Ecosystem metabolism (e.g., gross primary production [GPP], respiration [R], and net ecosystem production) integrates the activity of aerobic organisms and is an indicator of environmental change. Microplastics (i.e., particles 1–5 mm) are widespread in freshwater ecosystems and are colonized by biofilms and other organisms. However, microplastics' effects on ecosystem‐level processes including metabolism or other gas fluxes i...