Microplastic fate and behaviour in model freshwater mesocosms

Abstract

Microplastics (plastic particles <5.0 mm in diameter) have been detected in freshwater ecosystems worldwide, yet little is known about the fate and behaviour in Canadian freshwater systems. Knowledge gaps include the influence of ice formation, water quality (i.e., nutrients enhancing both biofilm and filamentous algae growth), and the presence of aquatic plants (cattails) on fate and behaviour of microplastics. I conducted two field-based mesocosm studies at the Prairie Wetland Research Facility at the University of Manitoba. The first mesocosm study was a long-term 622-day study where five different microplastic types (foams, films, fragments, microbeads and fibres) were added to understand long-term microplastic fate and behaviour during distinct seasons and overwintering. The second study was a short-term 251-day study using model constructed freshwater mesocosms where both films and fibres were added to understand whether nutrients, cattails and overwintering affected microplastic fate and behaviour. In both studies, distinct open water seasons (spring, summer, fall), over wintering and subsequent spring melt of ice lead to unique behaviours of different types of microplastics i.e., resuspending, and settling. In the second study, I found that biofilm and filamentous algal growth were enhanced by the nutrient addition were the likely drivers of microplastic settling and resuspension behaviour in the mesocosms through biofouling, defouling (biofilms) and trapping microplastics as enhanced filamentous algal growth occurs. Overall, substrate may not be the ultimate sink for microplastics as previously thought, as microplastic behaviour (e.g., aggregation, biofouling), and characteristics of the aquatic ecosystem (e.g., climate, ice crystal formation, and water temperature-density effects) will add to the complexities of microplastic behaviours and will determine their ultimate fate in freshwater ecosystems. This thesis has begun to advance our understanding of microplastic fate and behaviour in Canadian freshwater systems, their cyclical/seasonal dynamics, biofilm as a driver of microplastic behaviour, resuspension/settling dynamics due to ice formation and subsequent melting, and the potential for floating wetlands using cattails as a bioremediation technique.