Spatial and Temporal Distribution of Large (1–5 mm) Microplastics on the Strandline of a Macrotidal Sandy Beach (Polzeath, Southwest England) and Their Association with Beach-Cast Seaweed

Abstract

Microplastics (MPs) are ubiquitous and persistent contaminants of the marine environment, but a clear understanding of their cycling, fate, and impacts in coastal zones is lacking. In this study, large MPs (1–5 mm) were sampled spatially and temporally from the strandline of a macrotidal, sandy beach (Polzeath) in southwest England. MPs encompassing a diversity of sources were categorised by morphology (foams, nurdles, biobeads, fragments, fibres, films) and quantified by number and mass, with a selection analysed for polymer type. A total of about 17,600 particles of around 350 g in mass were retrieved from 30 samples over a period of five months, with an abundance ranging from 35 and 2048 per m². The space- and time-integrated average mass of MPs on the beach strandline was about 2 kg and was dominated (>90%) by fragments, nurdles, and biobeads of polyethylene or polypropylene construction. Nurdles, biobeads, fragments, and, to a lesser extent, fibres were correlated with strandline seaweed abundance, which itself was correlated with previous storm activity. Relationships with seaweed abundance were also supported by visible associations of these MP morphologies with macroalgal deposits through entanglement and adhesion. These observations, coupled with a lack of MPs below the sand’s surface (50 cm depth), suggest that the majority of MPs are transported from an offshore stock with floating organic debris, resulting in a transitory strandline repository and a habitat enriched with small plastics.

1. Introduction

Microplastics (MPs) are plastic particles < 5 mm in diameter, but encompassing various morphological forms, which have either been manufactured in this size range (primary) or have been weathered or fragmented from larger items to this size (secondary). Microplastics are ubiquitous and persistent environmental contaminants that have attracted a great deal of scientific interest, especially in the ocean. In these environments, MPs have a diverse array of sources, including atmospheric deposition (mainly fibres), riverine inputs, discharges from industry, agriculture, urban runoff and wastewater treatment, tourism and littering, shipping (including spillages), and fishing and aquaculture [1,2,3,4,5].

One of the main factors that determines the transport, weathering, impacts, and fate of MPs in the ocean is density. Thus, particles whose net density exceeds that of seawater (average density ~1.02 g cm⁻³) should ultimately sink to the seabed and become incorporated into the substratum [6], unless they are small enough to be retained in the microlayer by surface tension [7]. By contrast, particles whose densities are lower than that of seawater should be suspended or float on the surface, with scope for long-range transport and transitory beaching with other suspended or floating debris in the coastal zone [8,9].

Regarding beaches, the strandline is a critical zone, as this represents the first depositional habitat, generated by receding water levels, for low-density MPs and other litter before they are integrated into the standing stock of beached plastic or are returned to the sea [10]. As such, MPs along the strandline can provide insight into the type of plastic that is washed up from offshore and retained on the beach in the longer term or recycled within the coastal zone [11]. By conducting spatial and temporal studies of MPs of different morphologies (shapes and forms), potentially valuable information on the environmental drivers and mechanisms for the onshore–offshore transport and accumulation of different types of MPs can be gained [12].

In the present study, we determine the number, morphologies and, for a selection, polymeric construction of large MPs (1–5 mm; [13]) along the strandline of a sandy, windward, mid-latitude, and macrotidal beach (Polzeath, southwest England). Sampling was undertaken both spatially and temporally over a period that encompassed various tidal conditions and weather events. To assist our interpretation of the results and their drivers, we concurrently estimate the quantity of beach-cast wrack (organic matter and mainly seaweed) and, for comparison, determine large MPs at depth in the sand profile and above and below the strandline.

2. Materials and Methods

2.1. Study Site

Polzeath Beach (Figure 1; 50.573 N, 4.915 W) is a sandy embayment (about 600 m wide) at the mouth of the Camel Estuary, north Cornwall (southwest England). The beach is flanked by rocky headlands to the north and south and Polzeath village (population ~1500) to the east, and faces the Bristol Channel (Atlantic Ocean) to the west. The beach is subject to two daily tides of up to 9 m, with tidal currents moving in a northwest and southwest direction. However, wave-induced currents are more important to sand transport and result in a net drift of material to the north [14]. The climate of the region is oceanic, and prevailing winds are from the southwest.

