| With rapid social and economic development, more and more dams have been built on all kinds of rivers for electricity generation, irrigation, flood control, and so on. This brings us great economic advantage and makes a more and more notable impact on the earth's environment at the same time. Now, more and more scientists pay their attention to the effects of dam reservoirs on the biogeochemical cycle of riverine nutrients and succedent aquatic ecological problems.The Wujiang River, flowing through a karst area in Guizhou Province, is one of the main water sources of the Changjiang Three-Gorge Reservoir. According to the plan of divert electricity from West to East, there will be 11 cascade reservoirs on the mainstream of the Wujiang River. In order to understand the effect of reservoirs on the biogeochemical cycle of nutrient, we measured nutrient concentrations of the Wujiang River basin in 2002 and two cascade reservoirs (the Dongfeng and Wujiangdu Reservoirs) during 2003-2004, located on the up-mid mainstream of the Wujiang River, by the state standard method of water and wastewater analysis. The main results are presented as follows.1. The nutrient concentrations in surface water of the Wujiang River fully reflected the karstic features. The dissolved silicon (DSi) and the dissolved organic carbon (DOC) concentrations in the Wujiang River were both below the global average values. Their average concentrations were 85.45μM and 84.58μM in the high flow period and 60.80μM and 53.86μM in the low flow period, respectively. The dissolved nitrogen (DIN) concentrations in the Wujiang River basin were much higher than the global average values and the main form of nitrogen was nitrate nitrogen (NO3-N). The average concentrations of NO3-N and ammonia nitrogen (NH4-N) in the Wujiang River were 147.52μM and 11.64μM in the high flow period and 158.00μM and 47.43μM in the low flow period, respectively. Compared to the Changjiang River and its other tributaries, the Wujiang River had much higher phosphorus concentrations. This may be attributed to that there are many phosphorus mines (such as the Wengfu phosphorus mine) and corresponding factories in the Wujiang River basin. The total phosphorus (TP) concentrations were 6.43μM in the high flow period and 4.18μM in the low flow period, respectively. The particulate phosphorus (PP) accounted for 74% of TP and phosphate (PO4-P) 14% in the high flow period. They accounted for 27% and2. 49% in the low flow period, respectively. The spatial distribution of nutrients (especial phosphorus) in the Wujiang River showed that the reservoirs had a certain effect on the biogeochemical cycle of nutrients in the lower reaches.3. The thermal stratification in the water bodies of the Dongfeng and Wujiangdu4. Reservoirs appeared in Spring and Summer. But it was not strong and had no stable thermocline. The bottom water was at a serious anoxic condition in the Wujiangdu Reservoir during the thermal stratification period especially in July. But, on the other hand, the bottom water of the Dongfeng Reservoir had relatively high dissolved oxygen concentrations during the whole study period. The mean chlorophyll a concentrations in surface water of the Dongfeng and Wujiangdu Reservoirs were 2.74mg/m3 and 6.33mg/m3, respectively, which suggested that the primary productivity of the latter was higher than that of the former. The DOC concentrations in the Wujiangdu Reservoir were slightly higher than in the Dongfeng Reservoir, but they were both very low (about lmg/L). Nitrate is the main form of nitrogen in the two reservoirs, which accounts for about 85% of total nitrogen (TN). The main form of phosphorus in the Dongfeng Reservoir is PP, which accounts for 62% of TP, during the whole study period. However, it was PO4-P in the Wujiangdu Reservoir, which accounts for above 80%, except PP in the upper layer water in the high-flow period. The mean TN concentrations of surface water of the Dongfeng and Wujiangdu Reservoirs were similar, 2.97mg/L and 2.82mg/L, respectively. And the mean TP concentrations were O.Olmg/L and 0.05mg/L, respectively, obviously the latter was higher than the former. The mean DSi concentrations were 3.78mg/L and 1.86mg/L, respectively, the former is obviously higher than the latter. The two reservoirs are both in the mesotrophic state and the trophic degree of the Wujiangdu Reservoir is higher than that of the Dongfeng Reservoir. According to the Justic's nutrient limit element rules, the two reservoirs are both of phosphorus limitation.5. The organic carbon contents of Dongfeng Reservoir sediments keep constant with sediment depth, with a mean value of 1.84%. While in the sediments of the Wujiangdu Reservoir, firstly, it dereased gradually and then keep constant with sediment depth. Its mean content is 2.69%, which is higher than in the Dongfeng Reservoir. The mean ratio of C/N in organic matter in sediments of the Dongfeng and Wujiangdu Reservoirs are 13 and 12, respectively. This suggested that the latter had more phytoplankton-derived organic matter than the former. Thevertical profiles of DOC, NH4-N and DSi are similar in porewaters of the Dongfeng and Wujiangdu Reservoirs. They all increased gradually at first and then remained constant with depth, except that the DSi concentration in the Wujiangdu Reservoir decreased gradually.. The vertical profiles of NO3-N, NO2-N and PO4-P in porewaters of the two reservoirs are similar, too. Their concentrations decreased gradually at first and then kept constant with depth, with the peaks at the sediment-water interface. Significant seasonal variations of nutrient concentrations in porewaters were observed, with the concentrations in summer and spring generally higher than in winter. We also calculated the nutrient fluxes at the sediment-water interface. The results showed that the sediments were the source of DOC, DSi, PO4-P, NH4-N and NO2-N and the sink of NO3-N in the overlying water. The seasonal variation of nutrient fluxes was observed in the two reservoirs. The NH4-N and DSi fluxes increased gradually from winter to spring and to summer. And the maximum PO4-P and NO2-N fluxes appeared in winter and the minimum in summer. The diffusion effect of NH4-N and DOC was largest, followed by that of PO4-P. Except NO2-N, the nutrient fluxes and their effects on the overlying water in the Wujiangdu Reservoir are higher than in the Dongfeng Reservoir.6. Using one box stabilization model, the input, output and net fluxes of nutrients in the two reservoirs were estimated. The net TN, TP, DSi and DOC fluxes in the Dongfeng Reservoir are -4844.4 ton.y"1, -178.3 ton.y"1, -11453.4 ton.y"1 and -1575.7 ton.y1, respectively, accounting for 13.7% of the input flux of TN, 54.7% of TP, 22.8% of DSi, and 20.0% of DOC. While in the Wujiangdu Reservoir, the net TN, TP, DSi and DOC fluxes are -182.9 ton.y"1, -276.0 ton.y"1 > -6560.6 ton.y"1 and -1567.9 ton.y"1, respectively, accounting for 0.4% of the input flux of TN, 49.5% of TP, 10.7% of DSi, and 12.4% of DOC. Both the two reservoirs are believed to be the sink of the nutrients except that the Wujiangdu Reservoir is believed to be the source of TP and PO4-P. The two reservoirs have a stronger influence on the transport of phosphorus, silicon and carbon and little influence on the transport of nitrogen.7. Compared to the Dongfeng Reservoir, the trophic degree of the Wujiangdu Reservoir is higher and the nutrient regeneration is more intense at the sediment-water interface of the Wujiangdu Reservoir, which offsets the retentionto a certain extent. So, the nutrient retention in the Wujiangdu Reservoir is lower than in the Dongfeng Reservoir. In respect of the influence on the downstream, the Wujiangdu Reservoir has a more serious effect on either nutrients or 8. phytoplanktons in the downstream than in the Dongfeng Reservoir. This reflected that the run time of the reservoir is longer the effects of the reservoir on the biogeochemical cycles of riverine nutrients may be stronger.
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