Plastic transport and storage dynamics in estuaries have important implications for environmental risk in these systems and also modulate the transfer from terrestrial to oceanic spheres. Here we synthesize fieldwork, theory, and experiments with numerical modeling to elucidate the source, transport, and fate of microplastics in an urban estuary in southern California. Riverine concentration-discharge models based on streamflow sampling are used to estimate microplastic flux at the two major riverine inputs to the estuary. Utilizing the Delft-3D hydrodynamic model coupled with particle tracking, microplastic transport is simulated for a dry and wet Water Year (October - September). Subtidal sediments collected from the estuary support modeled results of microplastic accumulation rates in bed layer sediments. Intertidal sediment cores collected within high and low areas of the saltmarsh and dated using fallout radionuclide analysis revealed the present and historical influence of stormflow event-driven suspended sediment (and microplastic) mobilization as well as the effect of dredging-based sediment management on microplastic accretion. Additionally, we investigate the importance of tides (i.e., bi-directional flow, phase, and range) and stormflow peak discharge on determining particle transport distance, flushing, and areas of peak accumulation. Local hydrodynamics and particle characteristics are also examined to contextualize observations of spatial partitioning of microplastic types. Finally results of this study are considered to inform future microplastic and sediment management in watersheds and estuaries.

How to cite: Gray, A., Murphy-Hagan, C., Brand, M., and Hapich, H.: Microplastic transport and fate in an Urban Estuary, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22100, https://doi.org/10.5194/egusphere-egu26-22100, 2026.

Coastal zones are recognised sinks for plastic debris, although studying the transport and dispersion of plastic debris in nearshore waters remains challenging. The understanding of coastal processes and how these can be applied to numerical modelling studies is not yet fully mature. Most Lagrangian numerical modelling studies of plastic debris transport to date have been conducted at the basin or sub-basin level, typically using low-resolution hydrodynamic data (>2.5 km). These resolutions are generally sufficient at larger scales, but at smaller scales they create uncertainties regarding mainly the beaching of particles on the coastline and poor resolution of coastal structures or complex geometries.

The LOCATE numerical model addresses these constraints by incorporating high-resolution hydrodynamic data in nested grid configurations, focusing on areas of high interest. Specifically, this has been applied to the Barcelona coastline, where the LOCATE model used a coupled current-wave dispersion module with a three-way nested grid for current data, of 2.5 km resolution from CMEMS within the study domain, a 350 m high-resolution grid from the Spanish Port Authority (Puertos del Estado) covering the Barcelona metropolitan area, and a 70 m resolution grid covering the Port and the urban coastline. The use of high-resolution hydrodynamic data with the LOCATE model was validated using drifter data. Complex geometric structures were resolved, demonstrating that particles had residence times over 18 times longer within the Port when using high-resolution data compared to only using low-resolution data. Furthermore, a beaching module that calculated the real-time distance of a particle to the shoreline was developed using high-resolution shoreline data. This provided much more realistic beaching patterns, compared to determining particle beaching using its advected velocity, as well as shoreline continuity between the different resolutions in the hydrodynamic data.

These adaptations alone, however, do not address coastline or beach processes not included even in the highest-resolution hydrodynamic data. For this, a probabilistic framework was assumed, where a beaching timescale of 26.35 h was determined using a sensitivity analysis for backtracking simulations initiated in nearshore environments where particles could only cross the land-water boundary at known discharge sources. A further condition based on a minimum particle trajectory distance was also introduced to avoid artefacts. This final probabilistic configuration allowed for particle advection near the shoreline with more realistic trajectories within complex shorelines, considering stochastic elements of particle transport at localised scales. These parameterisations highlight the adaptations required to coastal and nearshore Lagrangian numerical modelling and serve as an area of future research which may be transposed to other locations.

Acknowledgements

This study was carried out within the TRAP project (Participatory Strategies for the Management of Plastic Pollution on the Transboundary Coast), which was 65% co-financed by the European Union through the Interreg VI-A Spain-France-Andorra Programme (POCTEFA 2021-2027). The objective of POCTEFA is to strengthen the economic and social integration of the Spain-France-Andorra border region

How to cite: Hernandez, I., Castro-Rosero, L. M., Liste, M., Espino, M., and Alsina, J. M.: Dispersion of floating marine litter: Lagrangian numerical simulations at coastal scales, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14025, https://doi.org/10.5194/egusphere-egu26-14025, 2026.