The catchment of the Camel Estuary (about 500 km²) consists of suburban land, arable and horticultural land, broadleaf woodland, and improved grassland. Within the catchment, there are 28 active sewage outflows arising from wastewater treatment plants, combined sewer overflows, and primary settlement tanks, with a further five situated along the surrounding coastline [15]. Polzeath Brook, a 4.5 km stream flowing through agricultural land, and several drains and road runoff outlets discharge directly onto or close to the beach. The intensities and precise pathways of the outflows vary and, despite accentuation during heavy rainfall, visible sewage has not been observed on Polzeath Beach over the past few years [16].

Polzeath Beach is surrounded by protected, priority habitats, ranging from maritime cliffs and subtidal kelp beds to deciduous woodland and shellfish waters, and the beach itself is a popular tourist destination and hosts various watersports. During the tourist season, the sheltered southernmost part of the beach is used as a tidal carpark. Because of tourism, sewage effluents, fisheries, regular strandings of macroalgae, and an Atlantic-facing aspect, Polzeath is subject to visible littering from a diverse array of sources [16]. In order to maintain cleanliness, beach cleans are regularly organised by the local council and charities that aim to remove large pieces of litter from the back beach.

2.2. Beach Sand Sampling

Samples of sand were collected from the strandline at Polzeath, the highest point of the intertidal zone and first depositional habitat for large MPs. On eleven occasions (A–K) over a period of five months (June–November 2023) that encompassed different seasons, weather conditions, and tourist activities, the strandline was identified from macroalgal deposits or textural changes in the sand. Samples were collected at up to five locations across a 250 m section of the strandline (n = 30 in total; Figure 1) that were selected by distance using an online unbiased number generator. The top 2 cm of sand was retrieved with a stainless-steel trowel from a 1 m² quadrat, filling two 3 L polyvinyl chloride buckets (about 10 kg of sand in total). Other observations, including the presence of macro-litter and tourist activity, were also noted, and seaweed abundance and previous storm activity were ranked (0 = no seaweed visible, 10 = highest abundance observed; 0 = no recent storms, 1 = a storm within the week before sample collection, 2 = a storm within the two days before sample collection). On the last sampling occasion, three additional samples of the same area and volume were collected for comparative purposes: surficial sand from the intertidal zone below the strandline; surficial sand from about 20 m above the strandline; and, with the aid of a stainless-steel shovel and a graduated rule, sand above the strandline, but at a depth of 0.5 m.

2.3. Sieving and Visual Sorting for Microplastics

Samples were processed at the Polzeath Marine Conservation Centre, about 50 m beyond the beach. Sand was air-dried for several days at room temperature to eliminate the majority of moisture before being sieved through a 1 mm nylon mesh. Material retained on the mesh was visually sorted into plastics and other debris (mainly organic matter) using metal forceps. Plastics were identified by their hardness, colour pigmentation, morphology, and texture [17] using a Parco Scientific binocular stereomicroscope, and a hot-needle test was used to assist in the identification of less distinctive particles. Plastics were also scaled using Mitutoyo Vernier callipers, and any with a dimension above 5 mm (or fibres below 5 mm in diameter, but more than 5 cm in length) were discarded (e.g., bottle caps, cigarette butts, fishing rope).

Isolated MPs were subsequently sorted and categorised by morphology as foams (e.g., expanded and extruded polystyrene and polyurethane), nurdles (disc-shaped pre-production pellets), biobeads (mainly black and ridged, derived from sewage treatment; [18]), fragments (irregular and semi-flexible or hard particles), fibres (long and thin flexible threads or strands with a consistent width), and films (thin, flexible, and sheet-like layers). In the laboratory (University of Plymouth), MPs in each category were counted, and the combined weight was recorded on a two- or three-figure electronic balance.

