Polymer Composition Reveals Source- and Transport-Driven Microplastic Patterns in Caribbean Coral Reef Environments

1. Introduction

Microplastic (MP) pollution, defined as plastic particles smaller than 5 mm [1], has emerged as a pervasive and persistent form of contamination in marine environments. These particles originate from both primary sources, such as industrial pellets and microbeads, and secondary processes involving the fragmentation of larger plastic debris [2]. Due to their small size, durability, and widespread distribution, MPs are now recognized as a significant concern for marine ecosystems, affecting biogeochemical processes, food webs, and ecosystem health [3,4]. Coastal environments, which act as interfaces between terrestrial and marine systems, are particularly vulnerable, receiving substantial inputs of land-based plastic waste [5].

Coral reefs are among the most biodiverse and socio-economically valuable ecosystems on Earth, yet they are increasingly threatened by multiple stressors, including climate change, ocean acidification, and pollution [6]. In recent years, MPs have been identified as an additional and potentially significant stressor in reef environments [7,8]. Interactions between MPs and corals include ingestion, surface adhesion, and physical abrasion, which may lead to tissue damage, increased susceptibility to disease, and reduced physiological performance [9,10]. In addition, the structural complexity and hydrodynamic conditions of reef systems may promote the retention and accumulation of MPs, suggesting that coral reefs can act as both sinks and dynamic reservoirs within coastal environments [8].

Despite growing attention, significant knowledge gaps remain regarding the distribution and characteristics of MPs in coral reef waters. A major limitation of existing studies is the predominant focus on particles larger than 300 µm, whereas smaller MPs, expected to dominate in number and potentially exert greater ecological impacts, remain largely underrepresented [7,11]. This limitation is primarily driven by methodological constraints associated with conventional sampling and analytical techniques, which often lack the resolution required for reliable detection and identification of smaller particles.

Recent advances in vibrational spectroscopic techniques, particularly Laser Direct Infrared (LDIR) imaging, have enabled automated, high-throughput detection and chemical identification of MPs down to tens of micrometers [12,13,14,15,16]. These approaches overcome limitations associated with conventional methods and provide a more accurate characterization of small MP fractions, which are frequently underestimated in net-based surveys [11,17].

Beyond size-related constraints, there is an increasing need to integrate MP abundance with polymer-specific composition. While abundance data provide a measure of contamination levels, polymer identity offers critical insights into potential sources, transport pathways, and environmental behaviour. Polymer composition has been shown to reflect dominant inputs, including urban runoff, wastewater discharge, and industrial activities, as well as environmental processes such as hydrodynamic sorting and degradation [18,19]. However, polymer-resolved analyses remain limited in reef-associated environments, where most studies still focus primarily on particle abundance rather than compositional signatures.

Furthermore, the effectiveness of marine protected areas (MPAs) in mitigating MP contamination remains uncertain. While MPAs can reduce local anthropogenic pressures, their capacity to limit MP exposure is constrained by regional oceanographic transport, which can introduce particles from distant sources [20,21]. This highlights the need to disentangle the relative influence of local inputs and large-scale transport processes when assessing MP contamination in reef ecosystems.

The Cuban archipelago hosts extensive coral reef systems that are considered among the best preserved in the Caribbean basin [22]. This condition has been attributed to relatively low coastal development, the predominance of offshore reef systems, and the presence of large marine protected areas, particularly in remote regions such as Jardines de la Reina. However, these ecosystems are not exempt from anthropogenic pressures, which vary spatially and are more pronounced near urbanized and industrialized coastal zones.

Despite their ecological importance, no study to date has provided a large-scale, high-resolution assessment of small MPs (<300 µm) integrating abundance, polymer composition, and spatial variability across Cuban coral reef systems. This represents a critical gap in understanding both the magnitude and the drivers of MP contamination in reef environments.

To address this gap, the present study provides a comprehensive assessment of MPs (20–300 µm) in surface waters across 22 coral reef sites along the Cuban shelf using LDIR imaging. Specifically, the objectives were to (i) quantify MP abundance and characterize polymer composition, size distribution, and morphology; (ii) identify spatial patterns of contamination using multivariate analysis; (iii) investigate the influence of local inputs and regional transport processes on polymer composition; and (iv) evaluate differences between protected and non-protected reef systems. By integrating high-resolution analytical techniques with compositional and spatial analyses, this study advances the use of polymer-specific signatures as a diagnostic tool to better understand MP contamination in coral reef ecosystems.

