Physiological and behavioural responses of aquatic organisms to microplastics and experimental warming

Microplastics are an emerging contaminant of concern because of their potential to cause harm to aquatic biota, such as reproduction, growth, and survival, and there is a lack of knowledge about how microplastics can affect other sub-lethal responses, such as movement behaviour and respiration rates, which may have consequences for species interactions. Additionally, there is little evidence for the effects of microplastics under different climate warming scenarios. To address this knowledge gap, the effects of high-density polyethylene (HDPE) microplastics, in combination with different constant temperature regimes (10 °C, 15 °C, and 20 °C) and a fluctuating regime (10-20 °C over a 24h diel cycle) on the respiration rates, feeding rates, and movement speeds of Gammarus pulex and Asellus aquaticus were assessed. Respiration rates of G. pulex increased with temperature according to metabolic theory, but there was no evidence for increased respiration rates of A. aquaticus at higher temperatures. Overall, the respiration rates and movement speeds of G. pulex were higher than A. aquaticus but there was no evidence that microplastics independently, or in combination with experimental warming, influenced any of the responses tested. There is increasing evidence that some microplastic particles may not be harmful to aquatic biota, and the findings presented in this study indicated that further evidence about the effects of different microplastic types, in combination with other human-induced pressures, is required to better understand the hazards and risks associated with microplastic particles in the environment.