Impacts of Marine Plastic Pollution on Seagrass Meadows and Ecosystem Services in Southeast Asia

Abstract

Seagrass meadows provide ecosystem services (ES) that are critical for humanity. Southeast Asia (SEA) is a hotspot of seagrass diversity, and the region’s seagrass-derived ES have been valued at over $100 billion annually; however, the health and extent of seagrass meadows are in decline. Marine plastic pollution (MPP) is an escalating problem and a concern for vulnerable marine habitats such as seagrass meadows. To understand the impacts that MPP has on seagrass ecosystems and their provision of ES, we conducted a mixed methodology study that involved a systematic search of the literature and the synthesis of the results with a risk assessment. The results indicated that MPP negatively impacts seagrass through factors such as spatial competition, chemical leaching, and reduced rates of photosynthesis and rhizome growth. This can lead to a reduction in seagrass biomass, triggering the degradation of all seagrass-derived ES. A risk assessment of the microplastic impact results indicated that seagrass-derived ES are at high risk of decline from the current microplastic concentrations, which in turn indicates a potential threat to the well-being of those dependent on them.

1. Introduction

Plastic pollution is pervasive, and we have yet to understand its full implications [1]. Plastics enter our oceans at unprecedented rates, with no sign of slowing [2]. Plastics are found everywhere in the environment [3], and while plastics may break down to smaller sizes in the marine environment, they are not removed [4]. The sizes of plastic debris can be classified as macroplastic (>20 mm), mesoplastic (5–10 mm), and microplastic (<5 mm) [5]. It is commonly accepted that, unless the flow of plastics into the environment is halted, irreversible damage to ecosystems globally may be caused. There is mounting evidence of the detrimental effects of MPP on marine ecosystems [6,7,8,9,10]. Marine plastics predominantly originate from land, entering seas via rivers and along coasts [11]; therefore, coastal marine ecosystems are particularly susceptible to the impacts of MPP [12]. Seagrass meadows are some of the most widespread coastal marine ecosystems globally [13] and may trap plastic litter [14]; therefore, these habitats and the ES that they provide may be at risk.

Seagrass meadows provide many ES [15]; they increase the water quality by removing nutrients and particulate matter from the water column and stabilise marine sediments protecting the coastline [16]. Costanza et al. [17] used the natural capital approach in 2011 to value seagrass and algae ecosystems, which were found to be worth an estimated $28,916 (expressed in terms of 2007 ‘International $’ ha/yr). The seagrass extent in SEA was estimated at 3,676,260 ha in 2018 [18], which could be worth over $100 billion annually (2007 ‘International $’), according to Costanza’s valuation. There is growing interest in the importance of seagrass meadows as a global carbon sink [19]; seagrasses capture carbon from the environment, and some species sequester large amounts in marine sediments [20]. This interest extends to the development of carbon credits for the ‘blue carbon’ thought to be stored in seagrass meadows [21]. Seagrass meadows are also recognised as essential nursery grounds for commercially important fish [22,23], with Dorenbosch et al. [24] showing that the fish density and diversity increased in coral reefs closer to seagrass meadows, compared with reefs further from seagrass, highlighting the indirect benefits of seagrass ecosystems.

Seagrass ecosystems are highly biodiverse and productive [25], which is likely one of the drivers of ES provision [26]; however, climatic, environmental, and anthropogenic impacts threaten these habitats and their ES [27,28]. Seagrass ecosystems are susceptible to multiple pressures, such as overfishing, habitat degradation, pollution, and climate change, and are degrading worldwide [29]. Research highlights that these trends are not universal, with areas of seagrass in the Mediterranean Sea showing positive growth over a 5-year period [30]; however, globally, the net loss of an estimated 560,200 hectares of seagrass habitats has been observed [31].

Beaumont et al. [5] predict that MPP will lead to a decrease in ES provision across multiple marine habitats and species, and the impacts of MPP on the functioning and ES of seagrass ecosystems have been shown [32,33]. The level of provision of seagrass-derived ES is highly dependent upon the healthy functioning of these ecosystems [34]; therefore, it is vital to understand how human activities, including MPP, impact this habitat and how these impacts can be mitigated. The relationships between MPP and seagrass-derived ES provision are unstudied and this is a critical gap.

The SEA region is in a period of intense population and economic growth, driving coastal development and rising pollution levels (including MPP), which are having harmful effects on the coastal ecosystems found there [35]; however, the paucity of research and investment hampers the practical assessment of these impacts [36]. The seagrass diversity in this region is high, with almost 30% of the global species present, although this varies across the region [37]. Furthermore, SEA has a greater extent of seagrass meadows than any other region [38]; see Figure 1 for the distribution of seagrass in SEA. Despite international recognition of their importance [13], annual declines in seagrass meadows in SEA of 10% have been observed [39], comparatively greater than the global 7% decline [40].

SEA has some of the highest annual plastic emissions, hosting three of the world’s five most polluting nations [42]. The Philippines and Malaysia contribute the greatest amounts of MPP within the region [43]. MPP is often derived from riverine sources, with six of the eight major rivers in the region containing almost 30 times higher concentrations of plastic items than comparable European rivers [44]. Another important source of MPP in the region is the incoming global plastic waste trade after China banned plastic waste imports in 2017, which provided the opportunity for SEA nations to increase their waste trade imports [45]. However, without the infrastructure to manage these imports effectively, this imported waste is often poorly integrated into recycling systems [46,47], further adding to the region’s plastic emissions.

