Water temperature near the surface of a lake increases with increasing air temperature, which results in stratification. The strength of stratification substantially influences the transport of water parcels from the surface to the bottom of a lake. In recent years, the stratification in Lake Biwa—the largest freshwater lake in Japan—has been stronger. However, it is difficult to reproduce the stratification well in the simulations. In the present study, we built a hydrodynamic model for the purpose of analyzing the structure of the stratification in detail. Using the model, we evaluated the reproducibility of the seasonal and annual changes of vertical water distribution and flow field in Lake Biwa from 2007 to 2011. The hydrodynamic model results show that the vertical water distribution approximately agrees with the field observations based on the statistical analysis. The seasonal change of thermal stratification is reasonably reproduced by the hydrodynamic model simulations. In the simulation, there are mainly two circulation flows at the surface layer of the lake. The first flows anticlockwise and the second flows clockwise in the northern part of Lake Biwa. In order to compensate for the surface water flow, the water under the thermocline sometimes flows in the opposite direction under each circulation flow.

This study examines the potential impacts of climate change on Lake Biwa, Japan’s largest freshwater lake, with a focus on temperature, wind speed, and precipitation variations. Leveraging data from the IPCC Sixth Assessment Report, including CCP scenarios, projecting a significant temperature rise of 3.3–5.7 °C in the case of very high GHG emission power, the research investigates how these shifts may influence dissolved oxygen levels in Lake Biwa. Through a one-dimensional model incorporating sediment redox reactions, various scenarios where air temperature and wind speed are changed are simulated. It is revealed that a 5 °C increase in air temperature leads to decreasing 1–2 mg/L of dissolved oxygen concentrations from the surface layer to the bottom layer, while a decrease in air temperature tends to elevate 1–3 mg/L of oxygen levels. Moreover, doubling wind speed enhances surface layer oxygen but diminishes it in deeper layers due to increased mixing. Seasonal variations in wind effects are noted, with significant surface layer oxygen increases from 0.4 to 0.8 mg/L during summer to autumn, increases from 0.4 to 0.8 mg/L in autumn to winter due to intensified vertical mixing. This phenomenon impacts the lake’s oxygen cycle year-round. In contrast, precipitation changes show limited impact on oxygen levels, suggesting minor influence compared to other meteorological factors. The study suggests the necessity of comprehensive three-dimensional models that account for lake-specific and geographical factors for accurate predictions of future water conditions. A holistic approach integrating nutrient levels, water temperature, and river inflow is deemed essential for sustainable management of Lake Biwa’s water resources, particularly in addressing precipitation variations.

Climate change-induced extreme weather events, including prolonged droughts and intense rainfall, exert a significant influence on river inflows. These inflows act as vital conduits for nutrient transport, water quality modulation, and the regulation of thermal dynamics in lakes and oceans. In this context, this study conducts a comprehensive examination of the multifaceted effects stemming from river water characteristics, snowmelt water influence, and shifts in precipitation patterns on the stratification dynamics of Lake Biwa in Japan. To facilitate these investigations, a hydrodynamic model was developed to simulate thermal stratification in Lake Biwa. The results demonstrate that an increase in precipitation and river water flow, specifically doubling these factors, leads to noticeable cooling of the lake’s surface layer and a consequent destabilization of stratification during the stratification period. Conversely, halving these factors stabilizes stratification. Furthermore, elevating river water temperature by 5 °C raises water temperature near the upper thermocline, encouraging vertical mixing within the surface layer. Conversely, a 5 °C decrease induces significant temperature fluctuations and an unstable stratification extending from the surface to deeper layers. Notably, the spatial variance in water temperature within Lake Biwa is profoundly influenced by fluctuations in river water temperature. This study underscores the critical importance of considering river plumes in the study of material circulation, stratification dynamics, and ecological well-being in lakes and oceans. Given the mounting concerns related to eutrophication and the prevalence of anoxia in aquatic ecosystems, this research provides invaluable insights into assessing the impacts of river plumes on Lake Biwa’s stratification structure and seasonal dynamics.

Hypoxia in Lake Biwa, Japan remains a serious water environmental problem. One of the causes of hypoxia in the lake is the formation of a thermocline, which is largely affected by meteorological factors, such as (1) air temperature, (2) wind speed, and (3) precipitation. However, the effects of these three meteorological factors on the formation of the thermocline have not been clarified quantitatively. In this study, applying a three-dimensional hydrodynamic model to Lake Biwa, the effects of each of the three meteorological elements on the formation of the thermocline was quantitatively analyzed to clarify the governing factors of meteorological conditions in the formation of anoxic oxygen. Sensitivity analysis of the stratification structure in Lake Biwa was performed by changing the three meteorological factors of (1) air temperature, (2) wind speed, and (3) precipitation. As a result, the change in wind speed gives the greatest effect on the stratification structure, the change in air temperature makes the difference in the stratification structure from the surface layer to the vicinity of the thermocline, and the change in precipitation affects it less than the others.

In enclosed water bodies, water quality deterioration has emerged as a critical environmental issue. Eutrophication contributes to phenomena such as red tides and blue tides, raising concerns about foul odors and adverse impacts on surrounding aquatic ecosystems. Despite efforts to reduce nutrient loading through water quality management measures, reports of stagnant or a worsening water quality persist. One key factor is the accumulation of nutrients in deep layers. Nutrient-rich fluids form density currents along the lakebed, transporting nutrients and organic matter to deeper regions. This study investigates the hydrodynamic properties of a nutrient release from the lakebed in Lake Biwa using hydrodynamic and ecosystem models. The results reveal that a nutrient release triggers plume formation along sloping lakebed surfaces, facilitating the transport of nutrients and dissolved organic matter. Additionally, water circulation driven by density currents and nutrient concentrations along the slopes generate compensatory flows, leading to dynamic variability in Lake Biwa’s hydrodynamics.