3). Few quantities of blue items (6 %), yellow/orange/brown (4 %) and red/pink/purples (2 %) were found in all ranges.

Table 3

Frequency of the type of items and colours conforming the micro-, meso-, and microplastic item categories found in the plastic floating in the coastal waters of the SE BoB (results are given in %).

%	Microplastic	Mesoplastic	Macroplastic
By type of items
Fragments	69.9	41.1	7.6
Films	2.1	25.2	13.9
Pellets	0.4	1.7	–
Expanded polystyrene	4.0	0.5	–
Paints	1.9	–	–
Fishing lines	3.7	31.5	78.5
Fibres	16.9	–	–
Rubbery	0.7	–	–
Paraffins	0.2	–	–
By colours
White	18.4	15.9	14.4
Black, grey	10.9	13.0	16.7
Transparent	51.7	44.4	33.3
Green	4.3	13.9	24.4
Blue	6.2	5.3	6.7
Red, pink, purple	3.0	2.5	1.7
Yellow, orange, brown	5.5	5.0	2.9

Discussion

Plastic abundance in coastal SE BoB surface waters

Many numerical modelling studies underline that the BoB is a trapping zone for floating plastic (Rodríguez-Díaz et al., 2020; van Sebille et al., 2012). Others, particularly point out towards the SE BoB as one of the most important aggregation areas, especially during spring-summer seasons due to their seasonal wind and wave patterns (Declerck et al., 2019; Pereiro et al., 2018, Pereiro et al., 2019; Ruiz et al., 2022a), fact that is corroborated by scientific samplings (Ruiz et al., 2022b). However, most studies focus on oceanic waters, and coastal areas are overlooked. The importance of analysing coastal waters has been stressed by some model simulations (Onink et al., 2021), which point at coastlines and coastal waters as important sinks of floating plastic. Morales-Caselles et al. (2021) compiled global-scale data to show state that most of the macrolitter entering the ocean from land-based sources is retained in the coastal strip, where it can create secondary microplastics by different processes (Efimova et al., 2018). Here we demonstrate that the coastal waters of the SE BoB, especially in spring and summer months are indeed a neustonic plastic aggregation area, with observed average microplastic abundances of 1,117,403 ± 3,808,626 items/km2. Average neustonic abundances are two times higher than those observed by Ruiz et al. (2022b) for the BoB, and 6–33 times higher than those reported by Gago et al. (2015) for the surface coastal waters of the SW BoB for 2013 and 2014 (34,000 and 176,000 items/km2). Same trend is observed when comparing our findings to data reported for Portuguese coastal waters by Frias et al. (2014) (0.036 items/m3 versus 2.748 items/m3 of the present study) or other coastal regions, even subtropical gyres (Table 4). Therefore, it seems appropriate to hypothesise the existence of a floating plastic gradient from coastal waters of the SE BoB towards the more westerly areas. Nonetheless, it is of utmost importance to not only increase the modelling effort in coastal areas but also the sampling efforts to improve the resolution of data.

Table 4

Reported microplastic abundances in surface waters for different seas around the world, including litter aggregation areas of the open ocean, using manta (M) or Neuston (N) nets.