2.4. Fourier-Transform Infrared Spectroscopy

A total of 30 MPs, comprising 5 randomly selected from each morphological category, were analysed by attenuated total reflectance-Fourier-transform infrared (ATR-FTIR) spectroscopy in order to identify polymeric construction. A stainless-steel scalpel was used to scrape a piece of a few mm in size from the plastic surface that was then clamped against the diamond crystal of a Specac compression cell. Sample spectra were then acquired with 16 scans in the region 4000 to 600 cm⁻¹ and at a resolution of 4 cm⁻¹ using a Bruker Vertex 70 spectrometer. Spectra were compared with multiple reference libraries using Bruker’s Opus v7.5 spectroscopy software, with a hit quality above 75% assigned as a positive polymer identification.

2.5. Statistics

Concentrations of MPs are presented per m² or per kg of dry sand, with the mass of sand not measured because of the large volumes of material processed but estimated from a representative density (1.67 kg m⁻³; [19]). Descriptive statistics were computed and statistical analysis for difference testing and relationships performed in Minitab v19 on quantitative data after checking for normality using an Anderson–Darling test. An α-value for significance was set at 0.05.

3. Results

3.1. Environmental Variables

Table 1. Sampling dates (day month year) and codes (as capital letters), tidal range, mean daily wind speed, and storm activity (0 = no recent storms, 1 = a storm within the week before sample collection, 2 = a storm within the two days before sample collection), along with sample numbers and the estimated relative abundance of beach-cast seaweed in each sample. Note, samples 31, 32, and 33 (in parentheses) are below the strandline, above the strandline, and at 50 cm depth (and above the strandline), respectively.

Sampling	Date	Tidal Range, m	Wind Speed, mph	Storm Activity	Sample Number	Seaweed
A	17 June 2023	5.2	7	0	1	6
					2	6
B	24 June 2023	3.9	14	0	3	2
					4	1
C	19 July 2023	5.1	12	0	5	1
					6	3
D	4 August 2023	7.2	13	0	7	3
					8	0
E	11 August 2023	2.8	10	0	9	8
F	14 August 2023	4.6	10	1	10	5
					11	9
					12	4
G	15 August 2023	4.8	12	1	13	7
H	26 August 2023	2.7	6	1	14	4
					15	6
					16	9
					17	10
I	30 September 2023	7.6	23	2	18	6
					19	7
					20	9
J	22 October 2023	3.0	18	0	21	4
					22	4
					23	6
					24	5
					25	8
K	4 November 2023	2.8	19	2	26	5
					27	9
					28	10
					29	10
					30	8
					(31)	(0)
					(32)	(3)
					(33)	(0)

shows the sampling dates and the corresponding tidal range, mean daily wind speed, and recent storm activity, along with the number of samples collected on each occasion and the relative abundance of beach-cast seaweed present. The abundance of seaweed, and mainly Fucus vesiculosus, as a marker for deposition of natural low-density (buoyant) material, was highly variable, both between and within sampling dates, but tended to be greatest (above an abundance value of 9) following storm activity.

3.2. Number and Mass of MPs

Table 2. Numbers of large MPs (1–5 mm) by morphology in each sample (1 m² or about 10 kg on a dry weight basis), along with total numbers and summary statistics. Note, samples 31, 32, and 33 (in parentheses) are below the strandline, above the strandline, and at 50 cm depth (and above the strandline), respectively, and are excluded from the summations and summary statistics.

