Determination Of Nutrient Limitation In Tributaries Of Three Gorges Reservoir Area

Water eutrophication is an increasingly serious global environmental problem,which seriously restricts people’s production and life.The key to control eutrophication of water body is to find its limiting factor,and to realize the effective treatment of eutrophication through the accurate regulation of limiting factor.At present,it is generally believed that the excessive input of nitrogen and phosphorus nutrients is the root cause of water eutrophication,and according to many studies,due to the influence of local natural environment,eutrophication phenomenon has temporal and spatial differences,which should be explored in different regions and at different times.Under the influence of the operation of the three Gorges Dam,the hydrodynamic conditions in the three Gorges Reservoir area are complex and have a great impact on the eutrophication of tributaries.Therefore,it is very important to explore the response of eutrophication limiting factors under the hydrodynamic alternation in the three Gorges Reservoir area.In this study,the annual assessment of nutrients concentration,hydrodynamic alternation,and the eutrophication status was performed monthly throughout 2018 in the Yulin River,a tributary of the Three Gorges Reservoir.The relationship between the phytoplankton growth and eutrophication status as well as environmental parameters was analyzed.Thereafter,the determining of nutrient limitation for phytoplankton growth,which may provide information for the control of harmful algal blooms,were carried out in estuary of the Yulin River.Based on the in situ experiment,a laboratory scale studied was used to describe the role of turbulence in internal phosphorus release.The main conclusions are as follows:(1)Overall,the Yulin River maintained moderate eutrophical state in 2018,the level of eutrophication during the discharge period is slightly higher than that during the storage period,and in the transition period of storage and discharge,both the comprehensive nutrient status index and phytoplankton biomass(Chl-a)decreased,the correlation analysis showed that the concentration of chlorophyll a was significantly negatively correlated with the velocity of flow and the volume of flow,and the flow field diagram of the monitored section also showed that the hydrodynamic co nditions during this period were complex.It is shows that the sophisticated hydrodynamic condition can depressed the growth of phytoplankton and consequently alleviated eutrophical state.(2)The growth of phytoplankton was not significantly promoted by adding N or P nutrients alone in the experiment.The biomass of phytoplankton was 1.8 times and 2.3 times of the initial value,respectively.The maximum biomass of phytoplankton was 5.7 times and 8.2 times of the initial value,respectively.Experimental results show that the eutrophical of Yulin river is limited by both solube nitrogen and phosphorus concentrations,and phosphorus is the main limiting factor.Only when the nitrate concentration is reduced to below 0.95 mg·L-1 and phosphate concentration is below 0.22 mg·L-1,it is expected to control the growth rate of phytoplankton.(3)Hydrodynamic conditions promote the internal phosphorus release process of sediment overlying water,but there are a series of phosphate buffer mechanisms for its immobilization,which will not aggravate the phosphate load in the overlying water.The main mechanisms include: the binding of iron oxides and phosphates with the participation of dissolved oxygen,the absorption and utilization of phosphate by aquatic organisms(phytoplankton,microorganisms,etc.),the adsorption of phosphate by resuspended sediments(SS),etc.,and the various mechanisms play different roles under different turbulence intensity.If the turbulence intensity is high,the adsorption of phosphate by resuspended particles(SS)is the main removal mechanism,and when the turbulence intensity is low,The combination of iron oxide and phosphate and biological growth and absorption with the participation of dissolved oxygen are the main removal mechanisms.Therefore,under hydrodynamic conditions,the internal phosphorus release in sediments will not increase the phosphate load in overlying water.