On the hypothesis of microplastics resuspension by internal waves in a deep stratified lake.

Kimitsuki et al (SIL 2022; EGU 2023, this session) have reported near-surface microplastic particle concentrations from 26 quasi-monthly sampling dates over three years in Lake Biwa, Japan.  The concentrations show a high degree of intermittency, both in the shallow South Basin and the deep North Basin.  Kimitsuki et al have suggested that the dates of high particle concentrations followed relatively strong wind events, which are hypothesized to stir up bottom sediments together with deposited microplastic particles. In the South Basin, which has an average depth of 4 m, this hypothesis seems very natural.  However the sampling point in the North Basin was located in waters well exceeding 60 m depth.  If the sampled microplastic particles there originated from wind-induced resuspension events, it seems likely that the resuspension occurred at shallower bottom depths close to shore.

Through well-resolved hydrodynamic simulations, combined with particle tracking, the current work considers how resuspension at shallow depths, followed by advection during the days preceding sampling, might explain the two highest spikes in Kimitsuki et al’s sampled concentrations in the North Basin.  These spikes occurred on June 20, 2021 and July 4, 2021, when measured particle concentrations were respectively 54 and 46 particles/m³, as may be compared to the median value over the three-year campaign of 6.5 particles/m³.  Preceding both dates, both forward and time-reversed particle tracking suggest that the sampled microplastic particles could have been resuspended from the lakebed at depths of around 10 meters, near to the shore about 5 km northwest of the sampling point. At this location and depth, internal waves, associated with vertical undulations of the thermocline, were predicted to induce strong water currents near the lake bottom.  The simulated near-surface currents were then predicted to transport such resuspended particles offshore toward the sampling location.