1:45 PM - 3:15 PM
[O11-P36] The Vertical Distribution of Microplastics in Lake Biwa: Seasonal Change and Difference of Ingredients Perspective
Keywords:microplastics, Lake Biwa, Vertical Distribution
We can find many surveys of microplastics (hereafter MPs) in the surface layers of oceans and rivers, but few studies have been done focusing on vertical distribution. So I was very interested in Lake Biwa, which, unlike the ocean, has limited inflow of MPs from external water sources. This study investigates the vertical distribution and types of MPs in Lake Biwa. Seasonal variations in depth and Chemical composition were graphed, with the aim of contributing to the resolution of the microplastics problem.
Water samples (1 ton each) were collected once a month from depths of 0 m, 10 m, and 30 m, at the northern basin of Lake Biwa. The samples were filtered through sieves with mesh sizes of 0.1 mm, 0.5 mm, and 1.0 mm. Algae and plankton were removed using hydrogen peroxide (following the Ministry of the Environment, Water and Air Environment Bureau, Water Environment Division: Guidelines for Microplastic Survey in Rivers and Lakes), and the MPs were stained with Nile Red. The samples were then observed under a fluorescence microscope (4x magnification) with 470 nm light irradiated from a blue LED. Fluorescence reactions are used to identify the Chemical composition of MPs in a simplified manner (based on Suparnamaaya Prasad, Andrew Bennett, Michael Triantafyllou: Characterization of Nile Red-Stained Microplastics through Fluorescence Spectroscopy) . The number of MPs was counted at the same time. Sampling period was 2023, and chemical treatment and MP observation were performed in 2024.
The result is that, the number of MPs at depths of 0–10 m increased from summer to autumn, while the number at 30 m depth increased during winter. This is believed to be caused by rainfall and the presence of a thermocline. An investigation into the rainfall in Nagahama City showed that fluctuations in rainfall aligned with changes in total MPs volume. Additionally, the seasonal trend of vertical distribution of MPs showed a strong correlation with thermocline data off the coast of the Ado River. MPs entering the lake due to summer rainfall were found to accumulate above the thermocline at approximately 15 m depth (based on 2023 vertical water temperature distribution data off Ado River), and as the thermocline disappeared in winter, the MPs were presumed to have sunk below 15 m.
Next, I analyzed focusing on the variation of Chemical composition of MPs. Comparing PS (specific gravity 1.05) and PVC (specific gravity 1.4), it was observed that from summer to autumn, the majority of PS remained above the thermocline. In contrast, heavier PVC showed a tendency to decrease at the 0–10 m depth and sunk to deeper waters even before the thermocline disappeared in winter timing. Both PS and PVC showed near-zero counts at 0–10 m depth during winter. This indicates that even MPs with specific gravity similar to lake water (such as PS at 1.05) mostly sink in winter, and that the sinking speed is faster for MPs with higher specific gravity. Furthermore, although PVC (1.4) and PET (1.4) have the same specific gravity, their vertical distribution is difference. Observation revealed that PVC fragments had a larger surface area than PET fragments. This suggests that PVC fragments experience greater shape resistance than PET fragments, slowing their sedimentation. From these findings, it is concluded that rainfall, thermocline presence, specific gravity, and shape resistance are key factors determining the seasonal vertical distribution of MPs.
Water samples (1 ton each) were collected once a month from depths of 0 m, 10 m, and 30 m, at the northern basin of Lake Biwa. The samples were filtered through sieves with mesh sizes of 0.1 mm, 0.5 mm, and 1.0 mm. Algae and plankton were removed using hydrogen peroxide (following the Ministry of the Environment, Water and Air Environment Bureau, Water Environment Division: Guidelines for Microplastic Survey in Rivers and Lakes), and the MPs were stained with Nile Red. The samples were then observed under a fluorescence microscope (4x magnification) with 470 nm light irradiated from a blue LED. Fluorescence reactions are used to identify the Chemical composition of MPs in a simplified manner (based on Suparnamaaya Prasad, Andrew Bennett, Michael Triantafyllou: Characterization of Nile Red-Stained Microplastics through Fluorescence Spectroscopy) . The number of MPs was counted at the same time. Sampling period was 2023, and chemical treatment and MP observation were performed in 2024.
The result is that, the number of MPs at depths of 0–10 m increased from summer to autumn, while the number at 30 m depth increased during winter. This is believed to be caused by rainfall and the presence of a thermocline. An investigation into the rainfall in Nagahama City showed that fluctuations in rainfall aligned with changes in total MPs volume. Additionally, the seasonal trend of vertical distribution of MPs showed a strong correlation with thermocline data off the coast of the Ado River. MPs entering the lake due to summer rainfall were found to accumulate above the thermocline at approximately 15 m depth (based on 2023 vertical water temperature distribution data off Ado River), and as the thermocline disappeared in winter, the MPs were presumed to have sunk below 15 m.
Next, I analyzed focusing on the variation of Chemical composition of MPs. Comparing PS (specific gravity 1.05) and PVC (specific gravity 1.4), it was observed that from summer to autumn, the majority of PS remained above the thermocline. In contrast, heavier PVC showed a tendency to decrease at the 0–10 m depth and sunk to deeper waters even before the thermocline disappeared in winter timing. Both PS and PVC showed near-zero counts at 0–10 m depth during winter. This indicates that even MPs with specific gravity similar to lake water (such as PS at 1.05) mostly sink in winter, and that the sinking speed is faster for MPs with higher specific gravity. Furthermore, although PVC (1.4) and PET (1.4) have the same specific gravity, their vertical distribution is difference. Observation revealed that PVC fragments had a larger surface area than PET fragments. This suggests that PVC fragments experience greater shape resistance than PET fragments, slowing their sedimentation. From these findings, it is concluded that rainfall, thermocline presence, specific gravity, and shape resistance are key factors determining the seasonal vertical distribution of MPs.