Sampling area	Sampling location: open ocean (OC), coastal (C)	Abundance items/km2		Net mesh size (μm)	Reference
		Mean	Max
Bay of Biscay
South-East Bay of Biscay	C	739,395	23,560,179	N 500	Present work
Bay of Biscay	C, OC	363,732	3,476,222a	N 335	(Ruiz et al., 2022b)
Western Bay of Biscay	C	176,000		M 335	(Gago et al., 2015)
Atlantic Ocean
North-East Atlantic	OC	36,623	375,854	M 333	(Maes et al., 2017)
North-Atlantic Ocean, Azores	C	173,811	467,260	M 200	(Herrera et al., 2020)
North-Atlantic Ocean, Madeira	C	69,626	124,190	M 200	(Herrera et al., 2020)
North-Atlantic Ocean, Canary Islands	C	194,951	1,007,872	M 200	(Herrera et al., 2020)
Mediterranean Sea
North-West Med.	C	116,000	892,000	M 333	(Collignon et al., 2012)
North-West Med.	C	158,000	578,000	M 333	(Pedrotti et al., 2016)
West Med., Balearic Islands	C	900,324	4,576,115	M 333	(Ruiz-Orejón et al., 2018)
West Med., Mallorca	C	858,029		M 335	(Compa et al., 2020)
West Med., Spanish coast	C	108,000	500,000	M 335	(de Haan et al., 2019)
Central and West-Med., Adriatic Sea	C, OC	400,000	4,520,000	N 200	(Suaria et al., 2016)
Central and West-Med.	C, OC	147,500	1,164,403	M 333	(Ruiz-Orejón et al., 2016)
East Med. (Israeli coast)	C	1,518,384	64,812,600	M 333	(van der Hal et al., 2017)
All Mediterranean	OC	243,853		N 200	(Cózar et al., 2015)
Other regions
Arctic (Greenland & Barents Seas)	OC, C	63,000	320,000	M 500	(Cózar et al., 2017)
Baltic Sea, Stockholm Archipelago	C	110,000	618,000	M 335	(Gewert et al., 2017)
Australia	C	4,256	33,412	N-M 335	(Reisser et al., 2013)
Hong Kong waters	C	334,780	1,675,982	M 333	(Cheung et al., 2018)
Southern Ocean, Antarctica	OC	1,719	39,096	N 200	(Suaria et al., 2020b)
Weddell Sea, Antarctica	OC	1,838		M 300	(Leistenschneider et al., 2021)
Accumulation zones (gyres)
North Atlantic gyre	OC		580,000	N 335	(Law et al., 2010)
North-East Pacific gyre	OC	209,010	4,188,092	M 500	(Egger et al., 2020)
South Pacific gyre	OC	26,988	396,342	M 333	(Eriksen et al., 2013)

This figure has been calculated by the authors from the data collected in the BIOMAN survey (2017–2019), which results are shown in Ruiz et al. (2022b), contribution that shares authorship with the present contribution.

Marine litter and plastic abundances in coastal waters have been explored worldwide (some studies are shown in Table 4). The Mediterranean Sea, for example, in addition of being the largest European sea and semi-enclosed basin with dense coastal population, it is an accumulative basin for plastic litter (Cózar et al., 2015). The average abundances observed in the SE BoB are similar to those reported for coastal areas of the Mediterranean Sea. In the Central and Western Mediterranean Sea, the observed average surface water microplastics abundance ranged from 347,783 ± 457,128 items/km2 in the coastal waters of the Menorca channel (Ruiz-Orejón et al., 2019) to circa 900,000 items/km2 (858,029 ± 4,082,964 items/km2 by Compa et al. (2020) and 900,324 ± 1,171,738 items/km2 by Ruiz-Orejón et al. (2018)) in the coastal waters of the Balearic Islands; and they were even slightly higher than those of the coastal western Mediterranean Sea (de Haan et al., 2019). Factors including boundary effects, discharge of large rivers, large coastal population, and tourism were proposed by these authors to explain the source and accumulation in coastal areas (Ruiz-Orejón et al., 2016). However, their abundances were far from the 23,560,179 items/km2 observed in September 2019, near the coast of Saint Jean de Luz (France). Only van der Hal et al. (2017) observed such extreme concentrations near the coast of Israel (in the Eastern Mediterranean Sea) with a maximum of 64,812,600 particles/km2. This suggests that surface coastal waters of the SE BoB seem contaminated with slightly similar concentrations than those observed in the Mediterranean Sea. However, the heterogeneity in sample collection and processing methodologies and reporting makes the comparison between studies quite complex (Mendoza et al., 2020); this complexity get further increased when fibres are accounted as shown in Section 4.2. Thus, it is of utmost importance to consider standardization of the sampling and analysis methods (Cole et al., 2011; Cowger et al., 2020; Galgani et al., 2010). The neuston net used in this study had a 500 μm mesh size net whereas most of the studies cited in the comparison used smaller mesh sizes. Nevertheless, an increase in the observed abundance would be expected if a smaller mesh size net (i.e., 335 μm) would be used. In any case, the figures would still be higher than those reported by Ruiz et al. (2022b) as part of the BIOMAN survey for the surface waters of the BoB.

