Revealing the Migration Behaviors of Microplastics in the Intertidal Environments

Qing Huang^1,2Keyu Tao²Yan Li^1*

• ¹School of Marine Sciences, Sun Yat-Sen University,, Guangzhou, China
• ²College of Materials Science and Engineering, Nanjing Forestry University, Nanjing, China

In January 2020, the novel coronavirus pneumonia (COVID-19) continues to rapidly spread throughout the world. As of the post-pandemic (July, 2023), the World Health Organization (WHO) reports 767 million confirmed cases and 6.9 million deaths worldwide, with new variants like Omicron still causing regional outbreaks (WHO, 2025). World Health Organization (WHO) calls on taking strict interventions decisively to prevent COVID-19 further spread, such as wearing face masks, washing of hands, and staying at home (WHO, 2025). The measures would unavoidably bring the upsurge demand on plastics, especially face masks, single-use dishware and disinfection supplies. Despite vaccination efforts, the pandemic has generated over 8.4 million tons of plastic waste globally according to 2022 satellite estimates, primarily from medical equipment and packaging (Peng et al., 2021). This surge has disrupted the 2025 global plastic reduction targets outlined in the UN Environment Assembly resolution (UNEA-5.2), as nations prioritized single-use plastics for infection control.For better management of the plastic wastes, the Basel Convention on the Plastic Waste Partnership also urges progressive plastic management during the COVID-19 pandemic.However, some plastic wastes are still not disposed of properly and entered the ocean surroundings. The debris can be fragmented and degraded into microplastics (MPs; <5mm) and nanoplastics (NPs; <1μm) under the effects of solar radiation, temperature variation, wave slap and biological interactions (Peeken et al., 2018). With the effect of ocean currents and tides, these micro-(nano-)plastics (MNPs) would migrate and accumulate in the intertidal zone, a place with strong land-sea interactions (Lv et al., 2020). Intertidal zone can be roughly classified as the mudflats, sandy beaches, rocky beaches, vegetated marshes and mangrove swamps (Murray et al., 2019). It is extraordinarily important as it not only provides habitats but also feeds the organisms.During the pandemic, the OceansAsia organization recorded that masses of face masks with various types were washed up on Hong Kong Soko beaches (Figure 1) (Reuters, 2020). TheseMNPs contain many severe ecological hazards: They adsorb surrounded pollutants, disrupt microbial and invertebrate physiology, and accumulate in food webs, ultimately threatening biodiversity and ecosystem resilience. However, a summary on the interaction between MNPs and the intertidal environment is still very limited and not yet fully understood. Thus, the aim of the study is to figure out the interaction MNPs and intertidal zone with different geological conditions. Afterwards, it calls more attention to reducing the quantity of mismanaged the plastic induced by the COVID-19 pandemic in the future. Overall, this study will be of vital importance during the special period to predict the possible environmental risks brought by coronavirus disease prevention manners.Mudflats often composed of the deposition and accumulation of suspended mineral/organic particulate matters and soluble nutrients in lacustrine, riverine, and estuarine environments (Figure 1). With the exponential usage of plastic during the pandemic, it could be speculated that a large number of MNPs will be discovered as the severe fragmentation generate undergoing the unique hydrodynamics, strong solar radiation and human intervention (Wu et al., 2024). Two main routes for convergence plastics into the mudflats including natural process and anthropogenic activities. Natural processes contain atmospheric deposition, tidal action, surface runoff and river transportation (Wei et al., 2024). Anthropogenic activities would be another major route as mudflats are prime recreational sites. The domestic sewage generated from the activities would increase the secondary outbreak risks by propagating pathogens (Isobe et al., 2019).As mudflats can supply abundant minerals and nutrients, they serve a place for numerous plants growing, and finally form the biological beaches as a shelter or nursery grounds for fishes and birds (Figure 1) (Li et al., 2020). Biological beaches have also been regarded as hotspots of MNPs accumulation after convergence, generating an overlap between MNPs and some species (e.g. zooplankton, invertebrates, crustaceans, fish, seabirds and even marine mammals) (Wu et al., 2022b). After ingestion, most microplastics with large size existed in gastrointestinal tract for several days. However, some of the small MNPs (<100μm) would translocate and induce deteriorations in the various systems, such as the tissue, digestive, lymphatic and even the neurological systems (Wu et al., 2022a;Huang et al., 2024). During the whole process, MNPs could release the harmful chemicals like additives and even pathogens, such as the release ofHalofolliculina on plastics inducing disease outbreaks in organisms (Lamb et al., 