The Dynamics And Budget Of Nitrogen And Phosphorus In Reservoir Liuxihe

Liuxihe Reservoir,located at the Tropic of Cancer,is a typical large valley-type reservoir. This master thesis studied the nutrient dynamics and distribution last from 2000 to 2008 and estimated the annual nutrient budget including input by dryfall and wetfall,inflow,outflow, internal release and sedimentation.The predictive model by OECD and Vollenweider was applied to estimate phosphorus concentration in the reservoir.The hydrodynamics of Liuxihe Reservoir is drived precipitation with moonson.The annual precipitation is on average 2000 mm,most of which occurred in the summer,a typical flooding seasons in tropics.Main inflows to the reservoir come from two feeding streams,and the inflow dynamics is consistent with coming flooding season.The mean water residence time was about 170 days with a great variation between flooding season and dry season. Water residence time in the flooding season was only one-quarter of that in the dry season. The thermal stratification was monomictic,a short mixing happened in winter,which promotes the nutrient flux from surface sediment.The mean concentrations of TN,TP,Chlorophyll a and SD were 0.6 mg/L,0.02 mg/L, 1.93 mg/m³ and 2.9 m,respectively,indicating the reservoir was oligo-mesotrophic.However, there is a tendency of eutrophication through last nine years observation.Mass ratio of TN/TP and DIN/DIP were 30:1 and 78:1,respectively,the high ratios mean that growth of phytoplankton is surely limited by phosphorus.The high N/P ratio was attributed to red soil which high iron content combines tightly phosphorus,and well-vegetated watershed and low human population activity.The nutrients as well as other water quality variables show a significant temporal and spatial variation.In the early phase of the flooding season(April to May),nutrients and Chlorophyll a concentration was remarkedly higher than the other periods,and it decreases from the riverine zone to lacustrine zone as:riverine zone＞transition zone＞lacustrine zone, with a converse tendency for water transparency.The nutrient variability in these three zones shows a typically longitudual gradient.The longitudinal pattern of nutrients in the Liuxihe Reservoir was mainly due to the precipitation with monsoon and morphology of the water basin. Sedimentation was mostly affected by hydrodynamic factors such as inflow.The sedimentation rate in the transition zone is in consistency with the inflows.Sedimentation in the lacustrine zone is hardly affected by allochthonous particles from the inflow but by autochthonous particles.Phosphorous concentration in sediment indicates the information and sediment character.This reservoir has a certain phosphorus releasing rate,for the mostly anaerobic conditions of the sediments and high sub-tropical water temperature.Entogenous release to the overlying water is an important part of nutrients loading to the water body.In 2008,the total input of NO₃-N(382456kg),TN(792177kg),PO₄-P(3780kg),TP (318645kg),using the formula inputs-outputs=change in storage,retention rates are NO₃-N (3.2%),TN(32.1%),PO₄-P(-62.9%),TP(11%).Retention rates of TN and TP are much higher than the ion nutrients,and PO₄-P has high negative storage,which indicates a high utilization rate by the plankton in the oligotrophic water body.OECD and Vollenweider's models are applied to predict total phosphours concentration of the reservoir.The results showed that predictive models were suitable for Reservoir Liuxihe which was located between tropics and subtropics,although the models reserach was based on temperate waterbody.