2. Materials and Methods

2.1. Study Area and Sampling Design

A survey was conducted in 2023 along the Cuban shelf, covering approximately 6,000 km of coastline across both northern and southern regions of the archipelago. A total of 40 sampling stations distributed across 22 coral reef sites were selected to represent a gradient of environmental conditions and anthropogenic influence, including 12 MPAs, urbanized coastal zones, industrial regions, and relatively remote reef systems (Figure 1; Table S1).

At each station, duplicate surface seawater samples were collected (n = 80) to account for small-scale variability.

2.2. Sample Collection

Surface seawater samples were collected at approximately 20 cm depth using pre-cleaned 1 L glass bottles. Sampling followed international guidelines for MP monitoring in water (ISO 5667-27; ISO 16094-2).

At each reef, at least two stations were sampled, and three replicate samples were collected per station. Two replicates were processed for MP analysis, while the third was preserved for future analysis of smaller particle fractions (1–20 µm).

All individual analytical results were used to characterize each reef, without prior averaging at the station level.

2.3. Sample Handling, Transport, and Initial Processing

Samples were transported to the laboratory without filtration and processed under controlled conditions. Filtration was performed using a 20 µm stainless steel mesh under a laminar flow hood.

Microplastic extraction followed an adapted enzymatic digestion protocol [16], using Corolase 7089 at 55 °C for 48 h with intermittent ultrasonication. After digestion, samples were filtered, rinsed, and resuspended prior to final filtration onto 0.8 µm gold-coated polycarbonate filters for spectroscopic analysis.

2.4. Microplastic Identification and Characterization

Microplastics were identified using an automated Laser Direct Infrared (LDIR) imaging system (Agilent 8700) [16]. Spectra were acquired in the 1800–975 cm⁻¹ range at 8 cm⁻¹ resolution and processed using Clarity software.

Polymer identification was based on spectral matching against a validated reference library, and only particles with a hit quality index (HQI) ≥ 0.85 were considered.

Particles in the size range of 20–300 µm were included in the analysis.

2.5. Quality Assurance and Quality Control

All sample processing was conducted under controlled conditions to minimize contamination. Glass and metal equipment were used throughout, and all materials were pre-cleaned and filtered prior to use.

Procedural blanks were included with each batch of samples and used to determine the method detection limits following ISO 11843-6. Recovery experiments using spiked samples yielded an average recovery of 86.0 ± 3.5%.

2.6. Data Analysis and Statistical Treatment

Data normality was assessed using the Shapiro–Wilk test. As MP abundance data were non-normally distributed, results are reported as medians and interquartile ranges.

Differences among sites and contamination levels were evaluated using Kruskal–Wallis tests, followed by post-hoc comparisons. Differences between protected and non-protected reefs were assessed using the Mann–Whitney U test.

Polymer diversity was quantified using the Shannon–Wiener index based on relative polymer contributions at the reef level.

Hierarchical cluster analysis (HCA) and principal component analysis (PCA) were applied to explore spatial patterns in polymer composition using standardized data.

All statistical analyses were performed using OriginPro 2025, and LDIR data handling was conducted using YABE, a Python-based application developed within the framework of the IAEA NUTEC Plastics initiative [23].

3. Results

3.1. Microplastic Abundance

Microplastics were detected in 100% of the surface water samples collected from the 22 coral reef sites, confirming their ubiquitous presence across the study area. All measured abundances were above the method detection limit.

MP abundances did not follow a normal distribution (Shapiro–Wilk test, p = 0.0062) and are therefore described using robust statistics. The overall median abundance was 66 MPs L⁻¹, with an interquartile range (Q1–Q3) of 50–120 MPs L⁻¹. Values ranged from a minimum of 8 MPs L⁻¹ at JARE to a maximum of 218 MPs L⁻¹ at HABA. The distribution exhibited positive skewness (0.525) and slightly negative kurtosis (–0.199), indicating a right-skewed distribution with moderate dispersion.

