Seasonal variation observed in microplastic deposition rates in boreal lake sediments

Abstract Purpose The sediment trap method allows measurements of vertical microplastic flux rate into sediments and provides comparable information of the spatial microplastic deposition rates. Such data are essential for comparison of the microplastic pollution rates in different sedimentary systems and for future risk assessments. Materials and methods We monitored microplastic fluxes using sediment traps in a boreal lake seasonally during 1 year. The sites represent different level of exposure to anthropogenic activities, from construction work to the open water site. Microplastic fluxes were compared to sediment characteristics (organic content) and sediment accumulation rates. Results and discussion The highest annual microplastic deposition rate (2300 items m −2 year −1 ) was recorded at snow disposal site, a location where the snow collected from the city streets during winter is transported. The lowest rate was observed at the control site (660 items m −2 year −1 ) upstream from the city. Our results reveal the seasonal variation in microplastic deposition rates. In general, the highest microplastic flux rates were measured during growing season, accompanied with higher sedimentation rate. The low microplastic deposition rate during winter is likely explained by ice cover, frozen soil, and snow cover in the catchment. In contrast, microplastic concentration was higher in winter samples due to ceased sediment transport from catchment to lake. The sediment accumulation rate did not predict microplastic accumulation rate. Conclusion Our data suggest seasonal variation in microplastic deposition rates. The microplastic flux rates compared to their concentrations indicates that sites with high sedimentation rates can lead to underestimation of microplastic deposition and hence hamper recognition of hot spots. Graphical Abstract