Microplastics in freshwater ecosystems : effects and drivers

Microplastics (MPs) are recognised as a pervasive contaminant of freshwater environments and their key resource processing chains. However, the impacts of MPs on freshwater ecosystems remain unclear. In this thesis, I assessed the effects of MP exposure on freshwater microorganisms, macroinvertebrates, and ecosystem functions through a series of laboratory and field experiments. I further aimed to identify key drivers of MP effects, including particle properties and potential interactions with natural sources of variation and anthropogenic stress. MP effects were detected in all my experiments, most often altering microbial growth and activity, which then propagated to affect macroinvertebrate growth or ecosystem functions. These responses were often facilitated by MP properties associated with higher surface area availability, including high concentrations and irregular fragment shapes. MP effects on ecosystems were often smaller compared to natural sources of variation, including macroinvertebrate presence and resource quality. These factors also frequently modified MP effects, such as macroinvertebrate presence, which dampened some MP effects on microorganisms and ecosystem functions. Additionally, MPs had some complex interactions with another global anthropogenic stressor, warming, especially on ecosystem metabolism, although their impacts on other response variables were largely independent of one another. Furthermore, MP leachates stimulated cellulose decomposition in my field study. While I detected many MP effects occurring at concentrations that exceed those generally recorded in the environment, other MP effects also occurred at realistic concentrations, particularly those on microorganisms and macroinvertebrates. Overall, the findings of this thesis highlight the potential for MPs to affect real-world freshwater ecosystems, including as temperatures rise due to global warming. However, MP effects on microbial communities and ecosystem functioning may be less extreme than other common sources of environmental variation.