Threshold Of N And P In Mountainous And Plain Rivers In Taihu Lake Basin(Zhejiang Province)

The growth and reproduction of phytoplankton in waters is depended on several eco-factors such as total nitrogen (TN), total phosphorus (TP), COD, and et al.. In this study, we selected tow typical kinds of rivers of the Taihu Lake Basin (Zhejiang Province) to investigate the relationship between the ecofactors and phytoplankton levels on September in2011and June in2012. One is a typical mountain river named Tiaoxi, and the other is enclosed in a plain area named Jiaxing river net. Two primary productivities, suspension chlorophyll-and benthic chlorophyll-a, were used to characterize the amount of existing algae and nutritious level. SOM was applied to analyze the multiple responses of ecofactors to the primary productivities. The N and P threshold for the two different rivers were assessed based on the chlorophyll-a limitation in waters.Results showed that, in Tiaoxi River Basin, the mean concentrations of TN, TP, COD were2.2mg/L,0.20mg/L,31.2mg/L in main channel, and2.8mg/L,0.16mg/L,19.2mg/L in tributaries, respectively. Nitrate-N was the major form of the TN, and TP was mainly composed of particulate-P and organic P. On average, the values of N/P were23.1and7-22.6in the tributaries, and main channel, respecitively, indicating this river is under P limitation. In Jiaxing plain, the average concentrations of TN, TP, and COD were5.2mg/L,2.3mg/L, and109.3mg/L, respectively. The low ratio of N and P leads Jiaxing river net under the status of N limited.The suspended chlorophyll-a and benthic chlorophyll-a concentrations in main channel of Tiaoxi were6.8μg/L,21.6mg/m2, and in tributaries were3.5μg/L,41.8mg/m2, respectively. The Jiaxing plains were staying at high nutritent level with10.2μg/L of suspended chlorophyll-a and149.3mg/m2of benthic chlorophyll-a.During the observation period, the average temperatures were20.7℃and20.2℃in the main channel and tributaries of Tiaoxi River repectively. Oxidation-reduction potentials were both between100mv to300mv. The average pH values were7.7and7.8. And both had a large range of turbidity (10NTU-1106NTU). In addition, dissolved oxygens were5.2mg/L and6.1mg/L. The corresponding values of these parameters in Jiaxing river net were:temperature of25.2℃, pH of7.4(weak alkaline), oxidation-reduction potential average of164mv, low turbidity of61.1NTU and low dissolved oxygen of the average value4.0mg/L. In addition, self-organized mapping (SOM) analysis showed that temperature, TN, TP, DO, and suspended chlorophyll-a neuron diagram had a strongly related trend.Importantly, based on the relationship of suspended (or benthic) chlorophyll-a and N (or P) in rivers, this study obtained TN threshold (0.9mg/L,1.3mg/L), TP threshold (0.05mg/L,0.05mg/L) for the main channel and tributary of Tiaoxi. While in Jiaxing plains, the N and P threshold is1.4mg/L and0.3mg/L. N and P thresholds of the Tiaoxi River and Jiaxing plains have referential significance for non-point source pollution control for other similar rivers.