Influence of Particle Size and Fragmentation on Large-Scale Microplastic Transport in the Mediterranean Sea

Complex release dynamics of microplastic additives: An interplay of additive degradation and microplastic aging

This study investigates the complex dynamics of additive release from microplastics in aquatic environments under natural ultraviolet (UV) radiation, which is critical for assessing ecotoxicological impacts and developing pollution remediation strategies. We focused on release kinetics of additives (Dimethyl phthalate (DMP), Dibutyl phthalate (DBP), Di(2-ethylhexyl) phthalate (DEHP), Bisphenol A (BPA) and Decabromodiphenyl ether (BDE-209)) from polyvinyl chloride (PVC), polyethylene (PE), and acrylonitrile-butadiene-styrene (ABS) microplastics exposed to UV light, exploring the interplay between additive release, photodegradation, and microplastic aging. Initial results showed a consistent release pattern, but under UV exposure, the release became more complex due to additive degradation and changes in the microplastics' structure. Factors such as polymer type, microplastic size, additive content, and environmental conditions (UV or darkness) significantly influenced the release quantity and kinetics. UV-induced additive degradation altered the concentration gradient between the microplastic and water, while aging, marked by changes in surface chemistry and internal polymer breakdown, accelerated additive release. By applying Inner Particle Diffusion (IPD) and Aqueous Boundary Layer Diffusion (ABLD) models, we demonstrated how UV-induced degradation and aging affected key parameters like the diffusion and partition coefficients, impacting the overall release process. These insights lay the foundation for understanding the environmental risks posed by microplastic additives and developing strategies to mitigate their impact in aquatic ecosystems.

Impact of seasonal variations on microplastic accumulation and characteristics in sandy beaches of Sichang Island, the inner Gulf of Thailand

Microplastics (MPs) are pervasive environmental pollutants whose fate, transport, and ecological impacts require clarification. This study examines the abundance and characteristics of MPs (16-5000 μm) in sandy beach sediments on Sichang Island, the inner Gulf of Thailand, during the dry season and after a four-month wet season. On the western beach, exposed to monsoon winds and currents, MP abundance increased from 2295.38 ± 1227.44 pieces/kg in the dry season to 2386.63 ± 121.45 pieces/kg post-wet season. Conversely, the leeward eastern beach showed a significant decrease, from 686.29 ± 243.90 to 238.63 ± 121.45 pieces/kg. Polypropylene was the dominant polymer, with reduced variation in MP color and shape following the wet season. The proportion of smaller MPs (16-100 μm) decreased from 59 to 62 % in the dry season to 27-42 % post-wet season, while larger debris became more prevalent. These findings highlight seasonal shifts in MP accumulation and dispersal on sandy beaches influenced by hydrodynamic conditions.

Where Is All the Plastic? How Microplastic Partitions across Environmental Compartments within a Large Pelagic In-Lake Mesocosm

How microplastics transit within aquatic ecosystems and partition among environmental compartments is not fully understood. To increase understanding, we added microplastic fragments ranging in buoyancy (positive: polyethylene (PE), neutral: polystyrene (PS), negative: polyethylene terephthalate (PET)) and size (∼30 to 1400 μm) to surface waters of closed-bottom, in-lake mesocosms (10 m diameter, 2 m depth). To assess residence time, we measured microplastics in surface waters and the water column over a 9-week period. To measure fate, we measured microplastics in the surface water, water column, bottom detritus, and biota (biofilm on the walls, zooplankton, fish) at 9 weeks. The residence times of microplastics were longer at the surface than in the water column, with less dense and smaller particles having the longest residence times. After 9 weeks, nearly all microplastics were on the bottom, with only 3% on the surface, 0.4% in the water column, 2% in biofilm, and <0.01% in zooplankton and fish. The surface water and biofilm on the walls were larger reservoirs than the water column, suggesting that surface microlayers and biofilm on hard substrates are important, yet overlooked, reservoirs of microplastics in aquatic ecosystems. Results inform future hypotheses relevant to monitoring programs and risk assessments.

