The abundance and composition of Micro Plastics in the North and South Basins of Lake Biwa, Japan

Considerable amounts of plastic waste that flow into rivers and the ocean have become a major environmental problem. This is especially true for microplastics (MPs: Plastic particles less then 5mm in size) and different organizations are investigating their impact on the environment. While a lot of related research has focused on oceans and coastal seas, investigations of MPs in lakes and rivers remain sparse. Lake Biwa, the largest and the oldest lake in Japan is no exception. In order to better understand the situation, we conducted quasi-monthly sampling on Lake Biwa (26 times total from March 2019 to July 2022) by filtering one cubic meter of water from the lake surface at a specified location in the North Basin (average depth <d>= 43m) and a second location in the South Basin (<d>= 4m). FT-IR and Nile Red staining were used to evaluate the quantity and quality of MPs in the two locations.

We found that the particle concentrations were highly intermittent in both basins. Median particle concentrations were low: 2.5 particles/m³ in the South Basin, and 6.6 particles/m³ in the North Basin. But a few “spikes” in concentration were observed. Notably, more than 100 Small MPs (plastic particles sized less than 1mm, per m³) were observed on Nov. 29^th and Dec. 27^th, 2020, and on June 20^th, 2022, in the South Basin. We suggest that it is related to the weather that occurred in Lake Biwa. Within the 3 days preceding these sampling days, strong winds of more than 10m/s were observed. Because the South Basin is much shallower, it is more likely that the sediment on the bottom was lifted up by wind-driven waves, and deposited MPs were resuspended into the water column.

Overall, the median number of MPs was clearly higher in the North Basin. We suggest that it happened because of the gyre that exist during the stratified season in North Basin. There are 3 gyres in the North Basin which create a non-uniform distribution in plastic particles. In addition, we have to note that the residence time in the North and the South Basin are quite different. The residence time in the North Basin is 5.5 years, whereas in the South Basin, it is only about 15 days. Due to the presence of the gyre, the residence time in the North Basin will be longer (19 years to be precise). Therefore, we can presume that many MPs stayed in the North Basin.

Concerning the composition of MPs in Lake Biwa, Polyethylene (PE) and Polypropylene (PP) were dominant, together accounting for 90%. Since PE and PP are low-cost, light, easy to process, etc., they are the most commonly used type of plastics. Similar results for the composition of MPs have been obtained by other research groups as well.