Contributions to River Plastic Monitoring Across Scales -Volume II

Marcel Liedermann^1*Daniel González-Fernández²Freija Mendrik³Lauren Biermann⁴Tim van Emmerik⁵

• ¹Department of Landscape, Water and Infrastructure, BOKU University, Institute of Hydraulic Engineering and River Research, Vienna, Austria
• ²Department of Biology, University Marine Research Institute INMAR, University of Cádiz and European University of the Seas SEA-EU, Puerto Real, Spain
• ³International Marine Litter Research Unit, School of Biological and Marine Sciences, University of Plymouth, Plymouth, United Kingdom
• ⁴Plymouth Marine Laboratory, Plymouth, United Kingdom
• ⁵Hydrology and Environmental Hydraulics Group, Wageningen University & Research, Wageningen, Netherlands

Plastic pollution in aquatic environments has emerged as a critical global issue, with rivers playing a significant role in transporting plastic debris from land to sea. The complexity of riverine plastic transport is influenced by various factors, including the physical properties of plastics such as type, size, shape, and density, which affect their spatiotemporal distribution in flowing waters (Range et al., 2023). Rivers, however, not only act as conduits for plastic debris but also serve as temporary or perpetual sinks, particularly in tidal zones or compounded stretches, also contributing to the formation of localized accumulation zones (van Emmerik et al., 2022;Liro et al., 2022). These riverine plastic hotspots, while smaller in scale compared to oceanic counterparts, exhibit significantly higher concentrations of plastic, posing risks to ecosystem functioning and human health (e.g., Liro et al., 2022;Tasseron et al., 2024) Monitoring and quantifying plastic transport in rivers is essential for understanding the dynamics of plastic emissions and validating global estimates (Vriend et al., 2020). Despite advancements in detection technologies, such as deep learning and acoustic methods, challenges remain in achieving continuous and accurate monitoring data and in method harmonization (van Emmerik et al., 2023). New techniques like using the Acoustic Doppler Current Profiler (ADCP) for macroplastic detection (Boon et al., 2023) and instance segmentation models using deep learning architectures like YOLOv8 (e.g., Ahmed et al., 2023;Fan et al., 2024;Kataoka et al., 2024) are being developed to enhance detection capabilities and provide more reliable data. An important step was made towards harmonization of data evaluation and determination of plastic transport (Pessenlehner et al., 2024). These innovations are important in order to create simplified application possibilities and thus improve global datasets and their interpretation. The urgency of addressing plastic pollution is underscored by its detrimental impacts on ecosystems and human health, necessitating comprehensive research and effective policy measures. Plastic debris can lead to ingestion, entanglement, and suffocation of marine life, and its presence in the food chain poses severe health risks to humans, including exposure to toxic chemicals (e.g. Issac and Kandasubramanian, 2021). Global efforts, including the UN's push for a legally binding treaty (March et al., 2024;UNEP, 2023) and the EU's ban on single-use plastics, highlight the need for coordinated action to mitigate these impacts (European Commission, 2022;IISD, 2022) This special issue aims to highlight recent studies that explore innovative methodologies and provide insights into the mechanisms of plastic transport and accumulation in riverine environments. Methodological Improvements to quantify plastic fluxes and stocks are presented that advance the scientific field, while studies on the origin and fate of plastic help to develop the most sustainable avoidance strategies. The general discussion brings a sharpened focus on future research topics. By advancing our understanding of these processes, we can better support policy decisions and develop strategies to reduce plastic pollution, ultimately contributing to the preservation of aquatic ecosystems and the health of communities worldwide.In the study of Boon et al. ( 2023), authors propose a novel use of ADCP technology, typically used for flow velocity and sediment measurements, to detect macroplastic items suspended in rivers. Through controlled, semi-controlled, and field experiments, the researchers demonstrated that ADCPs can identify plastic items even amid natural scatterers like organic material, aquatic life, and air bubbles. Although distinguishing plastic characteristics (size, material, orientation) remains a challenge, the study suggests that item-specific echo signatures could enhance detection accuracy. The method, when calibrated with net-sampling data, shows potential for continuous, long-term plastic monitoring through existing, standard ADCP measurements. This could offer new insights into plastic transport trends and the effectiveness of mitigation strategies.focus on detecting floating macroplastic debris (FMPD) using advanced computer vision. They developed five instance segmentation models based on the YOLOv8 architecture, trained on over 7,000 images from seven rivers. The models achieved strong performance in both object detection and image segmentation tasks. Crucially, the study found that ground sampling distance (GSD) significantly impacts detection accuracy as smaller GSDs enhance segmentation, while larger ones aid object detection. Additionally, careful category selection reduced false positives. These insights are key for standardizing international FMPD monitoring protocols and improving models to estimate plastic transport, enhancing our understanding of land-to-sea plastic emissions. 2024) offer an in-depth exploration of riverine plastic transport dynamics within the Austrian Danube River, focusing on the critical role of spatiotemporal and dischargedependent assessments for precise plastic flux determination. By employing a net-based crosssectional multi-point approach and comparing an impounded section near Aschach with a freeflowing stretch near Hainburg, the study uncovered significant vertical and lateral variability in plastic concentrations. This underscores the importance of multi-point sampling for accurate assessments. The research adapts suspended sediment-monitoring methodologies and introduces a quantification approach, independent of the underlying sampling technique. Furthermore, the study calls for improved sampling techniques, especially for capturing small microplastics and more seldomly transported large microplastic items. The findings stress the role of hydro-morphological conditions in plastic distribution and advocate for basin-wide, seasonally aware monitoring to better support mitigation policies.In London, Ontario, researchers examined macroplastic and large microplastic accumulation in stormwater systems using LittaTrap™ devices. The study, authored by Kozikowski and Corcoran (2024), identified pedestrian traffic as a key contributor to debris levels, with areas like parks and community centers showing the highest counts. Seasonal variation was also significant, with summer having the highest plastic loads due to increased outdoor activity. Finer mesh traps proved effective at capturing microplastics, suggesting enhancements to urban stormwater systems. Interestingly, no strong correlation was found between plastic levels and environmental factors like wind or precipitation, highlighting the complex interplay of urban behaviors and environmental conditions. The study supports the need for broader geographic coverage and sustained monitoring. 