Long-term changes in major ion chemistry—similarities and differences between inflow rivers and the lake in Taihu watershed, China

The major ion chemistry in the Taihu watershed has dramatically changed due to human disturbances; however, little is known about the similarities and differences in the responses of the inflow rivers and Taihu lake to the disturbances. Using historical (1950s–1970s) and recent (2018–2021) water chemistry data of inflow rivers and the lake, as well as socioeconomic and land use data, we explored the drivers for the major ion chemistry change and different responses of the inflow rivers and the receiving lake. The results indicated that, compared with 1950s–1970s, all the major ions and TDS in rivers and Taihu lake significantly increased (by 91% for Mg 2+ and by 395% for Cl − in rivers; by 68% for HCO 3 − and 134% for Na + in the lake); however, their increases in major ion composition presented a clear difference, i.e., although current dominant cation remained Ca in inflow rivers, the second dominant cation has shifted from Mg 2+ (1950s–1970s) to Na + (2018–2021) for rivers, while for the lake, the second dominant cation has become frequently Na + (2018–2021), followed by Ca 2+ , indicating a clear salinization tendency. Furthermore, the change of some indicative ratio indices of inflow rivers and the lake in the past decades presented an apparent difference, i.e., the river systems had a higher increase rate in Ca 2+ /Mg 2+ and SO 4 2− /Cl − than the lake, while the lake had a higher increase in (Ca 2+ + Mg 2+ )/HCO 3 − , TH/TA, and Cl − /Na + than the river systems. Analyses indicated that increased human disturbances were the major driver for the similar increase in the TDS and major ions for both river systems and the lake, while the different algal biomass in the rivers and lake, the land use change, and declined hydrological connectivity in this watershed played important roles in the different alterations of the water chemistry indices. Comparison of major ion correlation change between the running and stagnant waters indicated a clear “lacunification” trend of inflow rivers in terms of water chemistry characteristics in this dense river-network region. Our work revealed the cause and effect of the fundamental water chemistry change in a rapid development region and will provide scientific basis for the integrated management and recovery in the watershed.