It is assumed that the eventual sink of global microplastic pollution is the deep sea. The primary vector for sediment and particulate pollutants to the deep sea are gravity currents down canyons along the coastline and at the shelf edge, and it has become recognised that these trap and transport microplastics. In order to quantify the potential storage within these marine environments, we develop a reduced complexity model of the transport of microplastic within turbidity currents. We find that the relatively simple model can produce turbidity currents similar to that observed within the Whittard Canyon, offshore Ireland. Based on this model we map the fate of microplastic within the canyon. Under most scenarios, the model implies that small microplastics, fibres and fragments, will be transported into the canyon with little material leaving the canyon. Our best fitting model would suggest that only 15% of the source microplastic will bypass the canyon and be exported to the deep ocean floor. Marine canyons might therefore be a major sink of microplastic pollution, and act as a sponge between the anthropogenic source and the abyssal plane. This could have severe impacts on the ecosystems within these environments.

How to cite: Armitage, J., Teles, V., and Rohais, S.: Submarine canyons as microplastic reservoirs: insight from a reduced complexity model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3560, https://doi.org/10.5194/egusphere-egu26-3560, 2026.

Marine plastic debris that is discarded into the ocean and eventually beaches is an acute problem for small island nations such as the Seychelles. To address this problem and enable anticipatory action, the sources of marine plastic debris need to be identified. Recent observations suggest that not all of the debris that arrives at the Seychelles is from terrestrial input. However, there is currently a lack of quantitative attribution of maritime debris sources.

Therefore, we investigate the potential shipping and fishing vessel plastic debris sources in the southwestern Indian Ocean that beach at the Seychelles. We use a 2D Lagrangian particle-tracking model, based on OceanParcels, with particles that are advected by currents from a 1/50° (~2 km) regional ocean model. We combine this with satellite-tracked fishing and shipping data to inform particle starting locations and weightings. This model resolution allows us to resolve island to sub-island accumulation patterns.

Spatially, model results suggest that debris beaching at the Seychelles from shipping vessels is concentrated along a limited number of high-activity shipping routes. The port destinations of these routes are consistent with the origin of plastic bottles inferred from labels in a previous study. Model results also show that 50-66% of fishing debris that beaches at the Seychelles is discarded within its own exclusive economic zone, depending on the island group.

Temporally, the season during which debris is discarded strongly impacts the likelihood of beaching. This seasonal profile varies in amplitude and phase between islands due to wind-driven surface current changes. Additionally, we find debris beaching patterns can vary substantially between islands and on a sub-island scale. This highlights the importance of higher-resolution models for investigating plastic accumulation at kilometre-scale islands. We also assess the model against observations including bottle drifters and seasonal accumulation data at the Aldabra Atoll, the latter of which is consistent with sub-island scale accumulation seasonality from the model.

Given that most marine debris originates from a few major shipping routes and from within the Seychelles exclusive economic zone, we suggest targeted enforcement of MARPOL Annex V could tackle the source of the issue.

How to cite: Albinski, A., Savage, J., Burt, A., Vogt-Vincent, N., and Johnson, H.: Shipping and fisheries are major sources of plastic pollution for the Seychelles, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20648, https://doi.org/10.5194/egusphere-egu26-20648, 2026.

Based on our modeling, more than 10 million tonnes of plastic have entered the oceans, yet their ultimate fate and long-term impacts remain uncertain. We developed a multimedia model constrained by observations from beaches, seawaters, and seabeds to reconstruct the size (≥>0.2 mm), source, age, and storage of marine plastics since 1950. We find that beaches are the primary mass reservoir, while most particles remain suspended in the water column or sink toward the deep ocean.  Continental emissions primarily contaminate nearby regions, whereas maritime sources disperse particles widely across ocean basins. Global mean ages of plastics range from 14 (10–16) years at the surface to 26 (20–32) years on beaches and 38 (35–42) years on seabeds on a mass-weighted basis, and are generally higher when weighted by particle numbers, highlighting the enduring legacy of historical plastic emissions. Persistent fragmentation of older plastics, combined with accelerating new inputs, produces a numerical dominance of small plastic fragments  (<1 mm) despite their negligible contributions to total mass (<5%). These findings establish a quantitative framework for the ocean plastic cycle,  facilitating the quantification of its ecological and climatic impacts. This underscores the urgent necessity for proactive measures to mitigate the environmental and socio-economic repercussions. 

