Effects Of The Three Gorges Dam On The Biogeochemistry Of The Plume Front In The Changjiang Estuary

The ecological effects of dam on estuarine and coastal waters are more and moreconcerned, which is the current hotspot of the marine ecological and environmentalresearch.The plume front of estuary is a high productivity zone, which plays a crucialrole in biogeochemical cycle in estuarine and coastal waters. In this thesis, theChangjiang estuary and adjacent coastal waters was chosen as the research object.Based on the data of field surveys before and after closure of the Three Gorges Dam,the effects of the Three Gorges Dam on the ecology and environment in theChangjiang Estuary and its adjacent coastal waters are studied, especially thebiogeochemical characteristics of the plume front. The main results are as follows:1. The suspended sediment front of the plume front zone in Changjiang Estuary shiftsto low salinity direction behind the Three Gorges DamThe concentration of suspended sediment in the plume front zone varies in alarge range, from the high concentration of suspended sediment basically reduces toabout20mg/L or less concentration, froms the suspended sediment front. Thesuspended sediment fronts in spring and summer are most obvious. In spring thesuspended sediment front is in around23~31salinity range in2002,23~30salinityrange in2007,22~28salinity range in2011,respectively.In summer the suspendedsediment front is in around20~25salinity range in2002,17~22salinity range in2006,15~20salinity in2010, respectively. In autumn the suspended sediment front isin around33salinity in2002,30salinity in2007,30salinity in2010,respectively.Thesuspended sediment front of the plume front zone shifts to low salinity directionbehind the Three Gorges Dam.2. The chlorophyll peak of the plume front zone in Changjiang Estuary shifts to lowsalinity direction behind the Three Gorges Dam The high chlorophyll concentration is basically in low suspended sediment-filledarea, the chlorophyll peak forms in20mg/L sediment concentration or less. Thechlorophyll peaks in spring and summer are most obvious. In spring the chlorophyllpeak is in around30~33salinity range in2002,29~32salinity range in2007,27~32salinity range in2011, respectively. In summer the chlorophyll peak is in around26~29salinity range in1986,1988and2002,around21salinity in2006,18salinity in2010, respectively. After the Three Gorges Dam the chlorophyll peak shifts to lowsalinity direction.3. The nutients front of the plume front zone in Changjiang Estuary shifts to lowsalinity direction and the distribution and formation around the nutients front changesgreatly behind the Three Gorges DamThe nutients front of phosphate is most obvious. The location of nutients front isbasically coincidence with the chlorophyll peak, showing that the nutrients front iscontroled by biological factor. The nutrients fronts in spring and summer are mostobvious. In spring the nutrients front is in around31~33salinity range in2002,29~32salinity range in2007,27~31salinity range in2011, respectively. In summer thenutrients front is in around20~27salinity range in2002,19~28salinity range in2006,18~24salinity range in2010, respectively. After the Three Gorges Dam the nutrientsfront shifts to low salinity direction, causes the concentrations of nutrients increasedaround the nutrient front.The results indicates that N/P ratio and Si/P ratio seriously deviate from Redfieldratio in the plume front zone of the Changjiang Estuary and its adjacent coastal waters,N/P ratio incereases, Si/N ratio decreases, especially in spring and summer. Based onthe survey of2010~2011behind the Three Gorges Dam, we get as follows: in springand summer the N/P ratio is both more than30, but the Si/N is less than0.5and0.7respectively around the nutients front.In the plume fornt zone of Changjiang Estuarythe inorganic nitrogen is excess, and the silicate and phosphate is relatively less.