Abstract: Surveys were conducted of duckweeds-grown water bodies scattered in industrial, residential, and farming areas in a number of cities or counties to investigate species of the duckweeds and water samples collected for analysis of extent of eutrophication of the water bodies, and further clarify relationships of the distribution of duckweeds with pH and contents of ammonium, nitrate, total nitrogen and total phosphorus in the water. Results show that duckweeds tend to flourish in calm water rather than flowing water, and in water bodies in the farming and residential areas rather than in industrial areas. In summers of Jiangsu Spirodela polyrrhiza, Lemma minor, Wolffia arrhiza and Spirodela oligorrhiza are the most common duckweeds found in water bodies, and no trace of Lemma perpusilla and Lemma trisulca was found during the survey. Spirodela polyrrhiza and Lemma minor were found in all the water bodies surveyed, with water pH varying in the range of 4.5-7.5. The results also show that duckweeds grow well in water bodies, high or low in N and P concentrations, ranging from 0 to 42,0.2 to 2.0, 0.2 to 45 and 0.02 to 13 mg·L^-1 for ammonium, nitrate, total nitrogen and total phosphorus, respectively. No evidence was found showing that any close relationship existed between duckweeds distribution and concentrations of nitrogen and phosphorus in the water. In industrial areas, there are a variety of factories, including chemical plants, clothing factories, pharmaceutical plants, mechinary accessories factories, etc., discharging a high volume of waste containing heavy metals and organic matters. In water bodies in or near the industrial areas, symbiosis of Spirodela polyrrhiza and Lemma minor and occasionally Wolffia arrhiza, too, is commonly observed. Therefore, in exploiting the effect of duckweeds removing nutrient substances, like N and P from water, priority should be given to native species of duckweeds dominant in the season and the complex symbiotic system of different species of duckweeds so as to maximize the production of duckweeds and their N and P removing rate.

Key words: duckweed, water environment, eutrophication, nitrogen, phosphorus