Posidonia Spheroids Intercepting Plastic Litter: Implications for Beach Clean-Ups

: This study represents the ﬁrst assessment of plastic waste within Posidonia spheroids on four sandy, Mediterranean beaches, each characterized by varying levels of anthropogenic inﬂuence. Fifty-ﬁve (68.7%) spheroids, out of eighty examined, included plastic litter. A total of 202 plastic items were isolated. Plastic abundance was 2.5 items/spheroid corresponding to 132 items per kilogram. The length of plastic items ranged from 0.1 to 50 mm. Fibers, tangled ﬁbers and fragments were the most common shapes. The spheroids exhibited a substantial capacity for trapping plastic waste, with notable differences among the beaches. Our results underscore the signiﬁcance of implementing a beach clean-up plan aimed at removing all spheroids to prevent them from disintegrating and releasing trapped plastic waste into the environment. Manual removal is recommended to safeguard the beaches, and this process should target all spheroids, regardless of their size. This study provides valuable insights that can inform marine litter monitoring programs, contribute to the development of tailored management measures, and support the implementation of speciﬁc action plans to mitigate Mediterranean microplastic pollution.

Seagrass meadows are recognized for their capacity to filter, trap, and store various particles, encompassing both organic and inorganic matter [20,21], among which plastics have been documented [22][23][24][25].These particles are typically deposited on the leaves and within the sediment or on the soil surface [24,26,27].Plastics have also been identified in wrack composed of other macrophytes, including seagrasses belonging to genera such as Halodule, Syringodium, Thalassodendron, Zostera, Heterozostera, Amphibolis, and Cymodoces [28].In Singapore, Seng et al. [29] detected microplastic items on seagrass blades.In addition, correlation between plastic abundance and EG size.The evaluation of plastic abundance within EGs is important as they can reflect the plastic pollution level of both marine and beached sediments.The relation between EG size and plastic abundance can have implications in the context of beach litter management.In accordance with Pietrelli et al. [48], we expect to find greater quantities of plastic items in the largest balls; moreover, with this study, we expect that the removal of EGs can be used as a strategy to mitigate the impact of microplastics in beaches.

Study Areas and Samplings
Mazara del Vallo falls within the geographical subarea 16-"South of Sicily"-that is affected by several anthropic activities (commercial and tourist ports, agricultural and industrial activities, mariculture facilities, oil refineries, and offshore platforms).The South of Sicily also represents an important crossroads for Mediterranean trade routes (including the oil traffic), due to its central position.The South of Sicily is one of the most important fishing areas in the Mediterranean Sea with the fishing fleet of Mazara del Vallo [49][50][51].The territory of Mazara is crossed by two rivers, the Mazaro and the Delia (also called Arena), the latter starting from Lake Trinità, an artificial lake created by the homonymous dam, located on the border with the Castelvetrano area.According to a study carried out by the Sicily Region, the ecological status of the Mazaro River is sufficient while that of the Delia River is good [52].There are also several streams (Iudeo, Bucari) and artificial canals used mainly in agriculture.EGs were collected from three beaches along the south-west part of Sicily close to Mazara del Vallo, namely Capo Feto (1), Tonnarella (2), and San Vito (3).
Capo Feto (37 Augusta Bay (Siracusa, Italy), located in the geographical subareas 19-Western Ionian Sea-in a harbor area with high marine traffic activity.This area has hosted a variety of different chemical and petrochemical plants, a commercial harbor, and bases for the Italian Navy and NATO activities.The Augusta Bay is an area with high anthropic impact, considered one of the most polluted in the Mediterranean Sea due to the high presence of heavy metals.The Gulf of Augusta is considered an elevated environmental risk site by the World Health Organization and classified as a "site of national interest" by the Italian Ministry of Environment (G.U.R.I., L. 426/1998).Spiaggetta del Sole (37 • 14 26.0 N 15 • 14 11.3 E) is a touristic beach within the city of Augusta (Figures 1 and 2A).The investigated areas are briefly summarized in Table 1.Four hundred EGs, one hundred for each site, were collected simultaneously on the beaches within a band 1 km long and 5 m wide starting from the shoreline, in summer 2022 before the beach clean-up.