Sample Number	Number of MPs
	Foams	Nurdles	Biobeads	Fragments	Fibres	Films	Total
1	15	183	89	280	3	0	570
2	11	169	93	270	6	0	549
3	4	83	27	108	10	0	232
4	16	0	1	100	7	5	129
5	10	15	1	52	11	0	89
6	64	92	26	40	10	5	237
7	20	13	4	63	6	0	106
8	0	6	1	27	1	0	35
9	13	42	29	310	7	0	401
10	50	305	85	845	6	2	1293
11	25	35	15	257	11	0	343
12	0	22	5	97	2	0	126
13	63	86	53	425	13	2	642
14	10	13	3	178	11	2	217
15	7	19	13	280	11	2	332
16	4	95	39	590	32	1	761
17	8	134	42	650	42	2	878
18	5	40	19	164	3	2	233
19	38	25	20	141	33	1	258
20	19	34	14	204	11	0	282
21	5	11	6	128	1	0	151
22	6	21	9	236	2	3	277
23	27	43	21	520	7	0	618
24	22	40	10	450	7	2	531
25	70	41	14	628	8	1	762
26	15	71	32	554	4	1	677
27	32	248	116	1439	5	0	1840
28	42	310	111	1575	7	3	2048
29	22	293	92	1325	9	2	1743
30	51	173	47	962	31	1	1265
(31)	(2)	(2)	(1)	(21)	(0)	(0)	(26)
(32)	(50)	(173)	(84)	(310)	(9)	(5)	(631)
(33)	(4)	(4)	(0)	(52)	(1)	(0)	(61)
total	674	2662	1037	12,898	317	37	17,625
mean	22.5	88.7	34.6	430	10.6	1.2	588
sd	20.1	94.8	35.2	421	10.2	1.4	542
median	15.5	41.5	20.5	275	7.0	1.0	372
min	0	0	1	27	1	0	35
max	70	310	116	1575	42	5	2048

shows the number of large MPs (1–5 mm) in each sample, categorised by morphology. Overall, and excluding the three non-strandline samples, 17,625 MPs were isolated, of which the majority were fragments of various colours and sizes (about 73%), nurdles (about 15%), and biobeads (about 5%). Microplastics were present in all samples and ranged from 35 to 2048 per m², or from about 3.5 to 205 per kg, and while fragments, biobeads, and fibres were always observed, films and foams, and, in one case, nurdles, were sometimes absent.

Table 3. The dry mass of large MPs (1–5 mm) by morphology in each sample (1 m² or about 10 kg on a dry weight basis), along with total mass and summary statistics. Note, samples 31, 32, and 33 (in parentheses) are below the strandline, above the strandline, and at 50 cm depth (and above the strandline), respectively, and are excluded from the summations and summary statistics.

Sample Number	Mass of MPs, g
	Foams	Nurdles	Biobeads	Fragments	Fibres	Films	Total
1	0.01	4.22	2.16	13.75	0.04	0.00	20.18
2	0.01	4.78	2.12	11.20	0.01	0.00	18.12
3	0.14	2.06	0.78	3.91	0.03	0.00	6.92
4	0.62	0.00	0.02	9.16	0.37	0.20	10.37
5	0.61	0.37	0.03	2.67	0.61	0.00	4.29
6	1.74	2.58	0.79	10.20	0.42	0.07	15.80
7	0.21	0.33	0.12	3.08	0.35	0.00	4.09
8	0.00	0.11	0.02	1.13	0.29	0.00	1.55
9	0.11	1.40	0.65	8.08	0.50	0.00	10.74
10	0.37	6.63	2.31	17.33	0.26	0.01	26.91
11	0.78	0.82	0.38	6.30	0.44	0.00	8.72
12	0.00	0.73	0.09	3.09	0.01	0.00	3.92
13	0.68	2.54	0.95	8.67	0.31	0.05	13.20
14	0.04	0.28	0.05	4.55	0.35	0.06	5.33
15	0.02	0.53	0.29	4.71	0.50	0.05	6.10
16	0.08	2.12	1.40	13.22	0.82	0.04	17.68
17	0.04	2.10	0.69	11.91	0.36	0.05	15.15
18	0.44	1.17	0.55	3.76	0.05	0.00	5.97
19	0.39	0.56	0.53	3.54	0.18	0.01	5.21
20	0.06	0.76	0.41	4.50	0.91	0.00	6.64
21	0.01	0.30	0.01	1.91	0.01	0.00	2.24
22	0.02	0.43	0.25	2.65	0.08	0.07	3.50
23	0.06	0.79	0.52	5.46	0.03	0.00	6.86
24	0.10	0.75	0.24	5.87	0.04	0.02	7.02
25	0.78	0.83	0.41	5.23	0.10	0.12	7.47
26	0.04	1.59	0.78	10.98	0.06	0.03	13.48
27	0.12	4.66	2.78	20.76	0.13	0.00	28.45
28	0.26	6.76	2.89	25.04	0.12	0.03	35.10
29	0.60	5.73	2.31	11.87	0.69	0.06	21.26
30	0.25	3.57	1.25	13.09	0.43	0.01	18.60
(31)	(0.01)	(0.09)	(0.02)	(0.56)	(0.00)	(0.00)	(0.00)
(32)	(0.35)	(4.26)	(1.98)	(8.70)	(2.40)	(0.04)	(17.73)
(33)	(0.01)	(0.11)	(0.00)	(0.65)	(0.80)	(0.00)	(1.57)
total	8.59	59.5	25.78	247.62	8.50	0.88	350.87
mean	0.29	1.98	0.86	8.25	0.28	0.03	11.70
sd	0.38	2.00	0.88	5.82	0.25	0.04	8.48
median	0.12	1.00	0.54	6.09	0.28	0.01	8.10
min	0.00	0.00	0.01	1.13	0.01	0.00	1.55
max	1.74	6.76	2.89	25.04	0.91	0.20	35.10

