Research On The Spatio-teraporal Distribution Of The Concentrations Of Nitrogen And Phosphorus And Exogenous Fluxes In Erhai Lake

Objective:There are 2565 square kilometers in Erhai basin, where has 4000 years civilization history. Erhai Lake, the second largest lake on the Yunnan province, southwest china, is located in Dali Prefecture between 25°35'N to 25°58'N, and100°05'E to100°07'E. Erhai Lake is a typical rift lake, situated in the Dali basin, which developed along the Yenjiang-Honghe great fault, and has been in existence since the late Pliocene. Erhai Lake is centre of Can Shang Mountain and Erhai Lake national nature reserve in china. Erhai Lake provides drinking water resource, also is basic of social and economical development in Dali basin, and is called mother Lake. The concentrations of nitrogen and phosphorus in Erhai at the key period are changing from low level to eutrophication to Erhai now. Eutrophication lead to the water quality lower, which deteriorated sharply, and the eutrophication levels are worsen, blue algae blooms are outbreaking frequently, which has been a serious threat to people's normal work and life. It is critical to research on the spatio-teraporal distribution characteristic of the concentrations of nitrogen and phosphorus and exogenous fluxes in Erhai Lake.Methods:This paper selects Erhai Lake and its main rivers as the research object. We determine the concentrations of nitrogen and phosphorus in water, which including Erhai Lake,26 the main rivers and dry and wet deposition from May,2009 to April,2010. The assessment on water quality and the exogenous source of the N and P in the Erhai Lake were discussed primarily.Results:1. The concentrations of total nitrogen (TN) in Erhai Lake was 0.20～ 0.96mg·L-1, the average was 0.62mg/L; Total phosphorus(TP) was 0.014～0.047 mg·L-1, the average was 0.03 mg·L-1. The Chemical consumption of oxygen demand (CODMn) was 2.65～4.86 mg·L-1, the average was 3.88 mg·L-1.The chlorophyll was 3.715～13.25 mg·L-1, the average was 9.66 mg·L-1; The transparency was 1.26～3.04m, the average was 1.69m;The Carson index (TSI) is 39.42～51.41, the average was 45.06 mg·L-1.2. The exogenous fluxse of the TN and TP was 813.779t and 50.150t from May,2009 to April,2010, the proportion in total pollution load was 67.69% and 67.32%. The exogenous fluxes of CODMn were 2033.915t. The maximum of exogenous fluxes was June in temporal distribution, the minimum was February. From spatial distribution,TN, TP and CODMn in the north of rivers was 421.95t, 18.82t and 1315.36t, the west was 284.368 t,24.831 t and 605.847 t, the south was 106.84t,6.47 t and 109.14t, the east was 0.619 t,0.030t and 3.575 t.3. The fluxes of TN and TP in dry and wet deposition was 388.5t and 24.35t, the proportion in total pollution load was 32.31% and 32.68%. The maximum of exogenous fluxes was July in temporal distribution, the minimum was December.Conclusions:1. The water quality was middle nutrition level in Erhai Lake in the whole year from May,2009 to April,2010. Spatio-temporal distribution of the concentrations of TN and TP in Erhai Lake was different from 2009 to 2010 year. Erhai water quality was Grade I at February, which was best water, August was Grade III and eutrophication, was worst one. The other was GradeⅡ～Ⅲ.2. Erhai main river fluxes, the proportion of the exogenous source of the TN and TP in total pollution load was 67.69%and 67.32% from May,2009 to April,2010. The minimum of exogenous fluxes in main river was in February, the maximum was June. Temporal distribution of regional fluxes was north>west>north>east. Miji River, was located on Erhai north, was largest of exogenous fluxes in the research 26 river. The government should take measure to emphasis governance to Miji River.3. From total pollution load, the proportion of the fluxes of TN and TP in dry and wet deposition was 32.31 % and 32.68%. The maximum of exogenous fluxes of nitrogen and phosphorus on dry and wet deposition was in July, the minimum was in December.