Evidence from experiments, modeling, and field observations for effects of increased salinization on re-distribution of sediment base cations in Taihu Lake, China.

Taihu Lake, the third largest freshwater lake in China, has experienced rapid salinization in the past decades; however, little is known about the impact of sodium (Na) on ion exchange in the lake environment. To explore the potential effect of increased Na on the migration of base cations (Ca and Mg) and resulting redistribution between the water and sediment, we used the adsorption-exchange experiment, MINTEQ modeling to explore the cation exchange induced by high Na input, and its impact on the redistribution of Ca and Mg in Taihu different media. The results indicated that exchanged quantity of Ca and Mg increased with time, and the exchange process reached 90% during 0-4 h and reached equilibrium after 24 h under 100 mg/L Na (the maximum Na concentration in Taihu sediment pore water). Our MINTEQ modeled result indicated that the exchanged quantity of Ca and Mg increased with the increasing Na concentration, with Ca being preferably exchanged over Mg at the same Na concentration. The MINTEQ model further predicted that, in the Taihu lake environment, the exchange adsorption would reach the equilibrium at the concentration of 6000 mg/L Na, with exchanged Ca2+ and Mg2+ accounting for 47% and 55% of the total exchangeable Ca and Mg in the sediment, respectively. Although current Na-induced exchange in the Taihu lake has been far from the equilibrium, the MINTEQ result confirmed the existence of this reaction and predicted the potential redistribution of base cations or Ca/Mg ratio in the lake sediment and water phase with further Na increase. Furthermore, our field observations not only confirmed the existence of Na-induced cation exchange in this lake environment but also were generally in agreement with our experimental and modeled results. The increased salinization-induced ion exchange would alter the re-distribution of base cations and the resulting potential ecosystem consequences should be given close attention in this large freshwater lake.

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 Mg2+ and by 395% for Cl- in rivers; by 68% for HCO3- 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 Mg2+ (1950s-1970s) to Na+ (2018-2021) for rivers, while for the lake, the second dominant cation has become frequently Na+ (2018-2021), followed by Ca2+, 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 Ca2+/Mg2+ and SO42-/Cl- than the lake, while the lake had a higher increase in (Ca2+ + Mg2+)/HCO3-, 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.