| The natural water chemistry of surface waters in the Taihu watershed has changed dramatically over the past 60 years due to the influence of human activities;however,there is no systematic study of the similarities and differences in the natural water chemistry response to anthropogenic activities and the change characteristics of the influx rivers and Taihu lake,which are connected but different aquatic systems in the watershed.In this study,we collected and analyzed historical water chemistry data(1950s-1970s),recent(2018-2021)sampling and testing data of the influx rivers and Taihu lake,sediment data of Taihu Lake,and socioeconomic and land use changes in the watershed to compare and analyze the natural water chemistry change characteristics of the influx rivers and Taihu lake in the past 70 years.The results show that,driven by anthropogenic factors and ecological differences,significant spatial and temporal differences in the increase of major ion concentrations,TDS concentration status,dominant anions and cations,and the ratio of some major ions have occurred in the current influx rivers and Taihu lake compared with those in the 1950s-1970s,as shown by:(1)The pH,conductivity and total dissolved solids(TDS)concentrations vary widely in different water systems and lake areas.Currently,the pH values of the influx rivers and Taihu lake are 8.29 ± 0.33 and 7.81 ± 0.36,respectively,and the water bodies are weakly alkaline.Compared with 60 years ago,the concentration of TDS in the influx rivers and Taihu lake increased by 184%and 121%,respectively,and the salinization trend in the river and lake water bodies was obvious.The TDS concentrations in the Taihu lake are still higher than those in the influx rivers,and the TDS concentrations in different regions are consistent with the resident population and GDP near different water systems,indicating that anthropogenic activities have an impact on the chemical ion concentrations in river and lake waters.(2)On the time scale,all parameter ratios in the influx rivers(except SO42-/NO3-,which did not change significantly)showed a significant increasing trend in the last decades(p<0.05);while for lake waters,there was a significant increase in Cl-Na+(p<0.05),but a significant decrease in SO42-/NO3-and SO42-/Cl-(p<0.05),and Ca2+/Mg2+slightly increased.Due to the influence of anthropogenic activities,the ratios of(Ca2++Mg2+)/HCO3-and TH/TA in the influx rivers and Taihu lake increased from below or close to 1 in the 1950s-1970s to significantly above 1 at present(2018-2021).By comparing the increase rates of ion ratios between the influx rivers and Taihu lake,it is found that the increase rates of ion ratios of Ca2+/Mg2+and SO42-/Cl-in influx rivers are higher than those in Taihu lake,while the increase rates of(Ca2++Mg2+)/HCO3-,TH/TA and Cl-/Na+in Taihu lake are higher than those in influx rivers.(3)The analysis of the water chemistry triangles and Gibbs plots shows that the main ion sources in the influx rivers and Taihu lake have gradually shifted from "rock weathering"control to "anthropogenic dominance" dominance.Moreover,the main dominant anions and cations(Ca2+,Na+ and HCO3-)in the influx rivers and Taihu lake have changed,reflecting the important influence of rock weathering and anthropogenic discharge on the surface water of the watershed.In addition,based on the changes in the scattering relationships between(Ca2++Mg2+)and HCO3-,(SO42-+NO3-)and(Ca2++Mg2+),it indicates that the input of anthropogenic acids in the watershed not only changes the supply mechanisms of the dominant ions in river and lake waters,but also contributed to the leaching of cations from the watershed soil.(4)The lower Ca2+/Mg2+ and higher SO42-/NO3-in the water column of Taihu Lake compared to the influx rivers are due to the difference in algal biomass in the rivers and lakes.The "greater Ca2+/Mg2+in the algae than in the water column","lower Ca2+/Mg2+during the algal bloom than during the weak bloom" and "higher Ca2+ in the acid extractable state in the sediments of the northwest lake than in the southeast ",these phenomena consistently suggest that the alkaline environment formed during the algal bloom outbreak led to the formation of precipitation and migration of Ca2+from the aqueous phase to the sediment phase,resulting in lower Ca2+/Mg2+in the lake water.In addition,chlorophyll concentration and SO42-/NO3-in the water column of Taihu Lake showed a significant(r=0.51,p<0.01)positive correlation,indicating that more algae in Taihu Lake would absorb and utilize NO3-resulting in higher SO42-/NO3-,further demonstrating that aquatic organisms and biomass have an important influence on the ion concentration of river and lake water chemistry.(5)The TDS concentration was(637.23 ± 65.56)mg·L-1 at the sampling sites of the influx rivers with poor hydrological connectivity and(290.22±58.57)mg·L-1 at the sites with good hydrological connectivity,which indicates that the obstruction of hydrological connectivity will facilitate the accumulation of ions in the influx rivers environment.The construction of sluice gates and hydraulic engineering facilities in the Taihu watershed impedes the hydrological connectivity of the river system,while the artificial regulation shortens the hydraulic residence time of the lake water.As a result,the increase of each ion concentration in the influx rivers is significantly(p<0.05)higher than that in the Taihu lake,the retention capacity of the watershed for SO42-is increasing,and the influx rivers have gradually shown the development trend of lacustrine.(6)Compared with the 1980s,the proportion of impervious area in the Taihu watershed to the total watershed area has now increased to 24%(area increased by 5145.15 km2),while the area of cultivated land has decreased to 45%(area dCcreased by 5573.74 km2),and this change has made it easier for major ions in the watershed to enter the water body with surface runoff leading to the increase of TDS in the water body.The concentration of SO42-in lake water,represented by SO42-,was significantly correlated with the increase in impervious area in the area,indicating that the increase in impervious area promoted the entry and accumulation of SO42-ions in surface water.This research work reveals the main causes and influencing factors of the changes in the water chemistry of rivers and lake in this rapidly developing region of Taihu watershed,and the research results will provide a scientific basis for the integrated management and restoration of the watershed. |
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