2024) investigated macroplastic pollution in the Tullahan River, a heavily polluted urban waterway in Metro Manila, Philippines. Through visual surveys and manual collection, the researchers found that food wrappers and thin polyethylene plastics dominated both floating and riverbank litter. Human activity, especially packaging habits and inadequate waste management, was identified as a primary driver of pollution. The study also noted that residential and industrial land use areas contributed most plastic waste. These findings call for stronger plastic regulation and waste management improvements Additionally the field data collected can enhance modeling accuracy and policy development. The research provides a foundation for targeted interventions in urban rivers heavily burdened by plastic waste.A study from the Sierra Gorda Biosphere Reserve (SGBR) in Mexico revealed the presence of microplastic pollution in protected freshwater ecosystems. Granados-Sánchez et al. ( 2024), collected samples from rivers and reservoirs, finding significant microplastic concentrations in both water and sediments. Fibers, primarily composed of polyethylene terephthalate, polypropylene, and polyester, were the most common plastic types identified via FTIR spectroscopy. The study also detected trace metals like aluminum and zinc on these microplastics, raising concerns over their role as pollutant carriers. Using the Pollution Load Index, the region was categorized as having a moderate level of pollution. The study linked pollution to waste management gaps, while also stating that wind and rain serve as major transport mechanisms. These findings stress the need for improved conservation and waste strategies, even in seemingly remote or protected areas.Gallitelli and Liro (2024) examine river garbage patches (RGPs):dense, localized accumulations of plastic in rivers that are distinct from the more diffuse ocean garbage patches (OGPs). While RGPs cover smaller areas, their plastic concentrations can be orders of magnitude higher than OGPs. As rivers serve both as transport routes and sinks for plastic waste, understanding RGP dynamics is vital for effective clean-up and pollution control. The authors argue that RGPs, due to their accessibility and relative stability, present valuable opportunities for targeted mitigation. They advocate for more research into plastic abundance and distribution in rivers, and suggest that citizen science could play a significant role in monitoring and cleanup. The study emphasizes a shift in focus from oceans to rivers, which often retain plastic until flood events mobilize it downstream. A better understanding of RGPs could bridge existing knowledge gaps and lead to more efficient plastic pollution management strategies.Based on the recent advances in river plastic research we identified three main avenues for future work. First, there is still a strong need for the harmonization of plastic monitoring methods to facilitate comparability across environmental compartments, plastic size ranges, and locations around the world. Harmonization efforts should focus on both developing guidelines and standards for data collection (e.g. Wendt-Potthoff et al., 2020;Isobe et al., 2024), and developing databases for standardized data storage, processing and reporting.Second, a deeper understanding of plastic entry, transport, and retention processes is still required. This challenge can be partly solved through increased availability of comparable data. In tandem, new conceptual and physical models may support improving our understanding of the role of hydrology, climate change, and anthropogenic factors on the current and future global plastic pollution budget. The origin and fate of plastic items and particles must be better understood to identify relevant sources, sinks, and impact on land, in rivers, and in the ocean.Finally, a special focus should be placed on the development of assessment tools to quantify the potential and actual efficacy of intervention measures. With global, regional and local initiatives to reduce plastic pollution in the environment, it is crucial to provide quantitative support for the selection, implementation and evaluation of interventions, ranging from prevention strategies to removal technologies. To effectively mitigate plastic pollution in the environment, a combination of improved monitoring, scientific knowledge acquisition, global data comparison, and intervention assessment are paramount.Ahmed, D., Sapkota, R., Churuvija, M., and Karkee, M. (2023). Machine vision-based crop-load estimation using yolov8. arXiv:2304. 13282 Boon, A., Buschman, F.A., van Emmerik, T.H.M., Broere, S., Vermeulen, B. (2023): Detection of suspended macroplastics using acoustic doppler current profiler (ADCP) echo. Front. Earth Sci. 11:1231595. doi: 10.3389/feart.2023.1231595 European Commission (2022). Single-use plastics. Available at: https:// environment.ec.europa.eu/topics/plastics/single-use-plastics_en (Accessed April, 2025)) Fan, Y., Mao, S., Li, M., Wu, Z., and Kang, J. (2024). CM-YOLOv8: lightweight YOLO for coal mine fully mechanized mining face. Sensors 24 (6), 1866. doi:10.3390/s24061866 Gallitelli, L. & Liro, M. (2024): Do river garbage patches exist?. Front. Environ. Sci. 12:1480391. doi: 10.3389/fenvs.2024.1480391 Granados-Sánchez, R.R., Sedeño-Díaz, J.E., López-López, E. (2024): Microplastic pollution and associated trace metals in freshwater ecosystems within protected natural areas: the case of a biosphere reserve in Mexico. Front. Environ. Sci. 12:1441340. doi: 10.3389/fenvs.2024.1441340 IISD (2022). Unea launches negotiation of plastic pollution treaty, science body on chemicals. Available at: https://sdg.iisd.org/news/unea-launchesnegotiation-of-plastic-pollution-treaty-science-body-on-chemicals (Accessed April, 2025