How to cite: Wang, X., Wu, P., Pang, Q., Zhang, Z., Peng, D., Xu, R., Wang, X., Xu, X., Zeng, E. Y., Cózar, A., Schartup, A. T., Lei, L., and Zhang, Y.: Plastics long shadow: legacy and fate of plastic pollution in the oceans, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4170, https://doi.org/10.5194/egusphere-egu26-4170, 2026.

The amount of plastic pollution in the environment has increased exponentially since its mass production began in the 1950s. As the vast majority of plastic cannot biodegrade, it instead slowly fragments through mechanical and chemical processes, producing microplastics (MPs; <5 mm) that are now found across the globe. Growing concern exists regarding the potential impacts of MPs on ecosystems. While considerable research has investigated MP pollution in aquatic and coastal environments, the quantity, distribution, and historical accumulation of MPs in UK salt marshes remain largely unknown.

Salt marshes are coastal wetlands with a unique range of flora and fauna, providing coastal protection and critical ecosystem services. However, they may also act as long-term sinks for MPs. Studies of MPs in salt marsh sediments are scarce, and very few have incorporated sediment dating using short-lived radioisotopes (210Pb, 137Cs, 241Am) to establish a history of plastic pollution.

In this study, sediment cores from four UK salt marshes (Welwick in the Humber Estuary, Lindisfarne on the Northumberland coast, Skinflats in the Firth of Forth, and Caerlaverock in the Solway Estuary) were collected and dated using 210Pb, 137Cs, and 241Am, with chronologies constructed using the Bayesian Plum model. MPs were extracted from the cores and identified, providing insights into the types, quantities, and temporal trends of plastics in UK salt marshes. At Welwick, an additional core was collected from an area with visible surface plastic to investigate the relationship between surface and subsurface MP accumulation. Welwick Saltmarsh was selected due to its location along the River Humber, a region known for high levels of plastic pollution.

To extract MPs, organic matter in the sediment cores was digested using 30% H₂O₂ in an ice bath, minimising polymer degradation compared to heated methods. MPs were extracted via density separation with 1.5 g/cm3 LST FastFloat and subsequently identified using Nile Red staining, fluorescence microscopy, and Raman spectroscopy.

The cores were found to contain sparse particles and fibres of polystyrene, polyethylene, polyethylene terephthalate and styrene-butadiene rubber. The core collected from Skinflats contained the greatest concentration of MPs (12 MPs), including a styrene-butadiene rubber particle likely derived from tyre wear associated with the nearby major road, whereas the remaining three cores contained only 2–3 MPs each. The oldest MPs found date from the 1950s. Overall, concentrations of MPs were surprisingly low.

This research is the first to examine historical microplastic pollution using dated sediment cores from multiple UK salt marshes across different estuarine systems. Although microplastics in soils can alter microbial communities and plant growth, the low concentrations observed here suggest minimal implications for salt marsh ecosystem functioning, including carbon sequestration processes and the success of habitat restoration or realignment efforts.

How to cite: Gilbert, A., Gehrels, W., Hodson, M., Kroger, R., and Blake, W.:  A History of Microplastic Pollution in UK Salt Marshes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19187, https://doi.org/10.5194/egusphere-egu26-19187, 2026.

Plastic pollution is a critical global challenge, with annual production surpassing 400 million tonnes and only a small fraction properly managed. The remainder accumulates in terrestrial and aquatic environments, with rivers acting as major conduits to the ocean. Causing ecological, social and economic impacts that affect the fulfilment of the Sustainable Development Goals (SDGs). Estuaries, located at the land–sea interface, represent key zones for intercepting this flux. In order to address this environmental problem, given the strategic location of estuaries, this study assesses the potential of tidal marshes to retain meso- and macroplastic debris in shallow estuaries.

