istorically the pH level of a lake or wetland was determined by the geology of the catchment area in which it is located. In recent years, however, human activity has also become a factor. Some chemical substances can have an extraordinary effect, as the following example illustrates.

Table 1. Examples of lakes with different pH levels

The pH of most lakes is in the range of 6-9 and varies with carbon dioxide levels. When photosynthesis by plant plankton in lakes and wetlands is highly active, CO₂ consumption by the plant plankton renders lakes alkaline; when little photosynthesis occurs, respiration by plants, animals and microbes raises lakes' acidity. The pH of lakes is therefore sensitive to the availability of sunlight, varying quickly with the weather, time of day, season and place. In eutrophic lakes, where photosynthetic production by plant plankton is high, pH distribution varies in a vertical direction: the upper waters are relatively alkaline, while the lower depths, where lack of sunlight reduces photosynthesis, are more acidic. Figure 1 shows the results of measurement by the U-20 in Lake Biwa, indicating acidity in the deep reaches of the lake.