Water environmental capacity of estuarine microplastics capped by species sensitivity threshold

Estuaries are biologically rich ecosystems and act as aggregation zones for microplastic (MP) during their transport from rivers to the sea, posing heightened ecological risks compared to other aquatic environments. However, limit criteria for MP discharge to guide risk management remain lacking. This study quantified the water environmental capacity (WEC) of estuarine MPs using species health-based microplastic concentration (MPC) thresholds. Classified MPs simulation employed probability density functions and shape factors to convert mass concentrations into particle counts during the flood season in the Yangtze River estuary. The 5 % species hazard concentration (HC5) with two ecologically relevant metrics was selected as the MPC threshold by correcting the species sensitivity distribution (SSD) curves for polydispersity and biological accessibility of environmental MPs. Lastly, the dynamic WEC framework was established by linking MP simulations to MPC thresholds. MP aggregation hotspots were found in the intertidal zone and maximum turbidity zone in southern branch, which is akin to locating the 'shortest plank' in bucket effect. The average rescaled MPC in hotspots reached 4.77 × 105particles/(d·m3), accounting for 14.38 % of the WEC safety threshold. This framework explored scientific basis for quantifying the MPs carrying capacity of estuarine ecosystems and allocation of plastic discharge rights.

Long-term variations in size and polymer type of meso- and microplastics in seabirds and on beaches since the 1980s

Marine plastic pollution is a global issue that requires innovative ways of monitoring and mitigation. Information on how the size, mass and polymer type of floating plastic items are changing over time may improve our understanding of the complex dynamics governing fragmentation rates, dispersal, longevity, input rates and abundance at the sea surface. Procellariiform seabirds directly ingest floating meso- and microplastics, which they retain in their gizzards. As a result, petrels can be used as biomonitors to document trends in the abundance and characteristics of marine plastics. We compare the characteristics of plastics collected from regurgitated Brown Skua Catharacta antarctica pellets containing the remains and plastics ingested by four petrel taxa breeding at Inaccessible Island, South Atlantic Ocean, at roughly decadal intervals from 1987─2024. To assess if trends persist across biotic (ingested) and abiotic (beaches) compartments, we compare this to the characteristics of meso- and microplastics (2-25 mm) sieved from South African beaches from 1984─2023. Plastics were collected from beaches far from local urban source areas in an attempt to track changes in plastic floating at sea rather than local, land-based sources. Overall, there was little evidence of trends in the size and mass of ingested or beached plastics. The average mass of industrial pellets from beaches decreased up to 2015, suggesting an old, gradually eroding cohort of legacy pellets, but increased in 2023 after two major pellet spills off the South African coast. Nearly all ingested and beached plastics were polyethylene (PE) or polypropylene (PP), but the ratio of PP to PE in hard fragments increased over time, while recent increases in PE:PP ratios in industrial pellets match recent pellet spills at sea. Identifying polymer types in ingested and beached plastics is valuable for future studies, as it may be useful for marine pollution management.

The distribution of subsurface microplastics in the ocean

Marine plastic pollution is a global issue, with microplastics (1 µm-5 mm) dominating the measured plastic count1,2. Although microplastics can be found throughout the oceanic water column3,4, most studies collect microplastics from surface waters (less than about 50-cm depth) using net tows5. Consequently, our understanding of the microplastics distribution across ocean depths is more limited. Here we synthesize depth-profile data from 1,885 stations collected between 2014 and 2024 to provide insights into the distribution and potential transport mechanisms of subsurface (below about 50-cm depth, which is not usually sampled by traditional practices3,6) microplastics throughout the oceanic water column. We find that the abundances of microplastics range from 10-4 to 104 particles per cubic metre. Microplastic size affects their distribution; the abundance of small microplastics (1 µm to 100 µm) decreases gradually with depth, indicating a more even distribution and longer lifespan in the water column compared with larger microplastics (100 µm to 5,000 µm) that tend to concentrate at the stratified layers. Mid-gyre accumulation zones extend into the subsurface ocean but are concentrated in the top 100 m and predominantly consist of larger microplastics. Our analysis suggests that microplastics constitute a measurable fraction of the total particulate organic carbon, increasing from 0.1% at 30 m to 5% at 2,000 m. Although our study establishes a global benchmark, our findings underscore that the lack of standardization creates substantial uncertainties, making it challenging to advance our comprehension of the distribution of microplastics and its impact on the oceanic environment.

