1:45 PM - 3:15 PM
[O11-P38] Environmental changes in Lake Biwa due to global warming: An attempt to determine overturn using lake bottom data
Keywords:Turnover, Bdellocephala annandalei Ijima & Kaburaki, Lake Biwa
1. Introduction
Lake Biwa, approximately 4.2 million years old, is one of the few ancient lakes in the world. It supports a diverse ecosystem, including over 60 endemic species. Among these, the Biwa giant flatworm (Bdellocephala annandalei), Japan’s largest freshwater planarian, prefers cold water environments. This species requires strict living conditions: bottom water temperatures between 6-8°C and dissolved oxygen (DO) levels above 5%, making it highly vulnerable to environmental changes.
This study aims to clarify the impacts of global warming and insufficient overturn-often referred to as "Lake Biwa's deep breathing"-on the habitats of benthic organisms in Lake Biwa.
2. Background and Objectives
The Junior Doctor Training Program by Biwako Trust has conducted continuous monitoring at a fixed station in the northern part of Lake Biwa for the past six years to investigate the effects of global warming. In 2019-2020,
warming led to a cessation of turnover, resulting in increased bottom water temperatures and reduced bottom DO levels. Although two turnover events after 2021 temporarily restored bottom DO, the bottom water temperature
necessary for Biwa giant flatworm habitat (6-8°C) was not recovered, and population declines continued.
This research aims to assess the habitat status of the Biwa giant flatworm as a bioindicator of Lake Biwa's environmental conditions. We also attempted to identify turnover events through pattern analysis focusing on two
turnover components: "mixing" and "circulation," using bottom water temperature and DO.
3. Methods
At the northern Lake Biwa observation point (35.4°N, 136.1°E, depth ~93m), 17 monitoring sessions were conducted from March 2023 to March 2025 using biological traps and a CTD (Conductivity, Temperature, Depth) device. Biological traps, constructed from old fishing nets, were used to capture and observe benthic organisms, including Biwa giant flatworms and Annandale amphipods. At the same time, bottom water temperature and DO were measured using the CTD device.
4. Results
Biological trap surveys in July 2024 confirmed the presence of the Biwa giant flatworm for the first time in approximately three years, although the specimens appeared debilitated. Despite recent studies*7) indicating confirmed overturn for five consecutive years, our observations continued to show irregularities in overturn timing and fluctuations in bottom temperature and DO. Clear identification of overturn events became challenging, though a partial recovery in bottom DO levels was observed.
To resolve this contradiction, we attempted to classify overturn events through pattern analysis by separating the process into "mixing" and "circulation."
5. Discussion
Using the scatter plot method of bottom temperature and DO proposed by Nagata et al.*8), our analysis demonstrated the effectiveness of identifying overturn events through two distinct processes: "mixing" and "circulation". Observations from 2023 and 2024, where incomplete overturn events coincided with recovering DO levels, suggest the significant influence of the "mixing" process.
6. Conclusion and Future Challenges
Global warming-induced incomplete overturn events may deteriorate habitat conditions for the cold-water endemic Biwa giant flatworm in Lake Biwa. Our group intends to continue long-term monitoring to safeguard the biodiversity of this unique lake ecosystem.
Lake Biwa, approximately 4.2 million years old, is one of the few ancient lakes in the world. It supports a diverse ecosystem, including over 60 endemic species. Among these, the Biwa giant flatworm (Bdellocephala annandalei), Japan’s largest freshwater planarian, prefers cold water environments. This species requires strict living conditions: bottom water temperatures between 6-8°C and dissolved oxygen (DO) levels above 5%, making it highly vulnerable to environmental changes.
This study aims to clarify the impacts of global warming and insufficient overturn-often referred to as "Lake Biwa's deep breathing"-on the habitats of benthic organisms in Lake Biwa.
2. Background and Objectives
The Junior Doctor Training Program by Biwako Trust has conducted continuous monitoring at a fixed station in the northern part of Lake Biwa for the past six years to investigate the effects of global warming. In 2019-2020,
warming led to a cessation of turnover, resulting in increased bottom water temperatures and reduced bottom DO levels. Although two turnover events after 2021 temporarily restored bottom DO, the bottom water temperature
necessary for Biwa giant flatworm habitat (6-8°C) was not recovered, and population declines continued.
This research aims to assess the habitat status of the Biwa giant flatworm as a bioindicator of Lake Biwa's environmental conditions. We also attempted to identify turnover events through pattern analysis focusing on two
turnover components: "mixing" and "circulation," using bottom water temperature and DO.
3. Methods
At the northern Lake Biwa observation point (35.4°N, 136.1°E, depth ~93m), 17 monitoring sessions were conducted from March 2023 to March 2025 using biological traps and a CTD (Conductivity, Temperature, Depth) device. Biological traps, constructed from old fishing nets, were used to capture and observe benthic organisms, including Biwa giant flatworms and Annandale amphipods. At the same time, bottom water temperature and DO were measured using the CTD device.
4. Results
Biological trap surveys in July 2024 confirmed the presence of the Biwa giant flatworm for the first time in approximately three years, although the specimens appeared debilitated. Despite recent studies*7) indicating confirmed overturn for five consecutive years, our observations continued to show irregularities in overturn timing and fluctuations in bottom temperature and DO. Clear identification of overturn events became challenging, though a partial recovery in bottom DO levels was observed.
To resolve this contradiction, we attempted to classify overturn events through pattern analysis by separating the process into "mixing" and "circulation."
5. Discussion
Using the scatter plot method of bottom temperature and DO proposed by Nagata et al.*8), our analysis demonstrated the effectiveness of identifying overturn events through two distinct processes: "mixing" and "circulation". Observations from 2023 and 2024, where incomplete overturn events coincided with recovering DO levels, suggest the significant influence of the "mixing" process.
6. Conclusion and Future Challenges
Global warming-induced incomplete overturn events may deteriorate habitat conditions for the cold-water endemic Biwa giant flatworm in Lake Biwa. Our group intends to continue long-term monitoring to safeguard the biodiversity of this unique lake ecosystem.