Median MP abundances varied markedly among sites (Figure 2), ranging from 25 MPs L⁻¹ at JARE to 175 MPs L⁻¹ at HABA. Low-abundance sites included JARE, PILO, and GOCA, whereas the highest values were recorded at HABA, RALU, and PUFR. Most stations exhibited intermediate values between 50 and 120 MPs L⁻¹, reflecting a heterogeneous spatial distribution. These differences were statistically significant (Kruskal–Wallis test, H = 52.6, p < 0.001). Post-hoc comparisons indicated that variability was primarily driven by contrasts between low- and high-abundance sites.

Based on median values, sites were grouped into three contamination levels: low (<50 MPs L⁻¹), medium (50–120 MPs L⁻¹), and high (>120 MPs L⁻¹). Significant differences were observed among these groups (Kruskal–Wallis test, H = 37.3, p < 0.001), with all pairwise comparisons being significant (p < 0.001), indicating a clear gradient of MP contamination.

Comparison between protected and non-protected reef sites revealed significantly lower MP abundances in protected areas (Mann–Whitney U test, p < 0.001). Median MP concentrations were 55.5 MP L⁻¹ in protected reefs and 104.5 MP L⁻¹ in non-protected reefs, indicating nearly a twofold increase in MP abundance in non-protected areas, although MPs were consistently detected across all sites.

3.2. Polymer Composition, Size, and Morphology

A total of 11 polymer types were identified, with polyethylene (PE, 27%), polyamide (PA, 26%), and polypropylene (PP, 24%) dominating the assemblage (Figure 3).

Polymer composition varied across contamination levels (Table S2). Low-contamination reefs were dominated by PE, PA, and PP (79%), whereas high-contamination reefs showed a reduced contribution of these polymers (56%) and increased proportions of polyurethane (PU, 19%) and polytetrafluoroethylene (PTFE, 8%).

Microplastics were predominantly found in smaller size fractions, with particles <100 µm accounting for approximately 80% of total MPs (Figure 4). Particle morphology was dominated by irregular fragments (89%), with minor contributions from rounded particles and fibres.

3.3. Similarity of Reefs Based on Microplastic Profiles

Principal component analysis (PCA) based on polymer composition revealed moderate structuring of sampling stations (Figure 5). The first two components explained 22.6% (PC1) and 19.4% (PC2) of the variance. PC1 separated stations with higher contributions of PTFE and PU from those dominated by common-use polymers such as PA, PE and PP, while PC2 discriminated PA from polyolefins (PE and PP), highlighting differences within common-use polymers. Three compositional patterns were identified: (i) PTFE- and PU-enriched sites, (ii) PA-dominated sites, and (iii) sites with balanced PE–PP composition. No clear separation was observed in the PCA space between protected and non-protected sites.

Polymer diversity assessed using the Shannon–Wiener index (H′), showed marked spatial variability across reef sites. Higher diversity was observed in sites such as MLGO, PUFR, HABA, and RALU, characterized by a more even distribution of polymer types, whereas lower diversity occurred in sites dominated by a limited number of polymers, such as ISSA (PA-dominated) and SLPR (PE-dominated). Most sites displayed intermediate diversity, with PE, PP, and PA consistently contributing alongside secondary polymers. However, no consistent pattern in polymer diversity was observed between protected and non-protected reef sites, as both groups included sites spanning the full range from low to high diversity.

4. Discussion

These results demonstrate that polymer composition provides a more diagnostic indicator of MP sources and environmental processes than abundance alone.

4.1. Spatial Patterns of Microplastic Abundance

This study provides the first high-resolution assessment of MP contamination (20–300 µm) across Cuban coral reef ecosystems, revealing the pervasive presence of MPs and pronounced spatial variability in their abundance. Microplastics were detected in all sampled sites, confirming that even relatively remote and protected reef environments are chronically exposed to MP pollution.