The mismanagement of domestic plastic waste is also a critical issue. The waste disposal facilities in this region are considered inadequate [48] to cope with the high levels of single-use plastic in circulation [49], and the geographical spread of the region presents specific waste management challenges [50]. Moreover, poverty in this region is high [51], leading to the proliferation of the single-use sachet trade [52]. While regional agreements have been made to tackle the plastic pollution issue, there is no evidence of MPP abatement [53].

The high levels of MPP in the SEA region have the potential to threaten globally important seagrass ecosystems and the associated ES of local and global significance. This report addresses a critical gap in knowledge and synthesises MPP’s impacts on seagrass ecosystems in SEA and the ES that they provide. Furthermore, we classify the risk level of MPP on seagrass-derived ES to inform future policy decisions.

2. Materials and Methods

To effectively analyse and compare seagrass-derived ES and the impacts on them, an analytical framework is required. With multiple attempts to create a universal classification system of ES, definitions can become obscured between different frameworks. The Common International Classification of Ecosystem Services of the European Environment Agency, Copenhagen, Denmark v 5.2 [54] was the most recently updated and was used as a reference point for this research. ES not relevant to seagrass meadows were excluded, and a simplified framework was developed, similarly to [55], with the final ES classifications shown in

Table 1. Simplified ecosystem service (ES) framework developed for this paper, derived from CICES v 5.2 [52] (see Supplementary Table S1 for details on how simplification was completed). Text colour denotes the ecosystem service section: green—provisioning, blue—regulating, and red—cultural.

Ecosystem Service Section	Simplified Ecosystem Service
Provisioning	Food provision
	Genetic material provision
Regulating	Water purification
	Climate regulation
	Coastal protection
	Life cycle maintenance
Cultural	Recreation/tourism
	Cognitive
	Symbolic/sacred

; we defined nine key ecosystem services provided by seagrass beds.

Three separate literature searches were conducted to identify (1) the ES provided by seagrass ecosystems globally, (2) the ES provided by seagrass ecosystems in SEA, and (3) the impacts of MPP on seagrass and their provision of ES. Search terms were developed to identify the literature that provided empirical evidence (Supplementary Table S2). The literature was searched using the Scopus database, and filters were applied (limited to article, English language, keywords: seagrass); all relevant hits were investigated for inclusion. Grey literature and secondary research were omitted. We supplemented the literature identified above with additional references using a snowballing methodology, whereby key references in the selected literature were reviewed and included in the corpus where appropriate and additional information was identified; these additional references informed the analysis of the plastic concentrations.

The literature reviews returned 155 articles documenting the ES provided by seagrass ecosystems globally, with 29 articles relating to seagrass-derived ES in SEA; of these 29 articles, five were in addition to the initial 155. Furthermore, the literature review on the impacts of MPP returned 48 additional articles (Figure 2).

The following information was extracted from the retained articles: (i) a list of ES that seagrass was demonstrated to provide, (ii) any effects on the functioning of seagrass discussed in the paper, and (iii) whether the paper evidenced the impacts of MPP on seagrass ecosystems. The ES identified from the literature were classified by section (provisioning, regulating, and cultural) and class type using the simplified framework (Table 1) and were given an evidence score of low, medium, or high depending upon the abundance of available literature documenting each ES (low = 1–5 articles, medium = 6–10 articles, high = 11+ articles).

Evidence of the interactions between MPP and seagrass ecosystems was extracted from the literature using a review format adapted from [6]. Impacts were classified as direct (direct interactions between MPP and seagrass ecosystems) or indirect (how changes to the condition of seagrass ecosystems affect their provision of ES) and categorised by the size of plastic: macro, micro, nano, or leachates. Direct impacts were also standardised based on the overall effect on the habitat, such as habitat degradation or process disruption. A scoring system adapted from [7] was applied based on the following attributes: the scale of the impact was scored from 1 to 4 (1 = 0–5%, 2 = > 5–15%, 3 = > 15–60%, 4 = > 60–100%), and the scale was defined as the % change in the measured attributes (e.g., growth metrics or the percentage of samples where plastics were observed). The direction of impact was scored as −1 or +1; the impact was defined as an increase/decrease in a species/functional process/ES. The overall magnitude of the impact was then calculated as

$$scale of impact * impact direction$$

The articles providing evidence of the direct interactions of MPP with seagrass ecosystems also underwent a quality assessment, adapted from [56], as detailed below, providing a basis for the robustness of the results to be judged.

Articles were scored out of 100 based on four equally weighted attributes, giving each article a quality score between 0 and 100. These scores were allocated as follows.

• Study design: primary data collection = 25; historical data = 20; regional knowledge = 15; time series = 10; secondary data = 5.
• Comparator: before, after, control, impact (BACI) = 25; before, after (BA) = 17; control, impact (CI) = 8; singular = 0.
• Spatial replication: multiple replications = 25; single replication = 12; no replications = 0.
• Temporal replication: multiple replications = 25; single replication = 12; no replications = 0.

