Chinese scientists claimed that they have discovered the cause of seasonal water quality deterioration in lakes.

Seasonal deterioration of water quality in lakes is an inevitable result of the coupling of the effect of oxidation of organisms deposited at the bottom of the lake and pollution caused by human beings, Wan Guojiang, a research fellow with State Key Lab for Environment and Geochemistry of the Chinese Academy of Sciences, announced in Guiyang on Sunday.

Many countries, including China, have reported sudden serious deterioration of water quality and the death of a large number of fish in a couple of days since the 1970s. In China, such cases have been reported in Beijing, Guizhou, Guangdong and Hubei.

Wan and his research team started probing lake water environments in the 1970s. They focused on the relationship between deposits at the bottom of the lake and water in the 1980s and they furthered their research on the oxidation of deposits and its effect on water quality in the 1990s. They named the sudden deterioration of water quality "black tide".

Wan and his colleagues studied water samples taken from the lakes of many countries around the globe and cooperated with foreign research agencies to conduct related research.

Wan said black tides often occur in lakes during autumn.

Research shows that during autumn, organic matter at the bottom of lakes begins breaking up under the effect of microorganisms, causing oxygen deficiency at the bottom of the lake, a drop of PH levels, and an increase in the concentration of nitrous acid radicals. Vicious circles intensify oxygen deficiency in water and the spread of sulphide turns water black and fetid, Wan said.

However, water quality turns for the better in two or three months, with the balance of oxygen consumption and the recovery in the water and the transportation of water currents, said Wan.

Wan said during the course of water quality changes, human activities such as discharge of industrial and domestic waste can cause the formation of black tides.