Microplastic addition has no detectable effect on ecosystem metabolism or diel dinitrogen flux in a large in‐lake mesocosm experiment under oligotrophic conditions

Abstract

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 is rarely studied. In June 2021, we added microplastic fragments to large, in situ mesocosms ( N = 9) in the Experimental Lakes Area, Ontario, Canada. Target microplastic concentrations were 0–29,240 particles L −1 , relevant for freshwaters. We recorded continuous measurements of dissolved oxygen and temperature in each mesocosm and the lake epilimnion, along with wind speed and light from June 3 to August 10, 2021. We manually collected dissolved nitrogen gas (N 2 ) samples hourly over one 24‐h period in August in three mesocosms (zero, low, and high microplastics), and the lake epilimnion and hypolimnion. Microplastic addition influenced dissolved oxygen more strongly at the end of the experiment, but had no detectable effect on gross primary production, R, net ecosystem production, N 2 saturation, or N 2 flux. Gross primary production and net ecosystem production increased during summer. Mesocosm gross primary production and R trended higher than whole‐lake rates, suggesting biofilms in mesocosms enhanced metabolism. Similarly, dissolved N 2 was more strongly undersaturated in mesocosms than the epilimnion, and supersaturated in the hypolimnion, showing controls on N 2 concentration varied among habitats. Microplastics had no detectable effect on ecosystem metabolism or N 2 dynamics in pelagic mesocosms in a boreal lake under oligotrophic conditions, but the spatial and temporal comparisons offered insight into drivers of ecosystem processes.

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