presents the data from Table 2 on a mass basis and to the nearest 10 mg. Excluding the three non-strandline samples, the total mass of MPs retrieved was about 350 g, and the distribution by morphology was similar to that on a number basis (about 71% fragments, 17% nurdles, and 7% biobeads). Dividing the total mass of MPs according to type by the corresponding total number of MPs by type provides a measure of the average mass of each type of MP. Accordingly, average masses are 12.7 mg for foams, 19.2 mg for fragments, between about 22 and 25 mg for nurdles, biobeads, and films, and 26.8 mg for fibres. Overall, the average mass of an individual MP retrieved from the strandline is 19.9 mg.

When all particles were considered in individual samples, however, the relationships between mass and number revealed differences according to morphology (Figure 2). Thus, linear relationships for nurdles and biobeads indicate a rather uniform mass amongst these particle types, whereas fragments and fibres display more dispersed relationships, reflecting a wider variety of particle masses and sizes. For foams and films, there were fewer data points, but there was evidence of distinct groups by mass. Specifically, for foams, there appears to be distinctively larger and smaller groups, although this distinction could not be related to any environmental variable, time of year, or seaweed coverage. For the samples taken away from the strandline, and excluding fibres, data points defining MP mass to number lie close to strandline data, although samples in the intertidal zone and below the surface were close to the origin. Regarding fibres, those detected away from the strandline appeared to be anomalously large.

3.3. Correlations

Because strandline MP numbers and environmental data sets were ranked or non-normally distributed, relationships between variables were analysed by Spearman’s rank correlation. Results are shown in Figure 3, where statistically significant relationships are identified. Among the environmental variables, wind speed and storm activity and seaweed abundance and storm activity were positively correlated. Among the MP morphologies, nurdles, biobeads, and fragments were highly correlated with each other and with total MPs. Between environmental variables and MPs, fragments and, to a lesser extent, total particles were both inversely correlated with tidal range and positively correlated with storm activity, and, with the exception of foams and films, all MP types were positively correlated with beach-cast seaweed abundance.

3.4. Polymer Types

The polymers identified in the 30 MPs analysed by FTIR spectroscopy are presented in

Table 4. Polymers, along with their densities (in parentheses and g cm⁻³), identified in thirty MP samples (five from each morphological category). PE = polyethylene, PP = polypropylene, EPS = expanded polystyrene, FPU = foamed polyurethane, RY = rayon, TPV = thermoplastic vulcanisate. Densities reported by Das [20], GP [21], Omnexus [22], and Vinyltec [23].

