Microplastic ingestion by fish of the River Thames and the River Stour-East Anglia.
Supervisor: Alice Horton (NOCS)
Microplastic contamination is widespread throughout aquatic environments. There is currently insufficient understanding of the factors influencing microplastic ingestion by higher trophic levels, specifically in fish. This study compared microplastic ingestion between species of benthic fish (European Flounder) and pelagic fish (Whiting and Herring) in two UK watersheds: The River Thames and Thames estuary and the River Stour – East Anglia. The gastrointestinal tracts of 53 fish were analysed using a novel digestion protocol. In total, 170 microplastics (MPs) with a mass of 6.5198 mg were detected during micro-Fourier Transformed Infrared Spectroscope (μ-FTIR) analysis. In the River Thames, 72% and in the River Stour, 80% of fish had consumed microplastics. Polypropylene (60%) and polyethylene (26%) were the most abundant synthetic polymers in all the habitats. For benthic fish, the average microplastic abundance (MPs/Fish) is significantly higher in the River Stour than in the River Thames (Mann-Whitney U, p < 0.05). In the River Thames, the average microplastic abundance (MPs/Fish) was not significantly different in pelagic and benthic habitats (Mann-Whitney U, p > 0.05). However, in the River Thames, the average specific microplastic abundance (MPs/Body Weight (g) and MPs/GIT Weight (g)) was significantly higher in pelagic fish (Mann-Whitney U, p < 0.05). Feeding strategies might be the potential reason for differences in microplastic abundance in benthic and pelagic species. Effluents from wastewater treatment plants and boat services in the rivers in proximity to highly urban areas can be the major sources of microplastic in these rivers. Further, due to tidal action, low-density microplastic can accumulate in the river sediments. Furthermore, when all sites are considered, there was a significant positive linear relationship in microplastic abundance with the total length of the fish (R2 = 0.068, DF= 51, p < 0.05) due to the higher feeding rate and size of the gut in larger fish. The present study reveals that microplastic ingestion by fish varies between watersheds depending on the fish habitat, feeding behaviour, species, and size. The understanding gained from this study will help to interpret findings on microplastic ingestion by fish species and identify the potential influence on health risks on these fish species in future studies.

Keywords: – Microplastic abundance, River Thames, River Stour, Benthic, Pelagic