Study On The Nutrients In The Lower Yellow River And Changjiang Drainage Basin

The Changjiang and Yellow River are two largest rivers in China. Nutrientsin these two rivers are transported into the sea with the powerful run-off, which have asignificant impact on the ecological and environmental conditions of the estuary andadjacent seas. Observations were carried out at the lower reaches of the Yellow Riverfrom2009to2011, and in the Changjiang and its tributaries from August to October2009and in the middle and lower Changjiang in June2010. Dissolved and particulatenutrients were measured using spectrophotometry to study their concentrations,composition, distribution, and principal factors, such as the Yellow River water andsediment regulation event and the Three Gorges dam project.The results show that the concentrations of various N forms were lower in theflood season than those in the dry season. Dissolved inorganic nitrogen （DIN） was thepredominant species of TDN. Particulate phosphorus was the major form of totalphosphorus affected by high content of suspended particulate matter. Phosphaterepresented68%of TDP. Both silicate and biogenic silicon were higher in the floodseason than those in the dry season. The BSi was18%-21%of the sum of BSi andSiO₃-Si. The particulate phosphorus and biogenic silica had a positive correlation withSPM. Nutrient composition was characterized by high DIN／PO₄-P and SiO₃-Si／PO₄-P ratios and low SiO₃-Si／DIN ratio. In the past decades, the dissolved inorganicnitrogen concentrations increased, dissolved phosphorus remained at a stable level,and silicate concentration slightly decreased. Nutrient transport fluxes from the riverinto the Bohai show significant seasonal changes. It was found that the loads of water,sediment and nutrients in the flood season were dominant in all of the year. Theconcentrations and composition of nutrients had an obvious change during thewater-sediment regulation event. Abundant water, sediment and nutrients weretransported to the Huanghe estuary and the adjacent Bohai during the water-sedimentregulation event. The N concentration and the ratios of DIN／PO₄-P decreased, whilethe concentrations of P and Si and the ratios of SiO₃-Si／PO₄-P and SiO₃-Si／DIN increased. Further more, the proportion of particulate phosphorus and BSi increased.It is expected that the terrestrial inputs in short periods during regulation event willlead to an extensive impact on the ecological system of the Bohai.The concentrations of various forms of dissolved inorganic N of the Jinshajiangsection, more than3000km from the estuary in the upper reaches, are relatively lower,which may be relevant to the light pollution in the Jinshajiang, and the concentrationsincreased greatly along the upstream to the estuary. NO₃-N was always the majorform of inorganic N. The average N concentration in tributaries was significantlyhigher than in the mainstream. The various forms of inorganic N concentration werethe minimum in the Yalongjiang. Nitrate transports were mainly affected by Humanactivities in the Changjiang catchment. The suspend particle matter remained at a veryhigh level in the river, it was up to250-530mg/L in the upstream more than3000kmfrom the river mouth,50-70mg/L in downstream, less than30mg/L in the Rivermouth.In the upper reaches more than3000km from the river mouth the dissolvedphosphorous concentration was much lower than the particulate P. Particulatephosphorus was the main form of phosphorus in the upper reaches, accouting for85％-95％of the total phosphorus. While particulate P declined dramatically at the damregion accounting for10％-25％of the total phosphorus. In the middle and lowerreaches, the proportion of dissolved phosphorus was higher than particulate P. Thereis a linear relationship between the particulate phosphorous and the SPM. The silicateconcentration was stable all along the river at100-120μmol·L-1with an average valueof109μmol·L-1. The dissolved silicate concentration decreased in the lower reachrelated to the growth of diatoms and retention of BSi due to dam construction. Thenutrient level in northern tributaries was obviously higher than in the southerntributaries. And the nutrient level in the tributaries was higher than that in themainstream. Comparing with the data before impoundment of TGD in April,1997,nutrient concentrations in these investigations have increased for some extent affectedby multi-stressors.The DIN／PO₄-P ratio in the Changjiang drainage basin was stable and morethan except in the Jinshajiang. The SiO₃-Si／DIN and SiO₃-Si／PO₄-P ratiosdecreased along the mainstream with the increase in nitrogen input. The nitrogen,phosphorus and silicon were effectively intercepted in the Three Gorges Reservoir. The DIN, DIP, TPP, DSi, and BSi retention efficiencies were25%,32%,32%,3.2%,35%, respectively. And the Three Gorges Reservoir was the source of DON and DOP.Therefore, N/P ratio increased in the lower reaches of TGD due to differentretention efficiencies of nutrients. However, due to the complement in the middle andlower reaches,,the concentrations of dissolved nitrogen and phosphorus quickly wentback to a high level and the silicate concentration was back to the original level.While particulate nutrient concentrations complemented slightly. The TGD projectwas playing an important part in the nutrient transport to the ocean.