Polymer (Density)	PE (0.91–0.97)	PP (0.90)	EPS (0.02–0.04)	FPU (0.05–0.96)	RY (1.53)	TPV (0.96)
Foams			3	2
Nurdles	4	1
Biobeads	5
Fragments	3	2
Fibres	2	2			1
Films	3	1				1
Total	17	6	3	2	1	1

. Overall, the polyolefins, polyethylene and polypropylene, were dominant (n = 23) and were encountered in all categories with the exception of foams, where expanded polystyrene or foamed polyurethane were present. Additional polymers identified were rayon in one small fibre and thermoplastic vulcanizate, a mixture of polyolefin and uncured rubber, in a film. In only one case (rayon) was polymer density (also shown in Table 4) greater than that of temperate, coastal seawater (about 1.02 g cm⁻³).

4. Discussion

Overall, our observations are comparable to those of Wilson et al. [24] for sixteen beaches from the southern Bristol Channel, all to the east of Polzeath, that were sampled for 1–5 mm MPs along their respective strandlines. Thus, 74% of large MPs were fragments and 13% were pellets-beads (compared with 73% and 21%, respectively, in the present study), and hard plastics were dominated by polyethylene and polypropylene (61% and 26%, respectively). However, Wilson et al. [24] did not consider temporal trends, and focussed on a single month when storms were absent. Moreover, the potential role of beach-cast macroalgae and MP distributions away from the strandline were not addressed.

This study shows that large MPs are heterogeneously distributed on Polzeath Beach strandline, both temporally and spatially. This reflects the multitude of sources of MPs (land-based, offshore, littering) and the complexity of environmental processes acting on their transport, morphological modification, and deposition (wind, tides, currents, weathering).

Nurdles and biobeads have similar sizes, shapes, densities, and sources (mainly land-based spillages and, for nurdles, spillages during transportation offshore; [18,25]); consequently, their abundances were highly correlated (Figure 3). Both MP types were positively correlated with seaweed abundance, and were often observed to be entangled among algal deposits and occasionally adhered to fronds and thalli of F. vesiculosus. This suggests that nurdles and biobeads and detached, free-floating seaweed are subject to the same environmental processes leading to their beaching on the strandline. Moreover, seaweed drifting for long distances (up to hundreds of km; [26]) may act to sweep up and concentrate these MPs, an effect facilitated by the polysaccharide-rich mucus on the algal surface [27,28]. Once seaweed is beached, it may begin desiccation and burial, be returned to sea with the next, higher tide, or undergo tidal- and wind-assisted transportation higher up the beach [29]. That only four nurdles and biobeads were observed at depth, but were abundant above the contemporary strandline suggests the latter, redistributional process is more important for MPs than burial, at least to the depth considered here.

Fragments, films, and foams are secondary MPs that, in the marine environment, are derived from the physical and chemical weathering of a range of less-well defined primary plastics [30]. Fragments were the most abundant type of large MPs and were the most diverse in terms of colour. A statistically significant inverse relationship with tidal height suggests that their principal source is offshore (although this may include both land-based and maritime plastics), with higher and more energetic tides impeding and lower and less energetic tides facilitating deposition along the strandline. While fragments are similar to nurdles and biobeads in terms of (mean) size and polymer construction, their stronger correlation with seaweed abundance, and their greater visible presence among algal deposits, likely results from their flatter and more angular structure that promotes entanglement and adhesion. More flexible and thinner films constructed of polyolefins are predicted to behave like fragments, but their abundance (0.2% of total MPs) was too restricted to draw any statistical inferences.

Compared with nurdles and beads, foams and fragments were more abundant (relative to their respective mean strandline numbers) at depth (Table 2). In theory, particles with greater sphericity (pellets and beads) are more capable of infiltrating porous solids like sand than angular fragments [31]. This, therefore, suggests that particles are distributed at depth through burial during storms or strong winds, for instance [32], and that upward migration of positively buoyant MPs (or reverse infiltration) is then more impeded for those with an angular form.

