A temperate deep lake, Lake Kuttara, Hokkaido, Japan (148 m deep at maximum) was completely frozen every winter in the 20th century. However, unfrozen conditions of the lake over winter occurred four times in the 21st century, which is probably due to global warming. In order to understand how thermal regime of the lake responds to climate change, its heat storage change was calculated by estimating heat budget of the lake and monitoring water temperature at the deepest point for September 2012–June 2016. As a result, temporal change of the heat storage from the heat budget was very consistent with that from the direct temperature measurement (determination coefficient R2 = 0.827). The 1978–2017 data at a meteorological station near Kuttara indicated that there are significant (less than 5% level) long-term trends for air temperature (0.024 °C/yr) and wind speed (−0.010 m/s/yr). A sensitivity analysis for the heat storage from the heat budget estimate and an estimate of return periods for mean air temperature in mid-winter allow us to conclude that the lake could be unfrozen once per about two year in a decade.
A deep temperate lake, Lake Kuttara, Hokkaido, Japan (148 m deep at maximum) was completely ice-covered every winter in the 20th century. However, ice-free conditions of the lake over winter occurred three times in the 21st century, which is probably due to global warming. In order to understand how thermal regime of the lake responds to climate change, a change in lake mean water temperature from the heat storage change was calculated by integrating observed water temperature over water depths and by numerical calculation of heat budget components based on hydrometeorological data. As a result, a temporal variation of lake mean water temperature from the heat budget calculation was very reasonable to that from the observed water temperature (determination coefficient R2 = 0.969). The lowest lake mean temperature for non-freeze was then evaluated at −1.87 °C, referring to the zero level at 6.80 °C. The 1978–2017 data at a meteorological station near Kuttara indicated that there are significant (less than 5% level) long-term trends for air temperature (+0.024 °C/year) and wind speed (−0.010 m/s/year). In order to evaluate the effects of climate change on freeze-up patterns, a sensitivity analysis was carried out for the calculated lake mean water temperature. It is noted that, after two decades, the lake could be ice-free once per every two years.
Lake Tonle Sap, the largest lake in Southeast Asia, has been closely associated with the lives and culture of Cambodian people and society. A great amount of aquatic resources arising from its high biodiversity has supported them since the Khmer Dynastic Time to the present. However, because problems of environmental pollution, and atmospheric pollution in particular, have become worse within a short space of time in Cambodia due to rapid development of tourism of the Angkor Monument Complex for instance, its harmful effect to the natural environment has been of recent concern. Rapid deterioration of the natural environment will probably trigger a certain change of the lake ecosystem, and the change might lead to a fatal damage to the plentiful aquatic resources. The present article describes the preliminary results of the research missions Evaluation of Mechanisms Sustaining the Biodiversity in Lake Tonle Sap from 2003 to 2005. On the basis of these results loss and damage of the natural environment and biodiversity of the lake are estimated, along with the progress of atmospheric pollution.
Lake Tonle Sap has the largest surface area of any lake in Southeast Asia. The lake's water level varied by 8 m between seasons in 2005, which resulted in dramatic seasonal changes in the surface area. The quality of the lake water can be divided into two contrasting types in the low- and high-water-level periods. Measurements of water quality were made during the low-water-level period. Na-HCO3 type water was a characteristic feature of the water quality in the foreshore areas from March to May of 2005. Of particular interest during the low-water-level period is the ratio of chloride ions to total dissolved solids. Changes in the quality of the lake water during low-water-level periods are caused in part by an increase in the influence of discharge from inflowing tributaries as the volume of lake water decreases. In addition, seasonal changes are caused by anthropogenic contamination from mobile villages of floating and folding houses situated around the lake margin. In contrast to the low-water-level period, the dominant composition during the high-water-level period was Ca-HCO3 type water. The water quality of the lake during the rainy season does not appear to be affected by human activity but is significantly affected by reverse inflow from the Mekong River to Lake Tonle Sap.