Finally, to give an overall degree of reliability to each documented interaction, a confidence level was calculated as follows:

$$(quality assessment score+abundance of research) * consensus$$

where the abundance of research is either 1 (singular study) or 2 (multiple studies), and the consensus is either −1 (disagreement across literature) or +1 (agreement across literature). The confidence scores were then categorised as low, medium, or high.

The plastic concentrations found in the seagrass ecosystems and coastal regions of SEA were extracted from the articles to enable a risk analysis. Articles reporting plastic concentrations were categorised by the size of plastic as (a) microplastic abundance in sediments, (b) macroplastics within the seagrass canopy, or (c) microplastics in surface waters, with the units normalised to (a) “particles kg⁻¹”, (b) “items m⁻²”, and (c) “items m⁻³”, respectively. For the risk analysis, we focused on the concentrations of microplastics observed within sediments in SEA due to the abundance of available records. The pollution load index (PLI) was calculated for each reported concentration. The PLI was initially developed for heavy metals [57] but has recently been applied to microplastics in various environments [28,58,59]. The PLI is calculated as

$$PLI={\displaystyle \frac{C_i}{C_{oi}}}$$

$${PLI}_{zone}=\sqrt[n]{{PLI}_1*{PLI}_2*\dots {PLI}_n}$$

where C_i equals the concentration of plastic particles at each location, and C_oi is a defined value (for this research, the minimum observed concentration of plastics was used). To quantify the overall PLI for the region, the PLI_zone was calculated using the above formula, where n is the number of samples. The calculated PLI_zone was then compared against the risk level categories in

Table 2. Categories of risk level for microplastic pollution adapted from Picó et al. [59], with permission from Elsevier 2024.

Category of risk level	1	2	3	4
Pollution load index value (PLI)	<10	10–20	20–30	>30

, which were based on research by Picó et al. [59].

3. Results

The literature searches identified 155 articles evidencing the ES provided by seagrass ecosystems (complete reference list in Supplementary Table S3). While the classification varied across publications, the results were simplified using the framework outlined in Table 1. Regarding the simplified ES categories created for this paper in Table 1, seagrass was evidenced as providing all ES globally and in SEA (

Table 3. Ecosystem services provided by seagrass ecosystems as determined by the literature searches (shading indicates the evidence score of low, medium, or high, depending upon the abundance of literature evidencing each ecosystem service).

Section	Class	Seagrass-Derived Ecosystem Services Globally	Seagrass-Derived Ecosystem Services in Southeast Asia	Evidence Scores
Provisioning	Food provision				Low
	Genetic material provision				Medium
Regulating and maintaining	Water purification				High
	Climate regulation
	Coastal protection
	Life cycle maintenance
Cultural	Recreation/tourism
	Cognitive
	Symbolic/sacred

). The evidence scores were high for all regulating services, food provision, and symbolic/sacred importance globally; however, some evidence scores were lower within SEA, which was likely because much of this research was skewed towards higher-income countries. The evidence scores for recreation/tourism, cognitive effects, and the provision of genetic material were low at both the global and SEA levels, although this may be because of the difficulties in documenting some services.

The literature search on the impact of MPP on seagrass ecosystems returned 48 articles for in-depth review (Supplementary Table S4). Of these 48, sixteen evidenced the direct interactions of MPP and seagrass ecosystems. Twenty-two articles quantified the concentrations of MPP found in seagrass ecosystems, 18 of which compared these concentrations with those in other coastal habitats. The remaining ten articles, plus an additional 16 from the ES searches, provided contextual evidence of how changes in the condition of seagrass ecosystems affect their provision of ES (Supplementary Table S5).

The literature showed that direct interactions between MPP and seagrass ecosystems varied, with some conflicting results. The presence of plastics increased the diversity of associated microbes by up to 65.8%, including introducing taxa that cause wasting disease in seagrass, which is linked to 90% reductions in the seagrass meadow area extent [60]. When considering seagrass detritus decomposition, a vital life cycle service, the presence of plastic disposable masks increased this by 64.1% compared to the control [61]. In contrast, another study showed that high levels of MPP reduced decomposition by 36% [62]. Chemical compounds used in plastic production are numerous and have been observed to be adsorbed by seagrass tissues. Bis (2-ethylhexyl) terephthalate (DEHT) is observed at concentrations of 1.121–9.19 mg/kg [63,64], while the growth of seagrass is retarded in the presence of Bisphenol-A (BPA) at levels of 300 ng L⁻¹ and upwards [65,66,67].

The number of seagrass leaves per shoot also diminished in the presence of microplastic and nanoplastics by 26% [68]. Concurrently, plastic bags caused a 50% reduction in the rhizome length [69], while Menicagli et al. [70] showed MPP to prevent vertical rhizome growth and increase the vulnerability of seagrass to invasive algal growth and sediment deposition. Furthermore, under high concentrations of microplastics (1000 mg L⁻¹), the dark respiration of seagrass and epiphytes was reduced by 30–50%, respectively; simultaneously, photon and compensation irradiance were reduced by ~50% [71].

