Freshwater fishes and microplastics in an era of multiple stressors.

Abstract

Microplastics, plastics < 5 mm in size, are environmental contaminants typically arising from the breakdown of larger plastics that may cause negative impacts when consumed by freshwater fishes. However, microplastic exposure occurs in combination with other interacting multiple stressors, for example urbanisation and parasite infection, and may potentially cause a range of synergistic negative impacts. Understanding the patterns of microplastic ingestion and the interactive impacts in relation to additional stressors is thus crucial in the management of freshwater fishes to implement effective mitigation measures. Here, a literature review, field studies, an experiment and a metanalysis were carried out to examine how microplastic loads and their effects relate to stressors and the ecological features of freshwater fishes. More specifically, the thesis aimed to determine: 1) if microplastic loads are predictable from biological and environmental features; 2) the impacts of microplastic exposure on host-parasite dynamics and feeding; and 3) the interactive effects of microplastic exposure and additional stressors. A literature review was first undertaken to understand the ingestion and effects of microplastics in freshwater fishes and to identify knowledge gaps and testable hypotheses for further study (Chapter 2). The literature suggested microplastic loads and effects are somewhat predictable based on the ecological traits of fishes while knowledge gaps remained around the ecological impacts of microplastic exposure and the potential interactive effects with other stressors. Field studies established baseline microplastic loadings in relation to the environmental loads and species traits across a spatiotemporal gradient in sediments, macroinvertebrates and fishes from a small urban river (Chapter 3) as well as macroinvertebrates and fishes from a larger river system (Chapter 4). Both studies revealed a low incidence of particles and that loadings were unrelated to abiotic levels and biological features. Data from Chapters 3 and 4 then informed chronic environmentally relevant microplastic exposures for an interaction experiment with parasite exposure looking at the combined effects on fish parasite load, morphometrics and feeding (Chapter 5). Parasite exposure reduced feeding and growth, however microplastic exposure had no single or interactive effect. Chapter 6 examined microplastic loads within juvenile eels to understand the potential impacts of microplastic contamination within several south west England eel populations. Microplastic incidence was very low and unrelated to eel length or location, suggesting little threat of microplastic contamination on juvenile eels. Finally, a metanalysis determined the combined effects of microplastics and additional stressors in freshwater fishes and whether the type and magnitude of effects varied with the interacting stressor, response, exposure or fish features (Chapter 7). Interactions were mostly additive across different stressor and response categories, except for ecological responses for the exclusive dataset which were antagonistic, and were unrelated to microplastic or fish features. Overall, the results suggest microplastic contamination is consistent across space, time and different taxa while the experimental and metanalysis data demonstrate largely additive interactive effects. Other stressors may currently have greater importance in the management of freshwater fishes therefore it is recommended that managers target these known stressors but continue to monitor the levels and impacts of microplastics.