Microplastic distribution and its implications for human health through marine environments

Microplastics are pervasive pollutants in the ocean, threatening ecosystems and human health through bioaccumulation and toxicological effects. This review synthesizes recent findings on microplastic distribution, trophic transfer, and human health impacts. Key findings indicate that microplastic abundance is highest in the Indian and Pacific Oceans, particularly in seawater and sediment. Morphologically, fibers and fragments dominate, with polypropylene, polyethylene, and polyester being the most prevalent polymers. Smaller particles (<1 mm) undergo long-range transport via ocean currents, while biofouling accelerates vertical sinking. Trophic transfer studies confirm microplastic ingestion across marine food webs. Human exposure is associated with seafood consumption, inhalation of airborne particles, and potential dermal contact, particularly in marine environments. These exposures can lead to adverse health effects, including inflammation, organ damage, respiratory issues, oxidative stress, and metabolic disruptions. Finally, this review explores potential strategies for minimizing human exposure to microplastic pollution in marine environments, paving the way for further research in this critical area.

From source to sink: part 1-characterization and Lagrangian tracking of riverine microplastics in the Mediterranean Basin

The Mediterranean Sea is one of the most critically polluted areas due to its semi-enclosed structure and its highly anthropized shoreline. Rivers are significant vectors for pollutant transfers from the continental to the marine environment. In this context, a 3D Lagrangian simulation of the dispersion of riverine microplastics (MPs) was performed, which included the application of a recently developed model that reassessed the MP fluxes discharged by rivers. MP physical properties from river samples were further investigated to approximate vertical displacement in modeled ocean currents. The use of a high-resolution circulation model, integrating Stokes drift, turbulent diffusion, and MP sinking and rising velocities, enabled us to establish stock balances. Our simulation suggested that 65% of river inputs may be made of floating MPs drifting in the surface layer and 35% of dense MPs sinking to deeper layers. The Eastern Mediterranean tends to accumulate floating MPs, primarily originating from the Western Mediterranean Basin, where major river sources are concentrated. After 2 years of simulation, modeled stranding sequestered 90% of the MP inputs, indicating relatively short average residence times from a few days to months at most for particles at sea. Although spatial distribution patterns stabilized after this period and a steady state may have been approached, the surface concentrations we modeled generally remained below field observations. This suggested either an underestimation of sources (rivers and unaccounted sources), by a factor of 6 at most, or an overestimation of MP withdrawal through stranding, to be reduced from 90 to around 60% or less if unaccounted sinks were considered.

Nanoscale insight into the interaction mechanism underlying the transport of microplastics by bubbles in aqueous environment

The ecological risk of microplastics (MPs) is raising concern about their transport and fate in aquatic ecosystems. The capture of MPs by bubbles is a ubiquitous natural phenomenon in water-based environment, which plays a critical role in the global cycling of MPs, thereby increasing their environmental threats. However, the nanoscale interaction mechanisms between bubbles and MPs underlying MPs transport by bubbles in complex environmental systems remain elusive. This work for the first time directly measured and evaluated the interactions between bubble and polystyrene microplastic (PSMP) under various environmental factors in aqueous media using atomic force microscope (AFM) combined with a Stokes-Reynold-Young-Laplace (SRYL) model. Since hydrophobic interaction was strong enough to act across the repulsive barrier, bubble-PSMP attachment always occurred at different NaCl concentrations, pH and hydrodynamic conditions, and a decay length D0 of hydrophobic interaction was determined as 0.65 ± 0.05 nm. No bubble attachment was observed during approach for aged PSMP (APSMP) with the weakened hydrophobic interaction (D0 = 0.33 ± 0.02 nm), while in 100 mM NaCl, APSMP-bubble attachment occurred during retraction due to the hydrodynamic suction effect. The decreased D0 arose from the increased oxygen-containing groups on APSMP surfaces that significantly reduced the hydrophobicity of MPs surface as evidenced by X-ray photoelectron spectroscopy (XPS) and water contact angle measurement. It was further evident from transport tests that aging plays a crucial role in MPs transport driven by bubbles. This work provides nanoscale information on the interaction mechanism underlying the MPs transport by bubbles, with implications to evaluate the fate of MPs in aqueous environments.

