Evaluating the properties, fate and individual-to-ecosystem level impacts of contrasting microplastics in freshwaters

Abstract

Microplastic particles (MPs) are fragments, fibres and other shapes derived from plastic polymers in the size range of 1–5 000 μm. Concern about the environmental impacts of MPs and their implications for human wellbeing has never been higher. Unfortunately, growth in the empirical understanding of the dynamics and impacts of MPs lags behind. This hinders the capacity of scientists, managers and policymakers to address public concerns about the true level of risk posed by MPs, and to develop effective management, policy and governance strategies for eliminating or reducing those risks. Research on the behaviour and impacts of MPs in freshwater ecosystems is especially deficient, despite their vulnerability to inputs of plastic waste (e.g. via storm water and other terrestrial runoff), and their capacity to act as key transport pathways through the landscape. This represents a substantial black box in our understanding of the dynamics of MPs from inland to the ocean. In seven research activities (5 mesocosm experiments, 1 field study and 1 literature review) we addressed two broad research questions :a) Initial fate and environmental interactions of MP particles in streams, including biofilm formation and sorption of chemical stressors b) Ecological impacts of MPs on resource consumption, growth and survival of organisms, and on key ecosystem processes. Among our key results addressing the initial fate and biofilm formation of MP particles, we found that (i) biofilm formation generally made denser particles more buoyant and caused more buoyant particles to sink faster, (ii) biofilms on polystyrene MPs supported more cyanobacteria than other polymers, and (iii) aquatic macrophytes increase MP retention by up to 94 %. Among our key results addressing the ecological impacts of MPs, we found that almost all MP shapes and polymers studied had one or more effects on stream microbial organisms and associated ecosystem processes (e.g. microbial respiration, detritus breakdown), and/or on the life history of a model macroinvertebrate detritivore. We also provide evidence that effects of MPs on microbial organisms can propagate up food-chains to affect consumer growth and fat storage. Some MP impacts were similar to those of naturally occurring organic and inorganic particles, whilst others represented a risk over and above that associated with natural particles. The number of MP impacts detected in our experiments provides sufficient basis for “moving beyond the precautionary principle” when motivating a need for monitoring and management – there is now sufficient evidence that MPs alter key aspects of the functioning of stream benthic food webs to motivate a need for action. Based on our results, we further provide a series of recommendations for monitoring, policy and management targeting MPs, and for future research.