Trophic states and nutrient storage of reservoirs in Chongqing

The trophic states of 35 reservoirs were investigated in the region of Chongqing, Southwest China. The results showed that the concentrations of nutrients and chlorophyll a were high and organic pollution and transparence (SD) were low in the water body. The concentrations of total nitrogen (TN) range from 0.52 mg/L to 5.94 mg/L and those of total phosphorus (TP) range from 0.002 mg/L to 0.598 mg/L. Chemical oxygen demand (COD) ranges from 0.75 mg/L to 9.3 mg/L and SD ranges from 0.48 m to 3.2 m. There was a significant positive correlation between COD and chlorophyll a, but a significant negative correlation between SD and chlorophyll a. In terms of the integrated nutrition state index, the eutrophication states of the reservoirs were assessed. Of the investigated reservoirs, about 22 reached the grade of eutrophication. Only one reservoir was in the state of oligotropher, the rest were in the state of mestropher. Integrated nutrition state indices range from 25.4 to 74.5. The storage capacity of nutrients in the reservoirs was calculated. The stored TN, TP and COD were about 4731, 206 and 10259 t, respectively. The main pollution sources are industrial and domestic wastes. With the development of aquaculture, the contamination level increases gradually.