Baalkhuyur et al. [72] and Goss, Jaskiel, and Rotjan [73] discussed how seagrass-associated organisms incorporate plastics from the system into the food chain via grazing, which may negatively impact community biodiversity and abundance. However, some positive impacts were also observed. Pietrelli et al. [74] demonstrated how seagrass detritus traps MPP and transfers it to the shore via egagropiles, ball-shaped masses of seagrass detritus, enhancing the water quality of surrounding areas. Furthermore, plastic bottles in a seagrass meadow were found to be correlated with increased community biodiversity and abundance [75]. Although positive impacts are documented, the primary standardised impacts of MPP on seagrass ecosystems are negative and include habitat degradation and disruptions to physiological and biogeochemical processes (Supplementary Table S6).

An overview of the impacts of MPP on seagrass ecosystems is presented in

Table 4. The overall magnitude of the ecological impact, quality assessment, and confidence scores for articles detailing direct interactions between plastics and seagrass ecosystems. Scoring systems as per Methods section.

Size of Plastic	How Plastic Interacts with Seagrass Meadows or Their Associated Organisms	Quality Score	Confidence Score	Overall Magnitude of Ecological Impact	Reference
Leachates	Plastic leachates adsorbed by seagrass tissues	83	High	−4	[63]
		83	High	−4	[64]
	Increased BPAs correlated with reduced protein content of seagrass tissues	83	High	−1	[67]
	Presence of BPAs correlated with reduced growth and photosynthesis	83	High	−3	[65]
		100	High	−2	[66]
Macro	Increased presence of plastic correlated with increased bacteria and pathogens	70	Medium	−3	[60]
	Presence of plastic correlated with increased decomposition and carbon liberation	58	Low	−3	[61]
	Presence of plastic correlated with increased spatial competition	75	High	−2	[70]
	Presence of plastic correlated with increased spatial competition and reduced rhizome growth	83	High	−3	[68]
	Seagrass detritus ‘traps’ plastics and transfers to shore via egagropiles	83	High	3	[74]
	Presence of plastic correlated with increased community biodiversity	100	High	4	[75]
Micro	Presence of microplastics correlated with reduced growth of seagrass	75	High	−3	[71]
	Presence of microplastics/nanoplastics correlated with reduced growth of seagrass	58	Medium	−3	[68]
	Presence of plastics correlated with reduced decomposition and nitrogen liberation	83	Low	−3	[62]
	Seagrass-associated organisms incorporate plastics into the food chain through grazing	50	Medium	−4	[72]
		50	Medium	−4	[73]

, which also documents the overall magnitude of the ecological impact alongside the quality analysis and confidence scores for each article. Considering the quality analysis of the articles documenting the direct interactions of MPP with seagrass ecosystems, Table 4 illustrates how the quality scores of the articles are generally high, with each scoring at least 50 points out of a possible 100. The confidence scores are also predominantly high; however, opposing results produced low confidence scores for two documented impacts. The overall impact magnitudes are observed across the full range from −4 to +4, with the majority of the impacts classified as negative. The consensus across studies was also high, with only two articles having conflicting results; most studies were in agreement that MPP negatively impacts seagrass ecosystems.

The impacts documented in Table 4 directly affect the provision of ES by seagrass meadows and associated organisms (Supplementary Tables S5 and S6); the primary response is a degradation of the condition of the seagrass habitat, likely driving a reduction in the provision of all ES. The degradation of seagrass habitats is a commonly observed impact and has been linked to reduced carbon sequestration potential [76,77,78], reduced productivity [79], reduced community abundance [80], reduced nutrient cycling capabilities [77], and reduced fishery incomes [81]. Furthermore, MPP in seagrass ecosystems is linked with reduced light availability, which can reduce the carbon sequestration capabilities and the associated community abundance [82,83].

An overview of how the ES provided by seagrass ecosystems are impacted by MPP is provided in

Table 5. Details of how the ecosystem services provided by seagrass ecosystems are impacted by marine plastic pollution, as demonstrated by the literature searches (colours indicate impact direction: green signifies a positive impact only, orange signifies neutral (or both positive and negative impacts), and red shows negative impacts only).

Section	Class	Ecosystem Services Provided by Seagrass Ecosystems Globally	Impact Direction
Provisioning	Food provision			Positive
	Genetic material provision			Positive and negative
Regulating and maintaining	Water purification			Negative
	Climate regulation
	Coastal protection
	Life cycle maintenance
Cultural	Recreation/tourism
	Cognitive
	Symbolic/sacred

, which shows that the provision of all ES is impacted negatively by MPP. However, life cycle maintenance and water purification may also be positively impacted; for example, increased community biodiversity was observed in one study, and seagrass was seen to transfer debris to the shore via egagropiles in another, which enhanced the surrounding water quality.

Articles that specifically assessed the concentrations of plastics found within seagrass ecosystems were analysed; twenty-two found evidence of plastics within the seagrass canopy or sediments (Table 6 & Supplementary Table S7). Eighteen articles compared the concentrations of plastics in seagrass ecosystems to those in other coastal habitats, including coral reefs, mangroves, and bare sediments, and 11 found seagrass ecosystems to contain more significant levels of MPP than the comparators. In contrast, three found seagrass ecosystems to contain lower levels of MPP than the comparators, while four found no statistically significant differences between the habitats assessed. The ways in which this ‘trapped’ plastic may interact with seagrass ecosystems is presented in Figure 3.

