Abstract
Macrophytes in marine environments have been shown to locally increase microplastic contamination by trapping particles within their shoots and associated sediments. However, this phenomenon remains largely unexplored in river systems, where aquatic vegetation could influence local levels of microplastic retention and downstream releases of particles. This study addresses this knowledge gap by investigating for the first time the capacity of submerged freshwater macrophytes, specifically water crowfoots (Ranunculus spp.), to retain microplastics in their shoots and their effect on microplastic accumulation in underlying sediments. To achieve this, two experimental approaches were used, integrating controlled flume experiments with field sampling in the River Itchen (United Kingdom).In hydraulic flume settings, microplastic retention in bed substrate was investigated in the presence and absence of artificial Ranunculus-like plants. Three types of microplastic polymers were used: polyethylene (PE) fragments, polyethylene terephthalate (PET) flakes, and polystyrene (PS) fragments, divided into two size categories: 0.5 – 1 mm and 1 – 2 mm. Measurements of flow velocity and turbulence profiles in the flume were performed. Results showed that percentages of microplastic retention in the bed substrate were not significantly influenced by the presence of vegetation.
Field work was conducted at two sites in the River Itchen, where sediment samples were collected both underlying Ranunculus spp. beds and in bare conditions. At one site, water samples and Ranunculus spp. shoot samples were also collected. Across both sites, the average microplastic concentration in sediments was 93.58 (± 13.53 SE) items/kg (d.w.). Overall, the presence of vegetation did not have a statistically significant effect on microplastic concentrations in the bed substrate. However, site-specific differences were observed, with one site showing microplastic concentrations beneath Ranunculus spp. that were 1.75 times higher than in bare areas. Vegetation was found to have a significant influence on the shape and size distribution of microplastics retained in sediments; however, polymer composition was not affected.
Analyses of plant samples demonstrated that Ranunculus spp. shoots can retain significant quantities of microplastics on their surface with an average of 2,010 (± 250 SE) items/kg (d.w.). It was estimated that approximately 2,100 microplastic items could be retained in Ranunculus spp. shoots over the 12.2 m2 area of river surveyed. At the site investigated, microplastic contamination was highest in macrophyte shoots, followed by bed sediments, and lowest in surface water (0.32 ± 0.09 SE items/L).
This thesis demonstrates for the first time that submerged macrophyte shoots can act as significant microplastic retention structures in rivers, highlighting a previously overlooked environmental matrix. The findings reveal a novel process by which microplastics can be locally retained in river systems. Microplastic accumulation in aquatic vegetation could elevate risks of exposure to aquatic organisms and reveals a critical knowledge gap in understanding the role of rivers as sinks and pathways for microplastic pollution.
| Date of Award | 11 Dec 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Jonathan Potts (Supervisor) & Michelle Hale (Supervisor) |