The observed spatial variability reflects the combined influence of local anthropogenic inputs, coastal geomorphology, and hydrodynamic conditions, which together control the accumulation and redistribution of MPs in reef environments [7,8]. In this context, the results highlight that MP distribution is not governed by a single process, but rather by the interaction between source intensity and environmental retention capacity. Low-contamination sites (<50 MPs L⁻¹), including JARE, PILO, and GOCA, were consistently associated with remote reef environments characterized by minimal direct anthropogenic influence. These sites are located far from major urban centers, lack significant riverine inputs, and are not affected by industrial or port activities. In addition, all low-contamination sites correspond to marine protected areas, were reduced local pressures and enhanced water exchange likely limit MP accumulation. Similar patterns were observed in reefs with slightly higher but still relatively low abundances (e.g., ISSA, PUES, VITA, and CACO), suggesting that low contamination levels are primarily controlled by the absence of local sources rather than by complete isolation from regional inputs.

In contrast, the highest MP abundances were consistently observed in reef systems adjacent to semi-enclosed bays subject to intense anthropogenic pressure. Sites such as PUPA (Puerto Padre), RALU (Cienfuegos), and HABA (Havana) are directly influenced by urban, industrial, and port-related discharges, as well as riverine inputs, which are widely recognized as major pathways of MPs to coastal environments [24,25]. The semi-enclosed nature of these bays promotes the retention and accumulation of contaminants due to reduced water exchange and continuous pollutant inputs, resulting in elevated MP concentrations in adjacent reef areas. This pattern is consistent with previous studies demonstrating that coastal embayments can act as accumulation zones for marine debris and MPs [24].

An important exception to this general pattern was observed in the reefs SAFE and PUFR, which exhibited relatively high MP abundances despite being located within marine protected areas and characterized by limited local anthropogenic influence. This suggests that local sources alone cannot fully explain contamination patterns in these systems. Instead, regional oceanographic processes likely play a significant role. Both sites are influenced by circulation associated with the Yucatán Channel, a key pathway connecting the Caribbean Sea and the Gulf of Mexico and characterized by strong northward flow [26,27]. This circulation facilitates the large-scale transport of water masses and suspended particles, including MPs, across the Caribbean basin [21,24]. The elevated abundances observed in these otherwise low-impact sites therefore support the hypothesis that regional transport processes can override local protection measures and introduce MPs into remote reef environments.

Overall, these results indicate that MP abundance in coral reef systems is controlled by a hierarchical combination of processes: (i) local inputs associated with urban, industrial, and riverine sources; (ii) geomorphological and hydrodynamic conditions that regulate retention and accumulation; and (iii) regional oceanographic transport that redistributes MPs across the basin. This framework highlights the need to consider both local and large-scale processes when assessing MP contamination in coastal ecosystems and explains the widespread presence of MPs even in protected reef environments.

4.2. Global Context: Positioning Caribbean Coral Reef Systems Within the Global Microplastic Spectrum

Comparisons of MP abundances across studies remain inherently challenging due to differences in sampling strategies, analytical techniques, and size detection limits. To reduce methodological inconsistencies, the present comparison was restricted to studies applying automated micro-spectroscopic techniques (e.g., LDIR, µFTIR, or µRaman) with comparable lower size thresholds (~20 µm). This approach minimizes biases associated with the underrepresentation of small MP fractions and enables a more robust cross-study evaluation. Based on these criteria, data from 25 peer-reviewed studies worldwide were compiled (Table S3), and the global distribution of MP abundances is presented in the Supplementary Information (Figure S1).

Within this harmonized framework, MP abundances observed in Cuban coral reef systems span a wide range of values, encompassing conditions from relatively low contamination to highly impacted environments. Low-contamination sites fall within the lower range of values reported for coastal systems worldwide, comparable to remote or minimally impacted marine environments. In contrast, highly impacted sites such as HABA and RALU fall within the upper range of global observations, overlapping with values reported for urbanized coastal zones and semi-enclosed systems influenced by strong anthropogenic inputs (see Figure S1).

This broad distribution indicates that Cuban coral reef systems do not represent a single contamination regime but rather capture the full spectrum of MP conditions observed globally. In this sense, the study area can be interpreted as a representative gradient integrating both diffuse background contamination and localized hotspots driven by urban and industrial sources.

Importantly, the coexistence of relatively pristine reef environments and highly contaminated sites within the same regional system highlights the pronounced spatial heterogeneity that characterizes MP distribution in coastal ecosystems. This finding challenges the common assumption that coral reefs, particularly those located within marine protected areas, are consistently low-contamination environments.

