The Influence Of Hydrodynamics On The Exchange Of Nutrients In Estuary Of Tributaries In Three Gorges Reservoir

From August2012to July2013, intensive monthly sampling of hydrodynamics and nutrients was conducted at the mouth of three tributaries (Caotang River, Meixi River and Zhuyi River). Water temperature, turbidity and Chlorophyll a were also measured during this period. The main results are as follows:In most mouths, a clear stratification had been formed at the horizontal and vertical profile of water column in estuary of tributaries in Three Gorges Reservoir. Significant differences were shown between the three tributaries. In the Caotang estuary, the average velocity of the water column input from Changjiang was0.083m/s, and the average velocity of the water column output from Caotang bay was0.095m/s. The annual mean water discharge input from Changjiang reached730.55m3/s. In the Meixi estuary, the average velocity of the water column input from Changjiang and output from Meixi bay reached0.079m/s and0.079m/s, respectively. The annual mean water discharge input from Changjiang was428.62m3/s. In the Zhuyi estuary, the average velocity of the water column input from Changjiang and output from Zhuyi bay reached0.072m/s and0.07m/s, respectively. The annual mean water discharge input from Changjiang reached426.75m3/s.The concentrations of nutrients at the surface of Changjiang showed significantly higher than those in Caotang, Meixi and Zhuyi bay. The annual average values of DIN, DSi and DIP concentrations were about129.3μmol/L,119.5μmol/L and3.42μmol/L respectively. Most nutrients, especially DIP, displayed peak values when the highest or lowest Changjiang monthly discharges occurred, suggesting the Changjiang discharges strongly influenced the seasonal variations of these chemicals. In the estuary of tributaries, DIN and DIP showed higher values (DIN about135-175μmol/L, DIP about5-6μmol/L) in February, March, April, and May, whereas DSi (about120-150μmol/L) showed higher values in other months.In Caotang, Meixi and Zhuyi bay, there was significant difference in water temperature (23-24℃) between surface and bottom, and the turbidity (20-40NTU) decreased slowly from the estuary to the upper reach of three tributaries in September and October. The Chlorophyll a concentrations of Meixi bay were about5-6μg/L, higher than Caotang and Zhuyi bay. In November and December, the average values of DIN, DSi and DIP concentrations were about110μmol/L,135μmol/L and2.6μmol/L respectively. For DIP, the highest monthly value (about5-6μmol/L) occurred in March, April and May. During this period, the concentrations of DIN and DIP increased sharply, whereas the concentrations of DSi decreased sharply (about80-90μmol/L). In the flood season (June, July and August), the turbidity and most nutrients showed significantly higher concentrations at surface than those at bottom. Chlorophyll a displayed the highest value (ranged from9μg/L to12μg/L) in June.During the year sampled, the water exchange of the estuary of three tributaries in flood season was stronger than that in the other mouths. But the annual discharge fluxes of nutrients from the Changjiang to the Caotang, Meixi and Zhuyi bay were small. The high concentrations of nutrients in the tributaries bay were the result of the Changjiang mainstream input.