The concentrations of plastics found within SEA and the units used to report them varied across the studies; however, the units were standardised for comparison. The macroplastics in the seagrass canopy ranged from 0.004 to 0.060 items m⁻² (mean 0.032 ± 0.028 items m⁻²) (Supplementary Table S7). The microplastics in seagrass sediments, however, ranged from 17.90 to 24,805.41 items kg⁻¹ (mean 5378.98 ± 9735.17 items kg⁻¹); furthermore, the microplastics observed in various coastal sediments ranged from 0.24 to 29,869.33 items kg⁻¹ (mean 5290.52 ± 8982.00 items kg⁻¹) (Table 6). The microplastics in surface waters ranged from 0.02 to 1830.00 items m⁻³ (mean 331.90 ± 670.46 items m⁻³) (Supplementary Table S7). Microplastics were also found attached to 75% of seagrass blades [73], with 80% of seagrass biota [84] and 7.7% of seagrass-associated fish [72] containing microplastics. Furthermore, plastic fibres were identified in 27.6% of seagrass-associated macroinvertebrates [85].

When focusing on the microplastic concentrations found within coastal sediments (Table 6), the calculated PLI ranged from 1 (used as the reference minimum value) to 124,455.54 (mean 22,123.99 ± 8129.23). The PLI_zone calculated for the SEA region was 1893.26, classified as extremely high as per Table 2. Besides the lowest recorded value, which was used as a reference for the PLI calculations, all observed concentrations returned a high PLI score. Geographically, the coastal sediment microplastic concentration results represent 23 data points across five countries; Indonesia returned the greatest number of data points (8), followed by Thailand (7), Vietnam (4), Singapore (3), and Malaysia (1). At a country level, Thailand returned the highest PLI at 29,869.33 and Indonesia the lowest PLI at 74.58, even when discounting the minimum reference value of 1; Malaysia returned the median PLI at 1250.00 (IQR 304.17–38,491.67). The PLI_zone by country, in size order, was as follows: Vietnam 7173.75, Thailand 5629.08, Malaysia 1250.00, Indonesia 640.49, and Singapore 521.11. For an additional country-by-country analysis, see Supplementary Table S8.

Table 6. Observed concentrations of marine plastic pollution observed within Southeast Asia in coastal sediments, items kg⁻¹ (mean of all samples and STD). The current risk level is calculated as per Table 2.

Type of Sediment	Mean ± STD (Items kg−1)	PLI	Risk Level	Sampling Location	Reference
Seagrass sediments	17.90 ± 10.83	74.58	4	Kodingareng Lompo Island, Makassar City, Indonesia	[86]
	93.59 ± 43.14	389.96	4	Changi, Singapore	[87]
	195.00 ± 66.98	812.50	4	Barranglompo Island, Makassar, Indonesia	[88]
	1783.00 ± 241.00	7429.17	4	Quang Ninh, Hai Phong, Vietnam	[89]
	24,805.41 ± 6645.14	103,355.88	4	Mare Island, Indonesia	[90]
Beach sediments	0.24 ± 0.28	1.00	1	Anday Beach, West Papua, Indonesia	[91]
	47.23 ± 15.37	196.79	4	Phuket, Thailand	[92]
	568.00 ± 743.00	2366.67	4	Singapore Strait, Singapore	[93]
	1364.00 ± 507.13	5683.33	4	Chonburi, Thailand	[94]
	9238.00 ± 2097.00	38,491.67	4	Da Nang, Vietnam	[95]
	11,480.00 ± 8617.44	47,833.33	4	Chanthaburi, Thailand	[94]
	24,725.71 ± 30,303.19	103,023.79	4	Trat, Thailand	[94]
	29,869.33 ± 50,830.53	124,455.54	4	Rayong, Thailand	[94]
Coastal sediments	206.00 ± 91.46	858.33	4	Gulf of Thailand, Thailand	[96]
	300.00 ± 0.00	1250.00	4	Straits of Johor, Malaysia	[96]
Intertidal sediments	205.89 ± 109.80	857.88	4	Lamongan, Indonesia	[97]
Mangrove sediments	28.09 ± 10.28	117.04	4	Maura Angke Wildlife Reserve, Indonesia	[98]
	36.80 ± 23.60	153.33	4	Various, Singapore	[99]
	407.50 ± 407.50	1697.92	4	Tien Yen Bay, Vietnam	[100]
	1309.10 ± 124.80	5454.58	4	Lach Huyen, Vietnam	[101]
Subtidal sediments	73.00 ± 38.81	304.17	4	Bandon Bay, Thailand	[102]
	170.65 ± 18.25	711.04	4	Jakarta Bay, Indonesia	[103]
	15,200.00 ± 4932.00	63,333.33	4	Semak Daun Island, Jakarta Bay, Indonesia	[104]

Table 6. Observed concentrations of marine plastic pollution observed within Southeast Asia in coastal sediments, items kg⁻¹ (mean of all samples and STD). The current risk level is calculated as per Table 2.