Photodegradation Controls of Potential Toxicity of Secondary Sunscreen-Derived Microplastics and Associated Leachates

The escalating environmental concern over secondary microplastics (SMPs) stems from their physicochemical evolution from primary microplastics (PMPs), yet the contribution of varying physicochemical transformations to the ultimate environmental risks remains unknown. In this study, a photomechanical degradation process was employed to convert the primary sunscreen-derived microplastics (SDMPs) into secondary SDMPs. While mechanical degradation caused physical fragmentation, photodegradation induced both physical and chemical alterations, introducing surface oxidation, chemical bond scission, and cross-linking to the secondary SDMPs. Employing a combination of alkaline digestion and pyrolysis GC-MS techniques, it was observed that both physical fragmentation and photooxidation led to heightened intracellular sequestration of MPs. Although the bioaccumulated SDMPs could be indicated by the enlarged lysosomes and fragmented mitochondria, toxicity of secondary SDMPs at the cellular level was primarily driven by chemical transformations post-photodegradation. A nontargeted analysis employing high-resolution mass spectrometry identified 46 plastic-associated compounds in the leachate, with photodegradation-induced chemical transformations playing a crucial role in the dissociation of hydrophobic additives and oxidative conversion of leached compounds. The toxicity of the leachate was exacerbated by photodegradation, with mitochondrial fragmentation serving as the primary subcellular biomarker, indicative of leachate toxicity. This study elucidates the pivotal role of photodegradation in augmenting the cytotoxicity of secondary SDMPs, shedding light on the intricate interplay between physicochemical transformations and environmental risks.

Disentangling the retention preferences of estuarine suspended particulate matter for diverse microplastic types

As a major source of microplastics (MPs) for global oceans, estuarine MPs pose challenges for numerical modeling due to their particle diversity, while hydrodynamics and suspended particulate matter (SPM) further exacerbate transport prediction uncertainties. This study employs a categorization framework to pinpoint 16 representative MPs types, precisely simulating their transport processes in the Yangtze River estuary (YRE). Furthermore, spatial links between SPM concentrations and MP types at 1800+ simulated sites were examined using ArcGIS and bivariate Local Indicators of Spatial Association (BI-LISA). Results indicate that low-density (≤0.95 g/cm³), small-diameter (<500 μm) fiber MPs are more prone to hetero-aggregation with estuarine SPM flocs, while MPs with opposite characteristics may move depending on their intrinsic properties. High-high BI-LISA clusters were observed both in river branches and at the confluence with the sea, the latter closely associated with the turbidity maximum zone that promote MP hetero-aggregation. The interaction of these currents and Yangtze (Changjiang) diluted water forms MPs clusters between 122.0°E and 122.5°E at the confluence of the South Branch, averaging over 870 μg/m3. Examining the trapping preferences of estuarine SPMs for various MPs through this classification framework can help to determine the bioavailability of environmental MPs to aquatic organisms and map the MPs baseline values for health risk quantification.

Determination of Seasonal Microplastic pollutıon Variation in Aquatic Environments: Case Study from İskenderun Technical University Pond

Although microplastic (MP) pollution in aquatic ecosystems has been a critical issue, a significant portion of previous studies has been limited to marine environments. Few studies have evaluated MP abundance in inland waters, and those that exist often ignore seasonal variations. This study aimed to assess the seasonal variations in microplastic (MP) abundance in the İskenderun Technical University (İSTE) Pond. MP abundance ranged from 0.5 MPs/L to 10.8 MPs/L, with a mean of 3.7 MPs/L. Fiber-shaped, black-colored, small-sized (< 1000 μm) MPs were commonly found on the surface water. Statistical analysis confirmed significant variations in MP abundance across seasons. The abundance of MPs in the surface water correlated with the mean annual precipitation. Atmospheric deposition and storm water runoff were major contributors to microplastic pollution in the pond. PP and PET particles were identified. The findings of this study contains preliminary results that can be used in the development of future legislations and regulations.

