Measuring of microplastics settling velocities and implications for residence times in thermally stratified lakes

Microplastic (MP) residence times are currently poorly constrained in lakes, especially at a quantitative level. In this work settling experiments with pristine and biofilm-colonized MPs were combined with model calculations to evaluate settling velocities, particle distributions and residence times in the epi- meta and hypolimnion of a hypothetical stratified lake broadly based on Upper Lake Constance. Settling velocities of various biodegradable and non-biodegradable polymers of various shapes, sizes and biofilm colonization were measured in a settling column. The settling velocities ranged between ~ 0.30 and ~50 mm s^-1. Particle sizes and polymer densities were identified as primary controls on settling rates. MPs that had been exposed to a lake environment for up to 30 weeks were colonized by a range of biofilms and associated extracellular polymeric substances; surprisingly, however, the settling velocity did not vary significantly between pristine and colonized MP particles. Simulated MP residence times in the model lake varied over a wide range of time scales (10^-1 - 10⁵ days) and depended mainly on the size of the particles and depth of the lake layer. Long residence times on the order of 10⁵ days (for 1 µm MPs) imply that for small MP particles there is a high probability that they will be taken up at some stage by lake organisms. It also suggests that insignificant amounts of small MPs will be found in the lake sediment unless some process increases their settling velocity as their residence time is considerably longer than the theoretical retention time of Lake Constance (~4.5 years).