Reconstruction Of Lake Nutrient Evolution Based On A Diatom-Total Phosphorus Transfer Function For Lakse In Songnen Plain

Lake eutrophication is one of the major environmental problems facing lakes in China and even the world.Excessive enrichment of nitrogen,phosphorus and other nutrient elements is the root cause of eutrophication in lakes.The lake group in Songnen Plain is one of the highest density of lakes areas in China.In recent years,the salinization and eutrophication of lakes have become increasingly serious in this area.In view of the ecological environment of lake groups in Songnen Plain,it is urgent to study the mechanism of water quality change in this area.The species of diatom were identied and counted at various points,and the hydrochemical types of the sample points in Songnen Plain were classified by sorting and analyzing the water sample data collected from June to August 2008,then two datasets were established:original dataset comprised of 80 sampling points and chloride-sodium group-I(C_I^Na)water type dataset comprised of 71 sampling points,respectively defined as Dataset-?and Dataset-?.The relationship between planktonic diatoms and environmental indicators in the two datasets was investigated by using Canonical Correspondence Analysis?CCA?,and the key environmental factor affecting the distribution of diatoms in the two datasets was identified,the diatom-key environmental factor transfer function was established by selecting the appropriate dataset.After that,with the support of dating technology,the transfer function was used to quantitatively reconstruct the key environmental variable in the historical period of Chagan lake and obtained its evolution sequence,and the process of environmental evolution was verified by the record analysis of other indicators.Moreover,the driving factors of water environmentalevolution were further analyzed and discussed.The main conclusions were as follows:?1?After identification and counting,91 species of diatoms?which appeared at least 2 sampling sites and contained more than 1%in at least one sample?were used to establish the Dataset-?,and 88 species of diatomswere used to establish the Dataset-?.Aulacoseira granulata,Aulacoseira ambigua and Cyclotella meneghiniana can be used as indicator species in both datasets,they are often found in eutrophic water bodies.?2?A series of sorting analysis was carried out on the diatoms and various environmental gradients in the two datasets,the result showed that the species distribution responded to the environmental gradients.Some nutrient-tolerant species such as Nitzschia austriaca,Pinnularia microstauron var.brebissonii,Navicula cuspidata,Gomphonema parvulum,Nitzschia levidensis var.salinarum and Fragilaria brevistriata were positively correlated with the total phosphorus gradient.?3?As indicated by the analytical results based on both datasets,the total phosphorus out of 15 environmental indicators which explains the maximum variability of diatom species was one of the most important and significant environmental parameters affecting the distribution of diatom species.Viewing from the interpretation value of the marginal effect variance of total phosphorus and the unimodal response of the species,it is more appropriate to establish the diatom-total phosphorus transfer function usingthe data from Dataset-?.?4?When using the data from Dataset-?to establish the diatom-total phosphorus transfer function,the method of partial least square weighted average regression?WA-PLS??component 5?that provided a higher correlation coefficient between the observed value and the predicted value?R²_Jack=0.759?was a better modeling method compared to the method of weighted average regression?WA?.After deletion of outliers,the prediction error of the diatom-total phosphorus transfer function was reduced?RMSEP=0.169?while the correlation coefficient between the observed value and the predicted value was increased?R²_Jack=0.823?.The final results suggested that the WA-PLS?component 5?model after residual component extraction was the final ideal model.The transfer function performed better compared to other regional TP models in the world.?5?The constructed diatom-total phosphorus transfer function was applied to a sedimentary core of Chagan lake.With the support of dating technology,the total phosphorus contents of Chagan lake from the late 1850s to 2016 were quantitatively reconstructed.For more than 150 years,the total phosphorus concentration in Chagan lake fluctuated between 0.088-2.073mg/L,with a large range of variation.?6?The results of quantitative reconstruction of nutrient level in Chagan lake in the past 150 years were compared with the characteristics of grain size composition,water quality monitoring records under partial time scale,climate records and human interference factors in this region,to analyze and discuss the change trend of nutrient state of reconstructed lake water.Further combining with the fitting results of the observed and predicted values of total phosphorus concentration in Chagan Lake in some years?1984,1985 and 2002-2016?,it was proved that the regional diatom-total phosphorus transfer function was applicable and the predicted resultwas relatively reliable.Also,the predicted result indicated that the lake water quality evolved from?class into worse?class and then gradually evolved into?-?class,the nutrient state of the lake changed from mild eutrophication to hypertrophication and then recovered to the present mild eutrophication state in the past 150 years.?7?The effects of exogenous phosphorus load and internal phosphorus recirculation on nutrient evolution in Chagan lake were analyzed.The variation trend of the predicted values of phosphorus concentration in water and the observed values of phosphorus concentration in sediments were emphatically compared,the cycling process of phosphorus between sediment and water and the self-regulating mechanism of lake aquatic ecosystem were discussed under different environmental conditions.