Navigating the difference of riverine microplastic movement footprint into the sea: Particle properties influence

As a critical source of marine microplastics (MPs), estuarine MPs community varied in movement due to particle diversity, while tide and runoff further complicated their transport. In this study, a particle mass gradient that represents MPs in the surface layer of the Yangtze River estuary was established. This was done by calculating the masses of 16 particle types using the particle size probability density function (PDF), with typical shapes and polymers as classifiers. Further, Aschenbrenner shape factor and polymer density were embedded into drag coefficients to categorically trace MP movement footprints. Results revealed that the MPs in North Branch moved northward and the MPs in South Branch moved southeastward in a spiral oscillation until they left the model boundary under Changjiang Diluted Water front and the northward coastal currents. Low-density fibrous MPs are more likely to move into the open ocean and oscillate more than films, with a single PE fiber trajectory that reached a maximum oscillatory width of 16.7 km. Over 95 % of the PVC fiber particles settled in nearshore waters west of 122.5°E. Elucidating the aggregation and retention of different MPs types can provide more accurate environmental baseline reference for more precise MP exposure levels and risk dose of ingestion for marine organisms.

Modeling of Microplastics Migration in Soil and Groundwater: Insights into Dispersion and Particle Property Effects

Migration of microplastics (MPs) in soil-groundwater systems plays a pivotal role in determining its concentration in aquifers and future threats to the terrestrial environment, including human health. However, existing models employing an advection-dispersion equation are insufficient to incorporate the holistic mechanism of MP migration. Therefore, to bridge the gap associated with MP migration in soil-groundwater systems, a dispersion-drag force coupled model incorporating a drag force on MPs along with dispersion is developed and validated through existing laboratory and field-scale experiments. The inclusion of the MP dispersion notably increased the global maximum particle velocity (vmaxp) of MPs, resulting in a higher concentration of MPs in the aquifer, which is also established by sensitivity analysis of MP dispersion. Additionally, increasing irrigation flux and irrigation areas significantly accelerates MP migration downward from soil to deep saturated aquifers. Intriguingly, vmaxp of MPs exhibited a nonlinear relationship with MPs' sizes smaller than 20 μm reaching the highest value (=1.64 × 10-5 m/s) at a particle size of 8 μm, while a decreasing trend was identified for particle sizes ranging from 20 to 100 μm because of the hindered effect by porous media and the weaker effect of the drag force. Moreover, distinct behaviors were observed among different plastic types, with poly(vinyl chloride), characterized by the highest density, displaying the lowest vmaxp and minimal flux entering groundwater. Furthermore, the presence of a heterogeneous structure with lower hydraulic conductivity facilitated MP dispersion and promoted their migration in saturated aquifers. The findings shed light on effective strategies to mitigate the impact of MPs in aquifers, contributing valuable insights to the broader scientific fraternity.

Seasonal dynamics, tidal influences, and anthropogenic impacts on microplastic distribution in the Yangtze River estuary: A comprehensive characterization and comparative analysis

Microplastics (MPs) are emerging contaminants with significant ecological and human health implications. This study examines the abundance, characteristics, and distribution of MPs in the Yangtze River estuary, focusing on seasonal variations, tidal cycles, and anthropogenic influences. Surface samples were collected using the Manta trawl method to ensure consistency with previous marine MP research. The study found an average MP concentration of 1.01 (± 0.65) n m-3, predominantly comprising low-density polymers such as polystyrene (38 %), polypropylene (33 %), and polyethylene (29 %). MPs were mainly fragments (34.9 %) and foam (30.7 %), with a prevalence of white particles. Seasonal analysis indicated significantly higher MP concentrations during flood seasons (1.32 ± 1.09 n m-3), nearly 1.9 times higher than during non-flood seasons (0.70 ± 0.28 n m-3). Tidal cycles also impacted MP distribution, with ebb tides showing increased concentrations (2.44 ± 1.30 n m-3) compared to flood tides (1.48 ± 2.07 n m-3). Furthermore, MP abundance showed a decreasing trend with increasing distance from urban centers, with significant correlations (0.52 < R2 < 0.65, P < 0.001). These findings underscore the necessity for seasonally adjusted monitoring and robust management strategies to combat MP pollution. The study advocates for the integration of diverse sampling methods and the consideration of environmental factors in future MP assessments, laying the groundwork for understanding the MP transport mechanism in the Yangtze River estuary and similar estuarine systems worldwide.