Type of Sediment	Mean ± STD (Items kg−1)	PLI	Risk Level	Sampling Location	Reference
Seagrass sediments	17.90 ± 10.83	74.58	4	Kodingareng Lompo Island, Makassar City, Indonesia	[86]
	93.59 ± 43.14	389.96	4	Changi, Singapore	[87]
	195.00 ± 66.98	812.50	4	Barranglompo Island, Makassar, Indonesia	[88]
	1783.00 ± 241.00	7429.17	4	Quang Ninh, Hai Phong, Vietnam	[89]
	24,805.41 ± 6645.14	103,355.88	4	Mare Island, Indonesia	[90]
Beach sediments	0.24 ± 0.28	1.00	1	Anday Beach, West Papua, Indonesia	[91]
	47.23 ± 15.37	196.79	4	Phuket, Thailand	[92]
	568.00 ± 743.00	2366.67	4	Singapore Strait, Singapore	[93]
	1364.00 ± 507.13	5683.33	4	Chonburi, Thailand	[94]
	9238.00 ± 2097.00	38,491.67	4	Da Nang, Vietnam	[95]
	11,480.00 ± 8617.44	47,833.33	4	Chanthaburi, Thailand	[94]
	24,725.71 ± 30,303.19	103,023.79	4	Trat, Thailand	[94]
	29,869.33 ± 50,830.53	124,455.54	4	Rayong, Thailand	[94]
Coastal sediments	206.00 ± 91.46	858.33	4	Gulf of Thailand, Thailand	[96]
	300.00 ± 0.00	1250.00	4	Straits of Johor, Malaysia	[96]
Intertidal sediments	205.89 ± 109.80	857.88	4	Lamongan, Indonesia	[97]
Mangrove sediments	28.09 ± 10.28	117.04	4	Maura Angke Wildlife Reserve, Indonesia	[98]
	36.80 ± 23.60	153.33	4	Various, Singapore	[99]
	407.50 ± 407.50	1697.92	4	Tien Yen Bay, Vietnam	[100]
	1309.10 ± 124.80	5454.58	4	Lach Huyen, Vietnam	[101]
Subtidal sediments	73.00 ± 38.81	304.17	4	Bandon Bay, Thailand	[102]
	170.65 ± 18.25	711.04	4	Jakarta Bay, Indonesia	[103]
	15,200.00 ± 4932.00	63,333.33	4	Semak Daun Island, Jakarta Bay, Indonesia	[104]

4. Discussion

The results presented here show that seagrass ecosystems provide ES; with all ES from the analytical framework being evidenced globally and within SEA, the evidence scores relating to the quantity of research and agreement across the literature for these results were primarily high. Various impacts of MPP on seagrass ecosystems were identified, most of which reflected a negative effect. These impacts included reduced growth metrics, such as the leaves per shoot and rhizome length, and variations in the rate of decomposition and nutrient cycling. Additionally, our results link the impacts of MPP to the effects that they may have on seagrass-derived ES; we found all ES from our analytical framework to be affected negatively, besides water purification and life cycle maintenance, for which the results showed both positive and negative effects. Finally, a risk assessment of the concentrations of microplastics within SEA found the risk to be high to extremely high in all areas.

4.1. Seagrass-Derived Ecosystem Services

Evidence highlights that seagrass meadows provide all of the ES considered within the applied analytical framework (Table 1 and Table 3), with climate regulation, life cycle maintenance, and water purification being the most commonly reported. These ES vary depending on multiple factors, including the genera present or the size or condition of the plant [33]. There has been substantial discussion on the use of marine vegetation biomass as a source of fuel energy [105,106,107,108], yet no evidence of seagrass being utilised as energy was discovered during this study. In line with the current policy norms that demand the quantification and valuation of ES [109], those ES that are more easily quantified, such as climate and water quality regulation, measured through the seagrass’s ability to capture carbon and cycle nutrients, have received the most attention within the literature. Other ES are more difficult to quantify or value; these include intangible relationships between people and seagrass, including cultural services such as spiritual well-being [110]. The challenges in the measurement and integration of these ES into accounts and decision-making likely explain why they have been subject to less attention than those ES that are more easily measured. Furthermore, it is suggested that placing financial value on these services may not reflect the full value of the benefits that people receive from them [111]. The evidence of seagrass-derived ES in SEA was limited compared to that in other parts of the world, most likely because much ecosystem service research is focused on the global north [112,113], yet we found evidence that each of the services within our framework was present in this region.

4.2. Marine Plastic Pollution and Seagrass Meadows

Plastics are ubiquitous in the marine environment, including within seagrass ecosystems. Seagrass ecosystems can trap MPP [114,115,116], which some debate should be an ES in itself [14]. This trapped MPP could potentially shade seagrass plants and contribute to reduced growth metrics [117]; however, Molin et al. [71] suggest that the positive buoyancy of plastics influences the residency time, and therefore the exposure times are relatively short. At other times, microplastics may be susceptible to sediment burial when trapped in seagrass ecosystems [118,119]. This research found some studies that identified that seagrass beds and their associated sediments act as a sink for MPP, although other studies did not find a difference in abundance between seagrass meadows and other shallow water habitats. Predominantly, seagrass meadows were found to trap more plastics than bare sediment; however, when compared to other biogenic habitats, the results were mixed. Linking the plastic abundance and impacts to ES provision suggests that seagrass-derived ES will generally be negatively affected to some extent by MPP (Table 5); however, there are still several unknowns, such as the effects on decomposition (Table 4). These impacts may be exacerbated by the seagrass trapping MPP, increasing the magnitude of the effect.

