The Geochemical Characteristics Of Nutrient Elements In River Water In Jiulongjiang Basin

The nutrient elements are the most basic compoment of life,and their biogeochemical cycle are the main processes in the earth's system.The river is a bridge between the continent and the ocean,and plays a vital role in the global biogeochemical cycle of the nutrient elements.Jiulongjiang River is the second largest river in Fujian Province,southest China.It is facing huge environmental pressure due to strong human activities.However,there is few researches on the content and migration of nutrient elements in river water in the whole Jiulongjiang River.Based on this,systematic sampling was carried out on the main channel and tributaries including Beixi Stream,Nanxi Stream and Xixi Stream during the wet and dry season of 2017²018.The temporal and spatial distribution and variation of nutrient elements species?nitrogen,phosphorus and silicon?in the Jiulongjiang River were studied.The aims of this study are to explore the distribution characteristics of the nutrient elements and the controlling mechanism,and related environmental effects,and to calculate the budget of nutrient elements from river to the ocean,at last,to provide important scientific evidences for the ecological environment manages/protections.The main conclusions are as follows:1.The pH of Jiulongjiang River averaged 7.2,and presented weak alkalinity;the mean value of TDS is 97.4 mg/L,which is much lower than the average value of the other rivers in the world?283 mg/L?,it represented as the weak mineralized water.The results of main ions analysis showed that Ca²⁺and HCO₃^-were main cation and anion species in Jiulongjiang River respectively.The chemical composition of the main elements in Jiulongjiang River are similar to the river with geologically dominated by silicate rocks.Water chemistry type of Jiulongjiang River was bicarbonate-calcium-group-I type.2.Compared with other rivers in the world,the contents of N,P and Si in the Jiulongjiang River water are all high.The contents of nutrient elements in the Nanxi Stream and Xixi Stream is generally higher than those in the Beixi Stream.The contents of N,P and Si from the upstream to downstream along the Beixi Stream and Xixi Stream showed an increasing trend,respectively.The longitudinal variation of N along the Nanxi Stream increased at the upstreams and decreased at the downstreams,while P and Si decreased steadily along the river.The contents of all nutrients in the Xixi Stream were significantly higher in the dry season than those in the wet season.Except for the contents of Si in the Beixi Stream and the contents of NO₃^--N and Si in the Nanxi Stream,which were higher in the wet season than those in the dry season.The contents of the other N forms and P in Nanxi Stream and the contents of N and P in Beixi Stream were higher in the dry season than those in the wet season.The main form of N and P in Jiulongjiang River are dissolved inorganic state,and NO₃^--N is the main component of DIN.The varaition of nutrients in the cascade developed midstream of the Beixi Stream,indicated that the interception effects on nutrient elements of dam retention were obvious,especially in the dry period.Biological absorption may be the main factor determining the decrease of DIP.The results of nutrition analysis showed that Jiulongjiang River was in eutrophic and Nanxi Stream plays severe eutrophication.The ratios of N/P,Si/N and Si/P far exceeded the ratio of Redfield,but the contents of N,P and Si in the river were much higher than the growth threshold of phytoplankton,so there were no strict limitations on the nutrients.P should be regarded as a potential limiting factor,while there is a tendency of weaken the potential phosphorus limit during the dry period.3.Xixi Stream is the main contributor of the budget of NO₃^--N,N H₄⁺-N,DIN,TN and TP in the Jiulongjiang Basin,and the Beixi Stream is the main contributor of DIP and DSi.The daily budget of nutrient elements in Beixi Stream and Xixi Stream are all higher in wet seasons than those in dry seasons,mainly due to the higher water flows in wet seasons than those in dry seasons.The nutrient yelieds of DIN,DSi and DIP in the Jiulongjiang basin is the highest in the world,indicating that the nitrogen and phosphorus loads in the watershed are extremely high,and the interference of human activities is extremely serious.Higher DSi yelied than other rivers in the world is probably related to geological background and the changes of land use in the basin.