A multi-scale approach was implemented, combining controlled hydraulic flume experiments, field campaign, and GIS-based spatial modelling in the Santoña Marshes (northern Spain). Laboratory tests quantified plastic entrapment by salt-marsh vegetation (Juncus maritimus, Halimione portulacoides and Spartina maritima) under variable hydrodynamic condition representative of tidal flows varying water levels, flow rates and wind speeds, and the most common plastic materials found in salt marshes. Field surveys tracked floating buoys simulating plastics, under natural conditions, providing qualitative evidence of retention pathways. Geospatial analysis integrated one-year tidal flooding probabilities with vegetation distribution, local bathymetry, plant height, tidal patterns and laboratory entrapment results to estimate the probability of plastic reaching vegetated areas and its potential retention over a year, identifying possible accumulation hotspots.

Results indicate high entrapment efficiency under controlled conditions (≈90% per species), supported by field observations. Spatial modelling revealed significant variability across the entire marsh. Quantitative values for vegetation trapping potential were obtained with a resolution of 10 m. These results were visualised by classifying the area into categories of high, medium and low potential retention. Zones with high potential retention are associated with longer flooding durations throughout the year, which increase the likelihood of plastics reaching vegetated areas. These zones often coincide with the presence of secondary channels and dense vegetation, which slow water flow and enhance plastic trapping. Conversely, areas with low potential retention are typically located near the coastline, where flooding occurs for shorter periods, limiting the time available for plastics to interact with vegetation.

This work delivers species-specific, quantitative evidence of plastic trapping in estuarine environments, offering critical insights for management strategies and informing numerical models aimed at mitigating plastic pollution at the land–sea interface.

How to cite: Pérez García, L., Núñez, P., Sánchez, M., Bárcena, J. F., Abascal, A. J., and García, A.: Assessing the Potential Entrapment of Meso- and Macroplastic Debris by Tidal Marshes in Shallow Estuaries, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7795, https://doi.org/10.5194/egusphere-egu26-7795, 2026.

Coastal dunes worldwide trap marine plastics within psammophilous vegetation. Such processes may impact dune formation by altering sediment dynamics and affecting plant health. This study aims to assess whether plastics act as active geomorphic agents and explores how strategic vegetation management could transform this challenge into Nature-Based Solutions (NBS) for coastal regions at risk from plastic pollution.

We aim to develop a comprehensive UAV-based framework to evaluate interactions among plastic, vegetation, and geomorphology in dune systems. High-resolution surveys produce orthomosaics and DSMs, enabling us to map plastic distribution alongside traditional dune features such as scarp development, sediment retention volume, surface roughness, and gradients of vegetation health.

The analysis is guided by three research perspectives:

• Plastic entrapment dynamics by dune-building species
• Vegetation response to debris accumulation
• Geomorphic feedbacks influencing dune profile evolution

By establishing quantitative links between plastic accumulation and measurable geomorphic change, this study aims to identify critical thresholds where pollution may transform protective dunes into erosion-prone features. Marine plastics are regarded as a quantifiable factor influencing dune evolution, providing insights for vegetation-based strategies to maintain coastal stability.

Keywords: marine plastics, coastal geomorphology, UAV remote sensing, Nature-Based Solutions, dune evolution

How to cite: Corbau, C., Pignoni, E., Lazarou, A., Coltorti, M., and Simeoni, U.: Marine plastics as geomorphic agents in coastal dune systems: UAV framework and nature-based solutions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10462, https://doi.org/10.5194/egusphere-egu26-10462, 2026.

Since the 1950s, the increase in plastic production has led to widespread mismanagement and substantial discharges of plastic waste into the marine environment. Once released, (micro-)plastics can drift, remain buoyant, sink, strand on coastlines, be ingested, or fragment into smaller particles. Understanding the marine life cycle of large (L-MP, 5mm–300µm) and small (S-MP, 300-1µm)  microplastics is therefore a major challenge, particularly regarding the “missing plastic” paradox, highlighting the imbalance between plastic inputs and surface observations in the ocean¹. Recent studies^2,3 show that a proportion of MP is distributed throughout the water column, where it can aggregate with organic matter, marine snow, and microbial and phytoplanktonic organisms, ultimately settling to the sea floor. This project investigates MP fate in the marine environment, and the roles of MP type, shape, and environmental conditions in their sedimentation in the Calanques National Park (CNP) and Marseille Bay, a highly urbanized coastal area under strong anthropogenic pressure.