The results show that the negative impacts of MPP on seagrass meadows include reduced rhizome lengths by up to 50%, with the majority of seagrass’ organic carbon stored in the rhizomes and roots; this could have a significant effect on the sequestration rates [19]. Further impacts include a reduction in respiration of 26%, a decrease in the decomposition rates of 36%, and an increase in carbon liberation, further reducing the standing stock (Supplementary Table S6). These impacts regularly lead to an overall decline in seagrass habitat quality and diminished ES provision. Conversely, MPP was found to increase the community biodiversity, primarily by providing habitats for organisms [75]; however, it was unclear from the single study documenting this whether the increase in organisms represented an actual increase in species richness or rather just the aggregation of organisms (Supplementary Table S6).

The results indicated other impact pathways of MPP on seagrass. These included the trophic transfer of attached microplastics through grazing [72,73], which has the potential to harm organisms in various ways, including introducing toxic pollutants, which can bioaccumulate and cause liver toxicity [120] and may then become incorporated into the human food chain. Seagrass tissues have been shown to adsorb [64] and accumulate [63] plastic leachates (BPAs and plasticisers); the impacts of this adsorption accumulation in seagrass are currently unknown, but knowledge of the negative effects on terrestrial plants is emerging [121,122], suggesting that adsorption and accumulation will negatively impact seagrass ecosystems.

When seagrass is exposed to BPAs, blade lengthening is inhibited [66], the tissue protein content is reduced [67], and the photosynthesis rates decrease [64], which all contribute to a decline in the overall health of the ecosystem. Of particular relevance is the presence of BPAs in marine surface waters, recorded in Malaysia from 1.32 to 1890.51 ng L⁻¹ (mean 59.01 ng L⁻¹) [123]. Research has shown that seagrass growth is retarded in the presence of BPAs at levels of 300 ng L⁻¹ and above (Table 4) [65,66,67]. Furthermore, microplastics/nanoplastics may reduce seagrass habitats by increasing the leaf loss rate [68]. In summary, MPP at the current concentrations contributes to multiple negative impacts on seagrass ecosystems, which can influence ES provision. Furthermore, these impacts on seagrass ecosystems correlate with the direct economic cost of the degradation of seagrass-derived ES [77,124,125].

The risk to ES was assessed using the sedimentary concentrations of microplastics, as studies of microplastics or macroplastics in the seagrass canopy are limited and do not present sufficient data points to robustly perform a risk assessment. Just two studies of macroplastic concentrations in the seagrass canopy were found; the mean concentrations were 0.004 items m⁻² (±0.001) [126] and 0.060 items m⁻² (±0.020) [127] (Supplementary Table S7). Considering microplastics in surface waters in SEA, we found five studies, one of which occurred in a seagrass ecosystem and recorded concentrations of 0.02 items m⁻³ (±0.00) [86]; other studies occurred in coastal or estuarine sites, with concentrations ranging from 0.18 items m⁻³ (±0.25) to 1830.00 items m⁻³ (±1902.00) [93,102,103,128] (Supplementary Table S7). This lack of abundance data for different seagrass compartments highlights a knowledge gap, as, without understanding the types and relative concentrations of the plastics, we are unable to determine how these plastics interact with seagrass meadows and the impacts on the associated ES.

4.3. Future Risks of Marine Plastic Pollution

Large amounts of plastic pollution are entering the oceans from land-based sources, with some research estimating that more than 10 million MT enters the marine environment annually [129], a value that is continuing to increase [130]. Lau et al. [130] modelled a range of scenarios for future plastic pollution leakage, including a business as usual (BAU) scenario, where the annual rate of plastic pollution entering aquatic systems was predicted to increase annually by 160% between 2016 and 2040, and a ‘system change’ scenario, where the annual rate was predicted to increase by 57% in the same period. In the ‘system change’ scenario, all waste reduction interventions were implemented, yet leakage into the marine environment continued to increase, albeit more slowly.

An important finding of this study is that the current PLI_zone score for microplastic pollution in the coastal sediments of SEA is already extremely high at 1893.26, compared to the risk categories set out by Pico et al. [59] (Table 2). It is worth noting that, while there is significant variation in the PLI_zone between countries, ranging from 521.11 (Singapore) to 7173.75 (Vietnam), all country-level scores would be classified as extremely high. While knowledge is lacking as to the effects of macroplastics on seagrass ecosystems, their degradation will contribute to increased microplastic concentrations [131]. This, in addition to the additional leakage predicted by Lau et al. [130], could potentially push the plastic concentrations beyond those predicted for the region. Seagrass meadows display reduced growth and increased leaf loss when exposed to microplastics and nanoplastics [68]; furthermore, the seagrass architecture could be altered by plastics in sediments [70], and these impacts will likely increase with increased MPP concentrations. Primarily, research on the impact that plastic has on seagrass has been conducted from the perspective of the canopy [61,62,63,64,65,66,67,68,69,70,71]; therefore, the full extent of the impact of these sedimentary microplastic concentrations on seagrass meadows and ES provision is still unclear.

Importantly, the abundance of microplastics in coastal sediments can be 200 times greater than that in the surrounding water column [132]; consequently, the potential impacts of sedimentary microplastics on seagrass health and the overall functioning of coastal ecosystems represents a critical knowledge gap. Furthermore, MPP is documented as posing a high risk to marine diversity [133] and to marine ES [6], with coastal areas being hotspots for interactions.