While a strong positive correlation between micro and mesoplastic concentrations was identified, it must be noted that the macroplastics concentration was not correlated to the smallest fractions. This result points to the fact that macroplastics behave differently to micro and mesoplastics, being their drift and retention probably driven by different physical processes linked for instance to their different buoyancy (e.g. the fraction of macroplastics with positive buoyancy would be much more influenced by the wind drag direct effect). From independent sampling conducted in the same area in summer 2018, Ruiz et al. (2020) demonstrated that floating marine litter tends to accumulate in marine litter windrows. During the neustonic sampling, several litter windrows were observed in French waters. Nonetheless, if a windrow was detected in the area, a nearby window-free area was selected as an alternative location to tow the neuston net. This ensured that the net was not clogged or damaged and that results were not altered by sampling within an aggregation structure. Thus, while the sampling in Ruiz et al. (2020) focused on the windrows and macrolitter, here the neuston net was trawled in windrow-free waters. The difference between marine litter within windrows (mean ± SD = 24,864,714 ± 26,159,598 g/km2 reported by Ruiz et al. (2020)) and windrow-free areas (i.e. neustonic sampling locations reported in the current paper) (mean ± SD = 2445 ± 8008 g/km2) was very prominent during summer 2018. This evidences the striking contrast between plastic concentration within and beyond aggregation structures as well as the capacity of the windrows to retain floating plastics in the SE BoB. Thus, the need of studying these type of aggregation structures when assessing the overall plastic concentrations at sea, in order to avoid underestimation of ocean plastic. The correlation between the smallest fractions could also enable the extrapolation of mesoplastic pollution through the microplastics abundances. This correlation was also observed for the Mediterranean Sea (Faure et al., 2015). Unlike surface waters, Masiá et al. (2021) identified significant correlations between mesoplastics and macroplastics along the southwest beaches of BoB, highlighting that size correlation can differ between different marine compartments. The no-correlation between abundance and monthly sampling stations suggests that sampling stations may be under the same influence for plastic pollution; nonetheless, a wider dataset for the same sampling periods and years would be required to identify monthly and seasonal trends.

As such, plastic contamination of the coastal waters of the SE BoB was observed to vary both spatially and temporally, especially between Spanish and French stations. In relation to months, in French waters the spring-summer period months of August (1,928,036 items/km2) and September (2,432,697 items/km2) tended to present higher abundances and July the least (297,455 items/km2), whereas in Spanish waters November (327,017 items/km2) presented abundances in the order of May in French waters (429,558 items/km2). Several aspects are proposed to explain this difference. (1) The fact of having different seasons sampled in the analysis (France: Spring-Summer; Spain: autumn), may have affected the results. Nonetheless, neustonic sampling carried out within la Concha Bay in Donostia-San Sebastian (June 2021) by the authors, following the same methodology presented in this study, revealed average abundances 465,566 ± 174,935 items/km2 (165 ± 137 g/km2) (Larreta et al., 2021), slightly lower than the ones observed in French coastal waters. Thus, the sampling seasonality effect remains unproved. (2) The year 2018 presented the average highest abundance, which may be related to the fact of 2018 being a particularly rainy year, and consequently it may have led to an increase in the release of material from the rivers, including microplastic transported by the river plume. However, not significant differences were found amongst years and stations within each subregion (i.e., Spanish and French waters). Lastly, (3) French stations were located closer to the coast than Spanish ones. And although Pedrotti et al. (2016) observed the highest concentrations of microplastic within the 1 km of coastal water along the French Mediterranean coast, no statistically significant differences were observed between the abundances and the distance to shore.

Composition of the observed neustonic plastic

Microplastic accounted for 92 % of the neustonic plastics collected on the surface of the SE BoB, in line with the findings for the BoB southern waters (Carretero et al., 2022; Gago et al., 2015). Regarding the type of items, a large proportion of microplastics were plastic fragments (69.9 %), consistent with previous studies published for other European regions (Adamopoulou et al., 2021; Faure et al., 2015; Maes et al., 2017). The type of object does not allow making clear statements about the origin of the particles, though plastic fragments were probably derived from the fragmentation of larger objects, favoured by continuous beaching and resuspension along Spanish and French coastal areas. Fibres (16.9 %) were in the lower range of data reported for the Baltic Sea (Gewert et al., 2017) or the Atlantic Ocean (Kanhai et al., 2017). Fibres can be generated from a number of sources. They can be derived from washing textiles and enter the ocean via wastewater (De Falco et al., 2019; Gaylarde et al., 2021; Salvador Cesa et al., 2017) or they can be originated from sea-based conventional activities such as fishing (Lusher et al., 2017; Xue et al., 2020). Despite fibres are regarded as a prevalent type of microplastic, few studies dealing specifically with this type of item are scarce. Besides, distinction between natural and synthetic fibres during lab sampling process is not always conducted so further identification accuracy is needed to draw any firm conclusion on fibres abundance comparison (Suaria et al., 2020a). In the present contribution all fibres were considered to be synthetic, and no FTIR was applied to define the origin. Plastics were mainly transparent, followed by white and grey/black colored items, similar to those found in and Black Sea and Caspian Sea basins (D'Hont et al., 2021) and the Western Mediterranean sea (de Haan et al., 2019). However, colour comparison and relevance of this information is limited by methodology or observer subjectivity (Martí et al., 2020).