Plastic Isolation
Once in the laboratory, samples were dried at room temperature (25 • C) and under low humidity for a week [42], then 20 EGs for each beach were selected to obtain the widest range size (Figure 2B).The samples were dried at room temperature for a week.
Selected samples were weighed (g) and the length (mm) of the three principal axes was measured with a digital caliper assuming that an EG is like an ellipsoid (Figure S1).After that, EGs were carefully disentangled manually into fibers under a laminal flow microbiological hood.The fibers were sieved at 8 mm, 5 mm, 1 mm, and 0.63 mm using stainless steel sieves, washed with water to remove sand and salt, and dried at room temperature [42].Then, the disentangled EGs were observed under a stereomicroscope in order to isolate plastic debris.For each suspected item, the hot needle test was undertaken.The hot needle test is an accepted and cheap method used to verify plastic particles on the bases of their response [16,53,54].The tip of a thin needle was heated, and each isolated particle was tested under a stereomicroscope.When particles melted after exposition to the hot needle, they were confirmed as plastic items [16].Isolated plastic items were classified based on their size (small-microplastics: 0.1-0.9mm; large-microplastics: 1-4.9 mm; mesoplastics: 5-25 mm; macroplastics: >25 mm) and shape (pellet, fiber, tangled fiber, foam, fragment, film, and sphere).

Prevention Contamination
To limit external contamination, the samples were processed in a restricted room and under a microbiological laminar hood.Moreover, all workspaces and tools were meticulously cleaned with ethanol and filtered deionized water.The operators wore cotton coats (100% cotton) and latex gloves.Moreover, the procedural blanks were run, and filters (cellulose nitrate membrane diameter of 47 mm) were put in open Petri dishes, both under the microbiological laminar hood and next to the stereomicroscope [14].

Data Analysis
The roundness of EGs was calculated using the equation: where W represents the width, H is the height, L stands for the length, and D max is the maximum dimension measured (see Figure S1).Roundness is 1 for a perfectly round object and less than 1 for any other object [55].
We compared the different sampling sites based on the frequency of occurrence (FO%) and abundance (number of plastic items per spheroid).To check for potential significant differences in the occurrence of plastic items between sites, we performed a chi-squared test (χ 2 ).Spearman correlation was performed to assess the correlation between: (1) plastic abundance vs. EGs weight; (2) plastic abundance vs. EGs length; (3) plastic abundance vs. EGs height; (4) plastic abundance vs. EGs width; and (5) plastic abundance vs. EGs roundness.The Kruskal-Wallis non-parametric test was used to test whether there were any significant differences in the plastic abundance between sampling sites.All graphs and statistical analyses were performed using GraphPad Prism 8.4.2.3.

Beach Comparison
Plastic items were found in the EGs of all beaches.In particular, the occurrence was highest in the EGs from Spiaggetta del Sole (100%), followed by San Vito (80%), Tonnarella (70%), and Capo Feto (25%).Significant differences in the occurrence of plastic items were only detected between Spiaggetta del Sole and Capo Feto (χ 2 : 62.5, p < 0.05).
As for colors, Capo Feto had a higher quantity of green and white debris, while Tonnarella predominantly featured white debris.In San Vito, transparent debris was the most abundant, while Spiaggetta del Sole had predominantly white debris (see Figure 6)."Concerning size, large microplastics were the most abundant in the EGs from Capo Feto beach; mesoplastics were more abundant in EGs from Tonnarella, S. Vito, and Spiaggetta del Sole beaches (Figure 4).Fibers and tangles fibers were the most frequent in all beaches investigated, except for Tonnarella, where fragments prevailed (Figure 5).As for colors, Capo Feto had a higher quantity of green and white debris, while Tonnarella predominantly featured white debris.In San Vito, transparent debris was the most abundant, while Spiaggetta del Sole had predominantly white debris (see Figure 6).

Discussion
Studies focused on plastic litter have been carried out in marine and coastal ecosystems [56,57]; but only recently has the analysis of litter ashore from the sea to the beach received the attention of the scientific community [42,48,58,59].