Two field campaigns were conducted in March and September 2025 in Marseille Bay and the CNP. Subsurface waters were sampled using in situ pumping, while surface waters were collected using manta trawls. In addition, anchored particle interceptor traps (PIT) were deployed for one month in Marseille Bay. Microplastics from water and PIT samples were extracted following adapted protocols for L-MP⁴, with adjustments to recover S-MP (>32µm) and to characterize them using micro-infrared spectroscopy⁵. Complementary geochemical (POC, PON) and biological (chlorophyll a, TEP) analyses were performed on PIT samples.

The campaigns provide robust estimates of subsurface L- and S-MP concentrations, showing a size-dependent shape and abundance distributions, with MP concentrations decreasing and fiber abundances increasing as size increases. Samples of manta and pump collected from the same location reveal differences in the abundance and polymer composition of L-MPs in surface and subsurface waters. PIT deployment provides the first MP sedimentation fluxes for this area, which are put in perspective with hydrodynamic, geochemical, and biological parameters to elucidate the role of aggregation in MP vertical transport. Overall, these findings highlight the importance of S-MP contribution (<300µm) in the diagnosis of MP pollution and provide the first estimates of MP vertical fluxes in Marseille Bay.

References

¹Isobe, A. & Iwasaki, S. The fate of missing ocean plastics: Are they just a marine environmental problem? Sci. Total Environ. 825, 153935 (2022).

²Rowlands, E. et al. Vertical flux of microplastic, a case study in the Southern Ocean, South Georgia. Mar. Pollut. Bull. 193, 115117 (2023).

³Galgani, L. et al. Hitchhiking into the Deep: How Microplastic Particles are Exported through the Biological Carbon Pump in the North Atlantic Ocean. Environ. Sci. Technol. 56, 15638–15649 (2022).

⁴Alcaïno, A. et al. Influence of the Rhone River intrusion on microplastic distribution in the Bay of Marseille. Reg. Stud. Mar. Sci. 73, 103457 (2024).

⁵Nguyen, T. T. et al. Spatial and seasonal abundance and characteristics of microplastics along the Red River to the Gulf of Tonkin, Vietnam. Sci. Total Environ. 957, 177778 (2024).

How to cite: Surmont, A., Vidal, L., Labille, J., Licari, L., Wong-Wah-Chung, P., and Lebarillier, S.: Distribution and Vertical Fluxes of large and small microplastics (MP) in the Calanques National Park (CNP) and Marseille Bay , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6645, https://doi.org/10.5194/egusphere-egu26-6645, 2026.

Marine plastic pollution research has traditionally focused on the abundance and impacts in surface waters, resulting in substantial knowledge gaps regarding their vertical distribution and ultimate fate. This study addresses these limitations by providing a comprehensive assessment of plastic transport dynamics across multiple compartments. Plastic concentrations and composition (shape, size, and polymer type) were quantified from the sea surface to the sediment using high-resolution sampling instruments.

Our results demonstrate an exponential decrease in plastic abundance with depth, while revealing particle retention within the mixed layer, immediately above the pycnocline. Notably, sediments constitute the dominant sink, containing the vast majority of the plastic load (98% of the total measured abundance). Furthermore, sediments accumulated the highest-density polymers and smallest particle sizes, suggesting that density-driven sedimentation and fragmentation processes play a key role in vertical transport.

These findings highlight the importance of understanding vertical transport processes as a critical step in identifying the ultimate sinks of plastic and assessing its potential long-term environmental impacts, thereby supporting the development of more effective mitigation strategies.

How to cite: Quintana, R., Manzano-Medina, S., Pérez-López, L., Oyón-Sanz, A., González-Gordillo, J. I., Marti, E., and Morales-Caselles, C.: Beyond the Surface: Vertical Distribution of Plastics in Coastal Areas of the Gulf of Cádiz, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14036, https://doi.org/10.5194/egusphere-egu26-14036, 2026.