Origin and distribution of the observed plastic

LIFE LEMA project was proposed to implement an effective way of managing floating marine litter in the SE BoB. A deeper understanding of plastic distribution and concentration was necessary to target the sites exposed to this pollution. This has been addressed as part of several contributions targeting microplastics in the BoB (Mendoza et al., 2020), floating marine litter abundance as part of ecosystemic surveys (Ruiz et al., 2022b), aggregation of floating marine litter in coastal marine litter windrows (Ruiz et al., 2020), modelling of plastic sea surface distribution having rivers as sources (Declerck et al., 2019), modelling of fishing activity related floating marine litter (Ruiz et al., 2022a), floating marine litter collection solutions (Andrés et al., 2021), and beaching forecasts for efficient cleaning services (Granado et al., 2019). The present contribution provides the needed information regarding Neustonic plastic abundance and distribution, and it complements the overall picture. The backtracking approach of the observed microplastics particles is proposed as particularly useful for local authorities to tackle the problem at its source.

Conclusions

This study demonstrates that the south-east Bay of Biscay (SE BoB) is a dead-end for plastic and it shows that plastic pollution levels in coastal waters of the SE BoB are similar to those in the Mediterranean Sea. Neustonic plastic samples have been collected from 40 stations in four years (2017–2020) across the SE BoB in Spanish and French coastal waters. SE French Atlantic coast seems more prone to accumulate microplastics than Spanish neighboring region, especially in spring-summer months. The region presents an average abundance of 739,395 ± 2,625,271 items/km2 (998 ± 4338 g/km2) and a median of 232,227 items/km2 (74 g/km2). French coast was mostly affected (with 5 times higher plastic abundances than in the Spanish coast), with an average of 1,213,110 ± 3,624,181 items/km2 in 2018–2020, whereas in the Spanish counterpart (which are neighboring areas) averages of 255,601 ± 347,919 items/km2. Microplastic represented 94 % of the plastic items in the samples, mesoplastic 5 % and macroplastic 1 % (in terms of weight the share was 47 % microplastics, 31 % mesoplastic, 22 % macroplastic). The most common items collected were fragments and fibres in the microplastic range; fragments, fishing lines and films represented almost the 98 % of the items for mesoplastics; and fishing lines for macroplastics. As for colour, almost half of the plastic present in the study area was transparent, followed by white, black-grey, and green (this latter one only for meso- and macroplastics). Micro- and mesoplastic abundance and mass concentrations showed a positive correlation. None was found for macrolitter, suggesting that macrolitter may be governed by different physical processes (Morales-Caselles et al., 2021). Neustonic sampling must be combined with marine litter windrows analysis to be able to estimate the floating marine litter concentration. Our results reinforce the importance of having local authorities setting solutions to prevent the entry of plastics and their seasonal formation.

CRediT authorship contribution statement

O.C. Basurko: conceptualization, sampling, data analysis, investigation methodology, writing original draft, supervision, and funding acquisition; I. Ruiz: sampling, statistical analysis, validation, review and editing; A. Rubio: conceptualization, investigation methodology, modelling supervision, writing validation, review and editing; B. Beldarrain: laboratory analysis and sampling; D. Kukul: laboratory analysis; M. Galli: laboratory analysis; A. Cozar: methodology, review and editing; T Destang: laboratory analysis, data analysis; J. Larreta: conceptualization, investigation methodology, validation, review and editing.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Oihane C. Basurko reports financial support was provided by LIFE programme. Irene Ruiz, Anna Rubio, Beatriz Beldarrain, Deniz Kukul, Joana Larreta reports financial support was provided by LIFE programme. Anna Rubio reports financial support was provided by Horizon 2020.