Filter feeders are key to small microplastic residence times in stratified lakes: A virtual experiment

Microplastic (MP) is potentially harmful to lake ecosystems, with its uptake into the food web largely controlled by its residence time in the lake water column. Here we combine laboratory and virtual experiments to quantify residence times of small MP (<15 μm) in two contrasting model lakes; Lake Constance (large lake) and Esthwaite Water (a small lake). We compare MP residence times in a purely physical system with MP transport controlled by sinking and mixing to a model where, in addition to physical processes, zooplankton package MP into faecal pellets that are then egested into the water column. The laboratory experiments showed that MP settling velocities increased from ~5 × 10^-6-10^-3 mm s^-1 for pristine MP to ~1 mm s^-1 for MP embedded faeces. Modeled lake residence times for the 0.5 and 5 μm particles were >15 years in the abiotic models, while in the biotic simulations they were reduced to ~1 year. There was little difference between abiotic and biotic simulations for the 15 μm particles. The ratio of the MP zooplankton uptake velocity to the sinking velocity (v_{_up}/v_s__epi) was used to classify biological vs. physical transport pathways. For the 0.5 and 5 μm particles v_{_up}/v_s__epi was ≫1 in all cases for both lakes, while for the 15 μm MP there was a transition between biological and physical processes dominating residence times depending on zooplankton numbers. Our results suggest that packaging of small MP in faecal pellets by zooplankton will control its residence time in lakes. Moreover, the majority of small MP will cycle through organisms before reaching the sediment, increasing the likelihood of negative ecological effects and transfer in the food web.