Calcite carbonate sinks low-density plastic debris in open oceans

The vertical settling of plastic debris in oceans is poorly understood. A large share of low-density microplastics (LDMPs) are largely absent from sea surfaces. The present study employs a model that considers the potential of an overlooked microbially induced calcium carbonate precipitation (MICP) process and new motion equations for irregular LDMPs. Here we show that the motion of LDMPs in the present model, exhibiting a damped oscillation pattern, is quite different from that in biofouling models. Furthermore, LDMPs in the size range of 10-200 µm are most likely to gain sufficient density at the biofouling/MICP stage to independently sink to the ocean floor with relatively small drag coefficients, potentially explaining the selective enrichment of LDMPs in the oceanic sediment. The size and shape exhibit strong non-linear effects on the settling patterns of LDMPs. Overall, the present study highlights the importance of calcite-mediated sinking of LDMPs in open oceans.

Short-term buoyant microplastic transport patterns driven by wave evolution, breaking, and orbital motion in coast

Recently, there has been a notable rise in social and scientific interest regarding microplastic pollution in coasts where waves significantly influence flow patterns and material transport. This study explores typical short-term movement of buoyant microplastics driven by surf zone processes including wave transformation, breaking, and orbital motion. To track microplastics, Lagrangian Particle Tracking Model (PTM) coupled with Eulerian wave-current interaction model appropriate for coastal hydrodynamics was used. From the simulations, several important findings were observed. (i) In alongshore uniform beaches, lighter and larger buoyant microplastics tended to reach beach more readily. (ii) Accurate predictions of microplastic transport in the surf zone required the consideration of wave breaking. (iii) In alongshore non-uniform coastal bathymetry, rip-currents can send buoyant microplastics offshore, beyond the surf zone.

Use of an uncrewed surface vehicle and near infrared hyperspectral imaging for sampling and analysis of aquatic microplastics

Data on MP in aquatic environments have low resolution in space and time. Scaling up sampling and increasing analysis throughput are the main bottlenecks. We combined two approaches: an uncrewed surface vehicle (USV) and near infrared hyperspectral imaging (NIR-HSI) for sampling and analysis of MP > 300 μm. We collected 35 water samples over 4 d in a coastal area. Samples were analyzed using NIR-HSI and Fourier transform infrared spectroscopy (FTIR). Spiked samples were used to determine recovery. We conclude that using a USV can mitigate issues of traditional trawls like scalability, repeatability, and contamination. NIR-HSI detects more polyethylene but less polypropylene than FTIR analysis and reduces analysis time significantly. Highly variable concentrations were found at both sampling locations, with mean MP concentration of 0.28 and 0.01 MP m-3 for location A and B respectively. USV sampling in tandem with NIR-HSI is an effective analytical pipeline for MP monitoring.

Model-based estimation of seasonal transport of macro-plastics in a marine protected area

Management of plastic litter in Marine Protected Areas (MPAs) is expensive but crucial to avoid harms to critical environments. In the present work, an open-source numerical modelling chain is proposed to estimate the seasonal pathways and fates of macro-plastics, and hence support the effective planning and implementation of sea and beach cleaning operations. The proposed approach is applied to the nearshore region that includes the MPA of Capo Milazzo (Italy). A sensitivity analysis on the influence of tides, wind, waves and river floods over the year indicates that seasonality only slightly affects the location and extension of the macro-plastic accumulation zones, and that beach cleaning operations should be performed in autumn. Instead, the influence of rivers on plastic litter distribution is crucial for the optimal planning of cleaning interventions in the coastal area.

