Turbidity is used to place a numerical value on the cloudiness of a body of water. Water that is obviously cloudy has a turbidity of 100NTU or more, whereas clear water has a turbidity of around 10NTU. High turbidity in lakes and wetlands may be caused by inorganic substances, or by organic substances such as plankton.

Bodies of water in which plankton flourish at abnormally high levels are called eutrophic. Eutrophication occurs in a large number of lakes where municipal water and industrial effluent are dumped. The recent problem of "water bloom" (in which the surface of the lake water is covered in a layer of green) points to the ruination of entire lake ecosystems. This phenomenon is caused by abnormally large blooms of [microcystis] and [anabena] from the proliferation of blue-green algae. In a similar phenomenon called "red tide," an excess of [perigenium], a vortex flagellate algae, and [uroglena], a green- yellow vortex flagellate algae turn the surface of the water a reddish-brown color. The volume of plant plankton varies according to environmental factors such as light, temperature and availability of nutrients. Because the light does not reach the underlying water layer, however, almost no photosynthesis occurs. In the results of turbidity measurement in Lake Biwa shown in Figure 4, high turbidity is seen from the upper layer to the thermocline. This indicates the existence of some sort of relationship between turbidity and chlorophyll levels.