Qingdao beach after an invasion of sea lettuce.

Many species of sea lettuce are a food source for humans in many countries. However, rotting leaves produce large quantities of hydrogen sulphide, a toxic gas. According to environmentalists, one of the main causes of this phenomenon is the excessive use of nitrates used by farmers to feed pigs. Algae use nitrates as a source of food. China is incidentally the biggest pork producer in the world (China almost produces more pigs than the rest of the world combined).

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China water pollution images, water pollution in China photos