The densities of expanded and extruded polystyrene and foamed polyurethane ensure that they float on the sea surface rather than in suspension near to the surface. While there is clearly scope for these plastics to interact with seaweed, the foams we observed were only loosely distributed within algal deposits and never attached or entangled. Their low density and loose algal association promote subsequent dispersal by wind to higher reaches of the beach [33]. Regarding the two groupings based on mass to number ratio (Figure 2), we note that the larger, heavier foams were polyurethane fragments collected just after a yacht was wrecked in Polzeath Bay (polyurethane is commonly used in boat cavities as an insulator and buoyancy aid; [34]).

Fibres are more challenging to sample or define consistently because their aspect and flexibility allow for relatively long particles to pass through the (much smaller) pores of a mesh. In the present study, fibres included a heterogeneous array of monofilament and twisted or braided filaments of thickness < 5 mm, but whose length was capped at 5 cm. With the exception of the smallest visible strands, which are likely derived from textiles, the major source of this category of MP is the fishing industry that, in the present setting, has a clear, offshore source through the abandonment and loss of rope, nets, line, and twine [35]. A weaker but nevertheless significant correlation was observed with seaweed abundance, but here a visible association was mainly established through the entanglement of fibrous plastics with fronds and thalli rather than adherence. MP fibres are readily retained by the interstitial spaces between sand grains, but infiltration is more limited than other shapes [36]. Nevertheless, a limited number of relatively large fibres were found below the surface and on the back beach at Polzeath.

Overall, a median mass of all categories of large MPs retrieved from Polzeath is 8.1 g m⁻², or 8.1 g per m of strandline. For a total length of strandline of 250 m, this is equivalent to a representative time- and space-averaged strandline mass of about 2 kg. An important question that arises is what is the fate of this plastic? The considerations and modelling studies of Lebreton et al. [37] and Turrell [38] suggest that floating plastic undergoes repeated episodes of stranding and release, with fractions undergoing loss through offshore transport and degradation into smaller MPs that evade our definition and detection. Under this scenario, broadly the same particles could be encountered over a period of time, whose precise composition is dependent on factors like tides, winds, currents, and seaweed abundance and casting. This would account for the identification of beached plastics that can be dated back many decades [39,40]. Alternatively, there could be an equilibrium established in which the stock of floating plastic offshore is continuously shifting, but inputs to the strandline are balanced by losses through burial and transport to the back-beach where intervention is more focussed. In reality, it is likely that both scenarios combine and that one or the other is more important for the different morphological categories.

More generally, could the broad findings of this study be extrapolated to smaller (<1 mm) MPs? On the one hand, beach studies suggest that this population is dominated by cellulosic and thermoplastic fibres and that pellet-beads, foams, and films are not abundant [41,42,43]; on the other hand, however, studies using macroalgae have shown that microscopic fibres have a high affinity for the polysaccharide-rich surface [27,28]. The association of MPs, and, in particular, smaller MPs, with beach-cast seaweed is a concern from an ecological perspective, as wrack is a hot spot of biological activity [44]. Here, a variety of organisms, including intertidal macroinvertebrates and higher-trophic-level consumers [45], will be exposed to relatively high concentrations of plastic.

5. Conclusions

This study shows that large (1–5 mm) MPs are heterogeneously distributed along the strandline of a sandy, windward, macrotidal beach in southwest England, both spatially and temporally. MP concentrations ranged from 35 to 2048 per m⁻², or from about 3.5 to 200 per kg of sand, and yielded a space- and time-averaged quantity of MPs along the entire strandline of about 2 kg. Microplastics were dominated by fragments, nurdles, and biobeads constructed of polyethylene or polypropylene, and the abundance of each morphological type was significantly correlated with the amount of beach-cast seaweed present. Moreover, many items were visibly entangled with or adhered to the fronds and thalli of F. vesiculosus. Although surficial MPs were observed above the contemporary strandline, they were infrequently found at depth (50 cm below the surface). The results suggest that most MPs are transported from offshore with floating organic debris, resulting in a transitory detrital plastic habitat. While many of the findings of this study are likely to be more generally applicable, further research into a broader range of beaches of different morphologies and hydrodynamics is recommended.