Furthermore, these results emphasize that MP contamination in reef systems cannot be interpreted solely as a function of local conditions. While elevated abundances are clearly associated with local anthropogenic inputs, the widespread presence of MPs across all sites, including remote and protected reefs, indicates a significant contribution from regional oceanographic transport. This dual influence of local sources and basin-scale processes reinforces the need for integrated monitoring frameworks that account for both source-driven inputs and large-scale connectivity.

Overall, the findings position Caribbean coral reef systems as dynamic environments where global background contamination, regional transport, and localized anthropogenic pressures interact to shape MP distribution. This highlights the relevance of the Caribbean as a key region for understanding MP dynamics in coastal ecosystems and underscores the importance of standardized, high-resolution assessments to improve global comparability.

4.3. Size Distribution and Morphology of Microplastics

Microplastic size distributions were strongly skewed toward smaller particles, with approximately 80% of detected MPs measuring <100 µm and the 20–50 µm fraction alone accounting for the majority of particles (Figure 4). This pattern was consistent across all sampling sites, indicating that the dominance of small MPs is a pervasive feature of reef-associated environments rather than a site-specific characteristic.

The predominance of small particles has important implications for both environmental assessment and ecological risk. Smaller MPs exhibit higher surface-area-to-volume ratios and greater bioavailability, increasing the likelihood of ingestion across multiple trophic levels and enhancing their potential for biological interactions [28,29,30]. In addition, their numerical dominance suggests that studies focusing on larger size fractions may substantially underestimate total MP abundance in marine systems.

Morphologically, irregular fragments overwhelmingly dominated the dataset (~89%), with only minor contributions from fibers and rounded particles. This fragment-dominated profile is consistent with advanced stages of environmental degradation, reflecting the progressive fragmentation of larger plastic debris under mechanical and photochemical processes. In contrast, fibers (commonly associated with wastewater and textile sources) are more frequently reported in studies targeting larger size fractions, suggesting a size-dependent bias in morphological composition.

Overall, these results highlight the importance of incorporating small MP fractions into monitoring frameworks, as their exclusion may lead to significant underestimation of contamination levels and misrepresentation of particle characteristics in coral reef environments.

4.4. Polymer Composition and Environmental Drivers

Polymer composition revealed clear spatial patterns associated with contamination gradients and provided key insights into the relative influence of diffuse background inputs and source-specific anthropogenic contributions. A total of 11 polymer types were identified, with PE, PA, and PP dominating the assemblage and jointly accounting for approximately 77% of the detected particles (Figure 3). These polymers were consistently observed across all sampling sites, supporting their role as ubiquitous background components of MP contamination in reef-associated environments. Their widespread occurrence is consistent with global production patterns and their dominance in marine environments, where PE, PP, and PA are among the most frequently reported polymers due to their extensive use, buoyancy, and environmental persistence [2,18].

Despite this general consistency, polymer composition varied markedly across contamination gradients. Low-contamination reefs were characterized by relatively homogeneous assemblages dominated by PE, PA, and PP, whereas highly contaminated sites exhibited a clear shift toward more complex and heterogeneous profiles, with increasing contributions from polymers such as PU, PTFE, and polyvinyl chloride (PVC). This transition reflects a shift from diffuse, regionally distributed sources to more localized and source-specific anthropogenic inputs under elevated contamination conditions [18,19]. This shift in polymer composition across contamination gradients highlights a transition from regionally homogenized background contamination to locally driven source-specific inputs, reinforcing the value of polymer-resolved analysis as a tool for source discrimination.

This pattern was particularly evident in the most impacted sites (PUPA, RALU, and HABA), which are directly influenced by semi-enclosed bay systems receiving substantial urban, industrial, and port-related discharges, as well as riverine inputs. Large rivers are widely recognized as major pathways transporting plastic debris and MPs from terrestrial environments to the ocean [31], while semi-enclosed bays can act as accumulation and transformation zones due to limited water exchange and continuous pollutant inputs [32]. The combined influence of these processes promotes both retention and redistribution of MPs, resulting in more heterogeneous and diagnostically distinct polymer assemblages in adjacent reef environments.