Discussion
Studies focused on plastic litter have been carried out in marine and coastal ecosystems [56,57]; but only recently has the analysis of litter ashore from the sea to the beach received the attention of the scientific community [42,48,58,59].
This study evaluated, for the first time, the presence of plastic litter entrapped in Posidonia EGs on four Sicilian sandy beaches.Although the survey is limited to a single year and a single season, it was possible to collect the first data on plastic abundance within EGs in the considered areas.EGs, during their development, can incorporate debris from the sea bottom, seawater, beach sediments, and in the air.Plastic occurrence in the EGs used in this study (68.7%) is higher than those reported by Sanchez-Vidal et al. (17%) [42] and Pietrelli et al. (53%) [48].Similarly, the plastic abundance reported in this study (2.5 items/spheroid) was higher than that reported in EGs in the central Tyrrhenian Sea beaches (0.6 items/spheroid) [48].Unlike what we expected, there was a weak correlation between the size of the EGs and the abundance of plastic litter.In fact, plastic items were found even in the smallest EGs, and sometimes even in greater quantities than in the larger balls.
In this study, we found that EGs act as a trap for plastic litter ranging from micro (<5 mm) to macro (>25 mm), which aligns with findings in P. oceanica meadows [60].Like other plants, including dune plants and mangroves [61][62][63], Posidonia EGs entangle a significant amount of plastic litter (132 items/kg).This underscores the valuable role of EGs in monitoring plastic pollution.
Regarding the shape of the plastic items, we predominantly found fibers, consistent with the results of the study by Sanchez Vidal et al. (filaments and fibers: 65%; fragments: 22%; film: 8%) [42].The occurrence of fibers can be linked to textile pollution, as textiles are currently a primary source of microfibers in the environment.Manshoven et al. [64] reported that wearing and washing synthetic textiles is regarded as being responsible for the discharge of between 0.2 and 0.5 million tons of anthropogenic particles into the oceans each year.Conventional wastewater treatment, including tertiary treatment techniques, may remove up to 98% of microfibers.Fishing is the primary marine-based source of plastic pollution [65,66].Abandoned, lost, or otherwise discarded fishing gear have been recognized as a major problem [67].Fishing ropes, nets, and lines are the main contributors to beached fishing litter.Beached fishing litter may release hundreds of microplastic pieces per meter [68].

Beach Comparison
The monitoring we conducted allowed us to identify differences between the beaches, which can be attributed to their different locations and varying degrees of use.The EGs collected at Spiaggetta del Sole exhibited the highest abundance of plastic items (4.8 items/spheroid).This may be attributed to the high level of urbanization along the coast, extensive fishery activities, and the presence of a wastewater treatment plant drainpipe, all of which can contribute to the accumulation of debris on the sea floor [38,40].The fragmentation and degradation processes acting on this debris can induce the formation of small particles.The ingestion of plastic items by mollusks, teleosts, and elasmobranch in the Ionian Sea has been reported [74,75].
Regarding the South of Sicily, Tonnarella and S. Vito beaches were also impacted by plastic litter and mainly by fibers.The South of Sicily falls in an area strongly affected by several anthropic activities (commercial and tourist ports, agricultural and industrial activities, mariculture facilities, oil refineries, and offshore platforms) and represents one of the most important fishing areas in the Mediterranean Sea.All these activities, together with environmental processes led to high marine litter abundance and density in the South of Sicily, as reported by Spedicato et al. [76] and Garofalo et al. [49].Moreover, a high percentage of the small spotted catshark (Scyliorhinus canicula) ingesting microfibers has been recorded by Mancia et al. [77] and Mancuso et al. [16] in the South of Sicily.
EGs from Capo Feto beach (SCI) showed the lowest plastic items abundance (0.5 items/spheroid).This result could reflect the protected status of the site, characterized by a lower anthropogenic impact than Tonnarella and San Vito beaches.