4.4. Focus on Southeast Asia

There is very limited evidence of the direct impacts of MPP on seagrass ecosystems relevant to SEA, suggesting a major deficiency in regional knowledge; however, a recent study has presented evidence that MPP is linked with the degradation of all coastal ecosystems in Indonesia [134]. Globally, studies of microplastic pollution in seagrass meadows only represent ~12.5% of all seagrass species [116]. However, seagrass species have similar life histories, biological traits, and habitat requirements [135], making them applicable for direct comparison; therefore, much can be inferred from the available research. Interestingly, whilst the Philippines and Malaysia have been reported as contributing the most MPP in the region [43], only one study of the coastal MPP concentrations was found for each country (Table 5 and Supplementary Table S7), demonstrating a further knowledge gap.

In SEA, large proportions of the population live by the coast and are dependent on marine resources for their livelihoods and nutrition [136]. Furthermore, coastal habitats such as coral reefs, mangroves, and seagrass beds help to protect from storm surges and natural disasters [16]. The connectedness of these tropical coastal habitats (seagrass, coral reefs, and mangroves) can increase their resilience and the provision of ES; for example, the fish abundance and diversity are greater in seagrass meadows closer to mangroves than in isolated meadows [137], and mangrove carbon sequestration increases with mangrove–seagrass connectivity [138]. Furthermore, connectivity with mangroves increases seagrass plants’ longevity and multiple growth metrics [139]. Therefore, any decrease in seagrass habitat quality could potentially lead to the degradation of the ES provided not only by seagrass meadows but also by other connected coastal habitats.

Stankovic et al. [140] predict that the seagrass meadows in SEA could vanish by 2060, and the protection measures in the region are suggested to be inefficient in conserving seagrass meadows, even in areas with greater marine protected area coverage [39]. Further measures to improve seagrass conservation in SEA have been suggested [141], including engaging with all stakeholders, inspiring partnership, sharing information, and developing a standard monitoring system. Regarding MPP, options exist to mitigate these threats, such as effective waste management, implementing circular chains of use and recycling, and using the recently launched plastic credits [142,143]. The risk assessment presented here can provide the building blocks on which to raise the profile of these issues and prompt investment in these tools and technologies, with the goal of avoiding the high economic and well-being costs of ES degradation.

4.5. Limitations and Future Research

This is an initial attempt at assessing the impact of plastic pollution on seagrass ecosystem services by examining the interactions of plastic with seagrass meadows when trapped in the canopy or sediments. The impact on ecosystem services was assessed linearly; therefore, we may fail to capture the true extent to which each ES is affected by each specific direct impact of MPP or how these effects may be compounded. Data were limited concerning the abundance of plastics within the seagrass canopy, particularly for macroplastics; as research grows on both the abundance of plastics in SEA seagrass meadows and the impact of these plastics, a more in-depth study of the risks of plastic pollution will be possible. Confidence in the extracted data was predominantly high (Table 4); however, further research is required to understand how MPP affects seagrass decomposition. Additionally, further data on the specific impacts of sedimentary plastics on seagrass meadows are required to better understand these risks. The use of PLI scores in microplastic risk assessments is growing; however, they are often combined with a hazard index (HI) score, which considers the specific toxicity of the polymers present to represent the risk better [144]. Whilst this research did not consider HI scores, Pico et al. [59] found the PLI to be the decisive index, rather than the HI; therefore, we conclude that our results are applicable. Nevertheless, calculating the HI alongside the PLI scores could provide a more accurate picture of the risks of MPP.

5. Conclusions

Global seagrass ecosystems provide multiple benefits to humans through varied ecosystem services. However, these ecosystems are threatened by multiple compounding anthropogenic pressures, including MPP. SEA accounts for a sizable portion of the global seagrass meadows and has the greatest diversity of seagrasses, yet it is faced with ever-increasing plastic pollution emissions. The evidence synthesised here suggests that MPP can reduce seagrass growth and functioning, which could, in turn, degrade ES provision. The current concentrations of microplastics in SEA, therefore, present a potential risk to seagrass health, which will likely trigger the degradation of critical ES, and the projected increases in MPP [130] will only exacerbate this risk. Our findings indicating that MPP presents a considerable risk to coastal and estuarine environments are supported by studies focused on other regions documenting the impacts of MPP on marine habitats [145,146,147].

The lack of data available to assess the risk of macroplastics highlights a gap in the current knowledge and could potentially suggest additional risks to seagrass-derived ES. Furthermore, the lack of MPP concentration data for some countries further impedes the analysis. Filling these knowledge gaps could provide information with which to aid the development of management practices to mitigate the threat of MPP in the region. Solutions to alleviate the pressure of MPP are subject to much debate globally, including within the complex and politicised negotiations for the United Nations Plastics Treaty [148]. Additional potential solutions include implementing effective waste management and recycling programmes and the use of financial mechanisms such as plastic credits.

The presented risks are of specific concern in SEA because so many of the region’s population directly rely upon these ES for life support. With seagrass ecosystems recognised as ‘coastal canaries’ [149] and as suitable indicator species for biodiversity and environmental variability due to their extensive distribution and timely detectable reactions, their global and regional declines [40,150,151,152], in light of the risks presented here, are of grave concern.