2018). It could be hypothesis that the plastics attached corona virus might have the possibility to be multiple outbreak sources of COVID-19 epidemic.Rocky beaches are where large rock predominates in the coastal zone. Recently, some rocks in beaches are contaminated by the sink of plastics, defined as "plastiglomerates" (Figure 1) (Das, 2023). In the geologists' perspectives, plastiglomerates might be a new indicator of the Anthropocene due to their omnipresence and persistence characters. Thus, more and more researches have been performed on plastic pollution on rocky beaches (Turner et al., 2019;Das et al., 2023). Recently, a new effect named "pyroplastic" was reported to generate when the burnt debris becomes part of an agglomerate with the rocks after cooling (Figure 1). The phenomenon has been documented in many beaches, such as Hawaii beaches of the U.S.A and Whitesand beaches of the U.K. Meanwhile, another contamination called "plasticrust" was reported to form when the surface of rocky was encrusted by plastics and formed the variable crusts (Gestoso et al., 2019). It has been observed on the mid-upper rocky shore in Madeira island of Portugal and Giglio island of Italy. The continuous hydrodynamics induced the crash between MNPs and rocks in these places. Moreover, the plasticrust phenomenon becomes more severe with increased coverage as time passes (mean±error bar: 9.46±1.77% in 20×20cm quadrats; n=10) (Gestoso et al., 2019). Different from that the "pyroplastic" is mainly made of polyethylene terephthalate (PET), polyethylene (PE) with blue and white colors is the predominant type of plasticrust contamination. As both PET and PE were mainly applied in domestic products, the origins of the two plastic contaminations can be narrow down to domestic wastes from their corresponding counties. Latest studies reported that the global plastic production increased from 370.7 million tonnes (Mt) in 2018 (pre-pandemic) to 413.8 Mt in 2023 (post-pandemic) (Zhu and Huang, 2025;PlasticEurope, 2025). Even in the post-pandemic, the public perceptions and attitudes towards health and hygiene practices have changed. Wearing masks in public, which was once considered unusual outside of specific cultural contexts or health scenarios, has now become widely accepted and normalized, resulting in an annual growth rate of approximately 10% (Zhu and Huang, 2025).Thus, coastal marine debris monitoring and action plans might consider plastiglomerates contamination into their marine environmental monitoring guidelines.Sandy beaches are landforms alongside the sea composed of loose particles. MNPs transportation are easily affected by the changing ecological conditions, such as tides, winds, waves, and thermohaline gradients (Figure 1). Drifting behavior is very common and hypothesized as the comprehensive results according to tide and wind speed. Thus, the three-dimensional oil spills model was found able to well stimulate and predict the drifting track of MNPs (Isobe et al., 2014). Parts of low-density MNPs (<1.00g•cm -3 ) are easily rushed back into marine again due to windand wave-induced currents, and roll-structures. The repeated MNPs migration continues according to their sizes (Hurley et al., 2018). Small-size MNPs have the opportunity to escape from the offshore trapping and enter the open sea, while large-plastic debris are selectively conveyed onshore again by a combination of Stokes drift and terminal velocity (Seeley et al., 2020). Meanwhile, the migration of MNPs with the densities (1.00-1.03g•cm -3 ) could be influenced by the thermohaline gradients. For some MPs with higher density (>1.03g•cm -3 ), they tend to settle and concentrate in benthic environments (Seeley et al., 2020). Most MNPs gradually buried by turbulent and persist in the deepest depth in the sandy sediments (Wu et al., 2022b).However, the mechanisms for MNPs burial are still unclear and also need more numerical simulation and experimental results. Nowadays, the numerical parameters set in simulating the migration of MNPs mainly based on the experiences or publications. Thus, future studies are urgent to enhance the accuracy of the parameters setting by combining laboratory studies and field investigations.The intertidal zone, especially the mudflats, rocky shores, and sandy beaches, has been severely contaminated by the upsurge of plastic wastes. Mudflats as "plastic sinks" concentrated MNPs up to 10 times higher than sandy beaches due to their nutrient-rich sediments and dynamic hydrodynamics (Lo et al., 2020). Notably, virus-laden MNPs could be adsorbed onto organic matter creating the pathogen reservoirs. Conversely, rocky shores show concerning geochemical changes: "plasticrusts" (dominated by PE/PET) and pyroplastics (derived from PET) on coastal rock surfaces, enshrining pandemic plastics into geological archives with half-lives exceeding centuries (Das et al., 2023). Sandy beaches serve as "sorting engines" because low-density MNPs (<1.0 g/cm³) re-enter marine circulation via Stokes drift, while denser particles infiltrate benthic food webs, with 22% of crustaceans showing neurotoxic nanoplastics (Huang et al., 2024). The