Temporal And Spatial Variation Of Nutrients And Chlorophyll A, And Their Relationship In Pengxi River Backwater Area, Three Gorges Reservoir

Temporal and spatial distribution of nutrients has become the attentive focus that researchers concerned and the key to control eutrophication. Main stream water intruding into tributary submerges amounts of cropland and forms a broad backwater area in tributary bay, which leads tributary become lake. The operation mode of Three Gorges Reservoir(TGR) can be divided into high water level period, water release period, low water level period and storage period. The submerged cropland is not only sink of nutrition but also a source of nutrition in dispatching background, which plays a role in supplying nutrients for water. As the largest tributary, nutrients concentration is influenced by runoff, mainstream, sediment and so on in Pengxi river. In order to explore temporal and spatial variation of nutrients and chlorophyll a and their relationship in the Three Gorges Reservoir, nutrients and chlorophyll a were monitored chronically and frequently in Gaoyang Lake in Pengxi River in the Three Gorges Reservoir from May 2013 to June 2014, and some major conclusions were showed as follows:(1) Nutrients concentration was higher in winter than other season in Gaoyang lake, but the water quality was better and it was mesotrophic because of low temperature. In late Spring and early summer, it’s water body was a degree of moderate eutrophication and reached severe eutrophication on 10 May. Temporal trend of eutrophic state index was consistent with chlorophyll a. However, nutrients concentration has big difference in different water level. NH4+-N was lowest in high water level and highest during algal bloom and it source of nitrogen cycle in the water; NO3--N and TN were highest in My 2014 and lowest when TGR keeping low level and they come mainly from mainstream. Nitrate N and dissolved phosphorus were the major composition of TN and TP, respectively, during high water level period in Gaoyang Lake with nitrate N accounted for 71.4%-95.4% of TN and dissolved phosphorus 42.7%-94.4% of TP, showing that the backwater from the mainstream of Yangtze is the main source of nitrate N and dissolved phosphorus in Gaoyang Lake. On the other hand, runoff was another source.(2) Thermal stratification in GaoYang lake was affected by variation of temperature and water level, it formed in Spring (from 2 March to 10 May,2014). The water temperature delamination began to appear in March, afterwards the water temperature difference increased gradually, it formedstable layered structure in April. The stratification structure was damaged by water disturbance that TGR water release induced in May, and the thermocline disappeared gradually. The depth of the mixing layer deepened, that was to say, the depth was from 0.32m on 10 May to 8.5m on 29 May,2014. The water temperature delamination was the most obvious in midsummer, the water temperature difference between surface and bottom was up to 10.4℃. The water temperature delamination disappeared gradually in late summer, and there was no delamination in Winter.(3) The water temperature delamination was the main factordetermined the vertical distribution of the nutrients and Chlorophyll a. Under the conditions that the water temperature was no delamination, the content of the nutrients and Chlorophyll a were both no significant differences among every layers, and the Chlorophyll a concentration was less than 3μg.L-1. With the occurrence and development of the water temperature delaminationin spring, the content of the Chlorophyll a in the surface increased from 3μg.L-1 up to 183.73μg.L-1in 69 days during breaking out water bloom. Subsequently, the structure of the water temperature delamination was damaged, and the content of the Chlorophyll a decreased. It indicated that the vertical distribution of the nutrients was related tothe water temperature delamination. Under the conditions that the water temperature was no delamination, the content of the nutrients had no difference among every layers. However, the content of the nutrients showed relatively large difference among every layers after the water temperature delamination.(4) The water quality of the most sections in Pengxi River was up to mild eutrophic water, and even to severely eutrophic water in several sections. The spatial difference of the water eutrophication was obviousbeweenprewater release and post. The water quality of the upper sections was bad in the initial stage of water release, and the TLI showed a increase trend from lower reach to upper reach. Nevertheless, the water quality of middle and lower reaches was bad, and the changing trend of the TLI showed a parabolictendency along the river. It indicated that algal bloom had serious influence on the change of the water quality, and the change of the water quality along the river was consistent to the change of the Chlorophyll a concentration.(5) The change of the Chlorophyll a concentration could accurately reflect the position of algal bloom and its severity. The peak of Chlorophyll a was present to the segment 6 and 7,where algal bloom were most serious, in the initial stage of water release. But the peak of Chlorophyll a was present to the segment 2 and 4 after water release. The region of algal bloom showed a migratory tendency towards to the downstream with decreasing water level.(6) As a whole, the nutrients content of the yangtze river mainstream in Three-Gorgearea was higher than Pengxi River, a tributary of Yangtze River, between pre water release and post. The nutrients content of Pengxi River showed a downward trend from lower reach to upper reach. The nitrate N concentration was more than 2mg.L-1 in the segment close to the river, and it was less than 0.1mg.L-1 in the upper reach. The DTP was up to 0.14mg·L-1 at the Pengxi River estuary, and it was less than 0.06mg·L-1 in the upper reach. This study showed that the inputs of the trunk stream might be the main source of the nutrients in Pengxi River. On vertical sense of the nutrients content, it was the highest in the middle layer of the segment close to the river in April, especially nitrate N and DTP and so on. This research explained that water in the yangtze river mainstream flowed from its middle layer to Pengxi River. Nevertheless, the nutrients content of the middle and lower layers was higher than the upper layer in July, it showed that water in the yangtze river mainstream flowed from its middle and lower layers to Pengxi River.(7) There were 41 phospholipid fatty acids(PLFA) in the sediments of Pengxi River in April, when it was the initial stage of water release.38 PLFAs were bacterium, 2 PLFAs were actinomycetes and 1 PLFA was fungi. While there were 35 PLFAs in July, when it had finished water release.32 PLFAs were bacterium, the actinomycetes and fungi were the same as in April. Overall microbe species were no obvious change between pre water release and post(P＞0.5). However, the total content of PLFAs in other segments increased significantly after water release compared with prewater release except for PX-2、PX-3、PX-7, where the velocity and sediments had little influence on the total content of PLFAs. The content of bacterialcharacteristic PLFAs had increased evidently in all segments after water release. The dominant bacterium are both anaerobic bacteria between pre water release and post, but the number of aerobic bacteria exhibited an increase after water release. This research indicated that the water level change had effect on the biomass of microorganism and the structural composition of bacterium.