The enrichment of less commonly reported polymers such as PTFE, PU, and PVC provides further evidence of source-specific inputs. PTFE is a high-performance fluoropolymer widely used in industrial and consumer applications, including non-stick coatings, electrical insulation, and high-resistance materials. Recent studies have shown that PTFE can release micro- and nanoplastics through thermal degradation and mechanical wear, suggesting previously underrecognized pathways of environmental contamination [33]. Similarly, PU is commonly associated with urban materials and infrastructure, and its increased contribution in impacted sites is consistent with inputs from urban runoff and wastewater systems [34,35,36]. PVC, which is widely used in construction and industrial applications, is typically denser than seawater and therefore less prone to long-range surface transport. Its presence and enrichment in impacted sites further support the influence of local inputs and retention processes within semi-enclosed coastal environments. The combined occurrence of these polymers provides a distinct compositional fingerprint of urban and industrial influence, distinguishing impacted reef systems from those primarily influenced by diffuse background inputs.

Multivariate analysis further reinforced these patterns. Principal component analysis (PCA) identified three main compositional groups: (i) sites enriched in PTFE and PU, (ii) PA-dominated sites, and (iii) sites characterized by a more balanced PE–PP composition (Figure 5). This separation supports the existence of distinct contamination regimes and highlights the combined influence of localized sources and regional background inputs. Notably, no clear separation was observed between protected and non-protected reef sites, indicating that protection status does not exert a strong control on polymer-specific composition. Importantly, the lack of clear separation between protected and non-protected reefs in compositional space further supports the dominance of regional-scale processes over local management measures in shaping polymer distributions.

Polymer diversity patterns further support these findings. Sites with higher diversity exhibited more even polymer distributions, consistent with the influence of multiple and overlapping sources, whereas low-diversity sites were dominated by one or few polymer types, reflecting more homogeneous inputs or selective retention processes [18].

Notably, SAFE and PUFR exhibited elevated MP abundances but were dominated by common polymers such as PE, PP, and PA, with no detection of less frequently occurring polymers such as PTFE or PU. This indicates that contamination at these locations is not driven by localized inputs but rather reflects the accumulation of regionally transported MPs. The predominance of widely distributed polymers is consistent with long-range transport and mixing processes across the Caribbean basin [20,21], including circulation associated with the Yucatán Channel. This finding provides strong evidence that elevated MP abundance does not necessarily indicate local contamination sources but may instead reflect the accumulation of regionally transported particles.

Together, these results challenge the assumption that MP contamination in coral reef environments is primarily driven by local sources. Instead, they demonstrate that MP pollution in coral reef environments is governed by a dynamic interplay between ubiquitous background contamination, localized anthropogenic inputs, and regional oceanographic transport processes. The observed decoupling between MP abundance and polymer-specific composition highlights the limitations of relying solely on abundance-based metrics to assess contamination sources. Instead, polymer-resolved analysis provides a more robust framework to distinguish between transport-driven accumulation and source-driven contamination.

These findings have important implications for monitoring and management strategies in coral reef systems, as they indicate that even remote or protected reefs may be significantly impacted by regionally transported MPs. Consequently, effective mitigation efforts must consider not only local pollution control but also transboundary inputs and large-scale circulation patterns. Furthermore, the integration of polymer composition with abundance and diversity metrics offers a powerful approach for improving source apportionment and understanding MP dynamics in complex coastal environments.

Future research should integrate polymer-resolved observations with hydrodynamic modelling to better constrain source–transport relationships in reef-associated environments.

5. Conclusions

This study demonstrates that polymer composition provides critical insights into the sources and processes governing MP distribution in coral reef environments, beyond what can be inferred from abundance alone. The dominance of PE, PP, and PA across all sites reflects a pervasive regional background signal, while the enrichment of polymers such as PU, PTFE, and PVC in impacted areas indicates localized anthropogenic inputs associated with urban and industrial activities.

The decoupling between microplastic abundance and polymer composition highlights the coexistence of distinct contamination regimes, where elevated concentrations do not necessarily correspond to local sources. In particular, sites such as SAFE and PUFR illustrate how regionally transported MPs can accumulate in reef systems while maintaining a homogeneous polymer signature.

These findings emphasize the importance of integrating polymer-specific analysis with abundance and diversity metrics to improve source identification and better understand MP dynamics in complex coastal environments. Effective monitoring and mitigation strategies in coral reef systems should therefore consider both local pollution sources and large-scale transport processes that influence contaminant distribution.