Hotspots of Floating Plastic Particles across the North Pacific Ocean

The pollution of the marine environment with plastic debris is expected to increase, where ocean currents and winds cause their accumulation in convergence zones like the North Pacific Subtropical Gyre (NPSG). Surface-floating plastic (>330 μm) was collected in the North Pacific Ocean between Vancouver (Canada) and Singapore using a neuston catamaran and identified by Fourier-transform infrared spectroscopy (FT-IR). Baseline concentrations of 41,600-102,700 items km-2 were found, dominated by polyethylene and polypropylene. Higher concentrations (factors 4-10) of plastic items occurred not only in the NPSG (452,800 items km-2) but also in a second area, the Papaha̅naumokua̅kea Marine National Monument (PMNM, 285,200 items km-2). This second maximum was neither reported previously nor predicted by the applied ocean current model. Visual observations of floating debris (>5 cm; 8-2565 items km-2 and 34-4941 items km-2 including smaller "white bits") yielded similar patterns of baseline pollution (34-3265 items km-2) and elevated concentrations of plastic debris in the NPSG (67-4941 items km-2) and the PMNM (295-3748 items km-2). These findings suggest that ocean currents are not the only factor provoking plastic debris accumulation in the ocean. Visual observations may be useful to increase our knowledge of large-scale (micro)plastic pollution in the global oceans.

Distribution of microplastics in rainfall and their control by a permeable pavement in low-impact development facility

Microplastics (MPs) released from plastic products in daily life are present in the air and could be transported to freshwater environments along with rain. Recently, low-impact development (LID) facilities, such as permeable pavements, have been used to treat non-point source pollutants, including rainfall runoff. While runoff is treated by LID facilities, the periodic monitoring of MPs in rainfall and the efficiency of removal of MPs through LID facilities have rarely been investigated. Therefore, this case study focused on monitoring MPs in rainwater runoff and permeate from a permeable pavement in Busan, South Korea, thus evaluating the removal efficiency of MPs by a LID system. The initial rainfall runoff and permeate through the LID system were sampled, and the amounts, types, sizes, and shapes of MPs in the samples were analyzed using micro-Fourier Transform Infrared (FTIR) spectroscopy. The results showed that the distribution of MPs in the initial rainfall was affected by population in tested area. Polyethylene was the most common type of MPs in all the samples. Polyamide was only found in the LID samples because of the pollution caused by water flows and pavement materials. Fragment type MPs was most commonly observed and consisted of relatively small-sized (under 100 μm) particles. LID facilities were able to capture approximately 98% of MPs in the rainfall through a filtration process in the permeable pavement.

Quantifying microplastic dispersion due to density effects

An experimental study was conducted on how polymer density affects the transport and fate of microplastics in aquatic flows. For the first time, polypropylene (PP), polyethylene (PE), polymethyl methacrylate (PMMA), polyetheretherketone (PEEK), and polyvinyl chloride (PVC) were chemically stained and tested using solute transport techniques and velocities found among rivers in the natural environment (0.016 - 0.361 m/s). The movement of 3D-polymers with densities ranging from 0.9 - 1.4 g/cm³ was quantified in a laboratory flume scaled to simulate open-channel flows in fluvial systems. Except for PP, in most conditions microplastics exhibited similar transport characteristics to solutes regardless of density and established solute transport models were successfully implemented to predict their transport and fate. Mass recoveries and ADE routing model demonstrated microplastic deposition and resuspension was associated with polymer density below critical velocity thresholds ≤ 0.1 m/s. When density becomes the dominant force at these slower velocities, concentrations of denser than water microplastics will be momentarily or permanently deposited in channel beds and microplastics follow the classical Shields sediment transport methodology. This data is the first to provide microplastic suspension and deposition thresholds based on river velocity and polymer density, making a key contribution to research predicting microplastic fate and organismal exposure.