Implications for Cleaning Up
The fact that plastics might be intercepted by EGs has useful implications for beach clean-up management.The removal of EGs should be carried out by being mindful of vegetation, in fact, the removal of the balls should be manual at least in those stretches of coast where the EGs are on the banquettes (which play an important ecological role within the beaches and should be preserved [78].This kind of management should be the best practice, especially in protected areas (i.e., Capo Feto).Moreover, it is necessary to implement a beach cleaning plan for the removal of all EGs, both small and large, in order to prevent them from flaking and releasing the marine litter trapped inside them back into the environment.In this study, the small EGs could include up to eight pieces of plastic debris.In other words, EGs should be removed regardless of their size as their removal can help to mitigate plastic pollution in the marine environment.The degradation time of EGs is not currently known [42].We can hypothesize that the dense outward shell of the EGs and the refractory character of their lignocellulosic fibers make EGs resistant to degradation.In any case, what happens to "old EGs" and relative plastic debris once ashore deserves further investigations.
The removal of the EGs would gain the approval of stakeholders (tourists, local administrators) that generally dislike their presence on the beach.EGs have structural, mechanical, and physical properties that make them useful in different applications.The uses of EGs might also help in the future to sustain the economic growth of all the countries of the Mediterranean Sea.The removed EGs could be used as a renewable substrate to produce bio-absorbents in environmental remediation, as insulation and reinforcing materials for building and construction, as new material to create paper, textiles, or biofuel, and for bioplastic generation for the next generation [79][80][81][82].

Conclusions
The analysis of four beaches along the Sicilian coast demonstrated a significant presence of plastic litter within Posidonia spheroids.In the future, additional data on a larger time scale should be acquired to understand the different dynamics of the beach.The fact that plastics might be intercepted by spheroids has useful implications for beach clean-up management.In general, beach clean-ups are carried out by volunteers and operators and are focused on beached macrolitter.Instead, specific clean-ups should focus on Posidonia spheroids where a large amount of plastics can accumulate.The removal of spheroids can represent an important step towards environmental sustainability as it helps to mitigate the severe negative impacts of plastic on the marine ecosystem and the health of the environment.To successfully promote environmental sustainability through the removal of spheroids from beaches, it is important to establish regular cleaning programs, involve local communities, and raise public awareness on the issue.Additionally, it is essential to address the underlying causes of plastic pollution, such as excessive plastic production and usage, through targeted policies and initiatives.The attention of administrations should used to set up "working groups" which analyze the situations and, depending on the case, adopt the right solutions.Additionally, the authors propose conducting citizen science initiatives for gathering Posidonia spheroids.Combining plastic pollution studies with citizen science not only enhances research capabilities but also fosters greater environmental awareness among the public, ultimately enabling the collection of substantial volumes of waste within a brief timeframe.

Figure 2 .
Figure 2. (A) Pictures of the investigated beaches; (B) images of selected EGs from the four beaches.Figure 2. (A) Pictures of the investigated beaches; (B) images of selected EGs from the four beaches.

Figure 2 .
Figure 2. (A) Pictures of the investigated beaches; (B) images of selected EGs from the four beaches.Figure 2. (A) Pictures of the investigated beaches; (B) images of selected EGs from the four beaches.

Figure 3 .
Figure 3. Box plot of plastic abundance in EGs from the four beaches.Pictures refer to plastic items found at each beach.

Figure 3 .
Figure 3. Box plot of plastic abundance in EGs from the four beaches.Pictures refer to plastic items found at each beach.

Figure 4 .
Figure 4. Size (%) of plastic items in EGs from the four beaches.Figure 4. Size (%) of plastic items in EGs from the four beaches.

Figure 4 .
Figure 4. Size (%) of plastic items in EGs from the four beaches.Figure 4. Size (%) of plastic items in EGs from the four beaches.Sustainability 2023, 15, x FOR PEER REVIEW 10 of 16

Table 1 . Table 1. Description of four investigated beaches. Capo Feto Tonnarella S. Vito Spiaggetta del Sole
well as an area deserving of special protection (92/43/CEE).In July 2011, according to the Convention on Wetlands of International Importance, inserted in the Ramsar wetlands' list, Tonnarella (37 • 39 36.5 N 12 • 34 03.9 E) and S. Vito (37 • 38 17.8 N 12 • 36 38.7 E) are beaches with high anthropogenic impact during the summer and represent important tourist destinations.The main features of the investigated beach are briefly summarized in • 39 35.3 N 12 • 31 40.6 E) is a Site of Community Importance (SCI), as

Table 2 .
Morphometric data of EGs sampled in the four investigated beaches.