Shape- and polymer-considered simulation to unravel the estuarine microplastics fate

Environmental microplastics (MPs) constitute various sizes, polymers, and shape components. In estuaries, such differences are related to the reliability of assessing the seaward fate of MPs, aggregation hotspots, and ecological risks. This study sets the MP particle mass gradient using the shape factor and size probability density function to categorically estimate the MP load in the surface layer of the Yangtze River Estuary (YRE), which is the largest contributor of plastics to the sea. During the high plastic input period in July, the optimized estimated MP load through the surface layer of the YRE was 9766 kg/month, which was overestimated by 821 kg/month based on the empirical average particle mass. While tracking MP transport classified by shape and polymer type, the resuspension of MPs that accumulate in the intertidal zone cannot be neglected. The average relative error of the simulation was as low as 19.6% after including the abovementioned factors. Finally, the simulation results of the sensitive regions were extracted to assess the new MP risk index, which considers shape, abundance, and polymer type. By introducing these essential tools, this study helps to understand the fate of riverine MPs entering estuaries, where valuable opportunities for removing MPs exist before they spread to the oceans.

Low-Density Plastic Debris Dispersion beneath the Mediterranean Sea Surface

Plastic is a widespread marine pollutant, with most studies focusing on the distribution of floating plastic debris at the sea surface. Recent evidence, however, indicates a significant presence of such low density plastic in the water column and at the seafloor, but information on its origin and dispersion is lacking. Here, we studied the pathways and fate of sinking plastic debris in the Mediterranean Sea, one of the most polluted world seas. We used a recent Lagrangian plastic-tracking model, forced with realistic parameters, including a maximum estimated sinking speed of 7.8 m/d. Our simulations showed that the locations where particles left the surface differed significantly from those where they reached the seafloor, with lateral transport distances between 119 and 282 km. Furthermore, 60% of particles deposited on the bottom coastal strip (20 km wide) were released from vessels, 20% from the facing country, and 20% from other countries. Theoretical considerations furthermore suggested that biological activities potentially responsible for the sinking of low density plastic occur throughout the water column. Our findings indicate that the responsibility for seafloor plastic pollution is shared among Mediterranean countries, with potential impact on pelagic and benthic biota.

Proximity to coast and major rivers influence the density of floating microplastics and other litter in east African coastal waters

Floating anthropogenic litter occurs in all ocean basins, yet little is known about their distribution and abundance in the coastal waters off east Africa. Neuston net and bulk water sampling shows that meso- and micro-litter (8567 ± 19,684 items∙km^-2, 44 ± 195 g∙km^-2) and microfibres (2.4 ± 2.6 fibres∙L^-1) are pervasive pollutants off the coasts of Tanzania and northern Mozambique, with higher litter loads off Tanzania. Densities of meso- and micro-litter at the start of the rainy season were greater close to the coast and to major river mouths, suggesting that much litter likely originates on land. However, the mass of litter increased with distance from the six major coastal cities. By number, 95% of meso- and micro-litter was plastic, but only 6% of microfibres. Our results highlight the need to reduce plastic use and improve solid waste management in the region.

Influence of Particle Size and Fragmentation on Large-Scale Microplastic Transport in the Mediterranean Sea

Microplastic particles move three-dimensionally through the ocean, but modeling studies often do not consider size-dependent vertical transport processes. In addition, microplastic fragmentation in ocean environments remains poorly understood, despite fragments making up the majority of microplastic pollution in terms of the number of particles and despite its potential role in mass removal. Here, we first investigate the role of particle size and density on the large-scale transport of microplastics in the Mediterranean Sea and next analyze how fragmentation may affect transport and mass loss of plastics. For progressively smaller particle sizes, microplastics are shown to be less likely to be beached and more likely to reach open water. Smaller particles also generally get mixed deeper, resulting in lower near-surface concentrations of small particles despite their higher total abundance. Microplastic fragmentation is shown to be dominated by beach-based fragmentation, with ocean-based fragmentation processes likely having negligible influence. However, fragmentation remains a slow process acting on decadal time scales and as such likely does not have a major influence on the large-scale distribution of microplastics and mass loss over periods less than 3 years.