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.

Accelerated carbonate dissolution caused by anthropogenic acidification - contrast of watershed soils to lake sediments in Taihu Region, China.

Although Taihu watershed is an "acid-insensitive" region, anthropogenic acidification has greatly changed the water chemistry in Taihu Lake. However, how soil carbonates responded to the long-term human-induced acidification received less attention. In this work, we investigated soil carbonate concentrations from different land uses in the upstream of the lake and sediment carbonate profiles in the lake, to explore the linkage of carbonates dissolution in the land and sedimentation in the lake. The result showed that the wheat-rice surface soil, the most acidification-impacted by fertilization and acid deposition, had significantly lower pH than vegetable and wetland soils (p < 0.05). Meanwhile, the carbonate concentration in wetland soils, only impacted by acid deposition, was significantly higher than that in wheat-rice and vegetable soils (p < 0.05). The pH profile of fertilized soils, with an increasing trend from the surface to bottom, further indicated the acidifying effect of fertilization. Although the average soil pH across all land uses was 6.6 in the upstream of the lake, remaining carbonate buffering system, the significant carbonate decrease especially in surface soils evidenced the definite carbonate dissolution by acidification, which is cumulative and irreversible. Contrary to the topsoils, the sediment carbonate concentration presented an increasing trend from the depth of 15 cm (denoting around the early 1980s) to the surface, indicating that lake sediment is a major sink of carbonate Ca and Mg from the watershed, particular under an alkaline lake environment caused by frequent algae blooms in the past decades. In addition, Ca/Mg ratio in the sediment, having higher values in a higher pH environment, was quite different from the watershed soil pattern, suggesting different biogeochemical processes Ca and Mg underwent during their transportation and sedimentation. The effects of acidification-altered re-distribution of carbonate Ca and Mg and Ca/Mg ratio in the terrestrial and aquatic environments deserve wider considerations of ecosystem consequence.

Acid deposition induced base cation loss and different responses of soils and sediments in Taihu Lake watershed, China.

Acid deposition and algae blooms have resulted in great changes in the water chemistry of Taihu Lake; however, there have been few calculated results to describe these processes. Here we used a mass balance model to estimate base cation losses and evaluate the model applicability in this intensively human-impacted watershed based on a long-term database (1985-2015). The results showed that carbonate weathering induced Ca2+ and Mg2+ losses in the watershed were responsible for the increased ion net reaction (INR) of Ca2+and Mg2+ in the lake. While the increase of K+ and Na+ were not appropriate to provide independents check on the mass balance model because they generally entered the lake as human discharges, not reflecting change of the geochemical process in the watershed as the watershed dominant bedrock is carbonate but not silicate. Acid deposition in Taihu region caused decrease in pH, lime potential, and different Ca, Mg species of surface soils. Our field measurements of sediments in the two lake parts showed that the sediment lime potential was significantly higher in the algae dominated lake area than in the hydrophyte-dominated area due to the in-lake alkalinization by algae blooms, indicating that algae blooms played a significant role in the acidification resistance. Meanwhile, the measured lime potential of the watershed soils was lower than that of the sediment, implying a potential risk of acidification in the watershed. This research helps understand the accelerated interactions between human activities and natural geochemical processes and accelerated water chemistry change at the watershed level.

δ15N and nutrient stoichiometry of water, aquatic organisms and environmental implications in Taihu lake, China.

Nitrogen pollution has become a worldwide problem and the source identification is important for the development of pertinent control measures. In this study, isotope end members (rain, nitrogen fertilizer, untreated/treated sewage), and samples (river water discharging to Taihu lake, lake water, aquatic organisms of different trophic levels) were taken during 2010-2015 to examine their δ15N values and nutrient stoichiometry. Results indicated that phytoplankton (primary producers), which directly take up and incorporate N from the lake water, had a similar δ15N value (14.1‰â€¯±â€¯3.2) to the end member of treated sewage (14.0‰â€¯±â€¯7.5), and the most frequently observed δ15N value in the lake water was 8-12‰, both indicating the dominant impact of the sewage discharge. Relationship analysis between N isotope value of nitrate and nitrate concentration indicated that different N cycling existed between the algae-dominated northwest lake (NW) and the macrophyte-dominated southeast lake (SE), which is a result of both impacts of river inputs and denitrification. Our nutrient stoichiometry analysis showed that the lake water had a significantly higher N:P ratio than that of algae (p < 0.05), suggesting that N is available in excess relative to the amount demanded by the algae. The long-term trend of the socio-economic development in the watershed further confirmed that the rapid population increase and urbanization have resulted in a great change in the N loading and source proportion. We suggest that although P control is necessary in terms of eutrophication control, N pollution control is urgent for the water quality and ecological recovery for Taihu lake.

Response of sediment calcium and magnesium species to the regional acid deposition in eutrophic Taihu Lake, China.

Acid deposition causes carbonate dissolution in watersheds and leads to profound impacts on water chemistry of lakes. Here, we presented a detailed study on the seasonal, spatial, and vertical variations of calcium and magnesium species in the overlying water, interstitial water, and sediment profiles in eutrophic Taihu Lake under the circumstance of regional acid deposition. The result showed that both the acid deposition and biomineralization in Taihu Lake had effects on Ca and Mg species. In the lake water, calcium carbonate was saturated or over-saturated based on long-term statistical calculation of the saturation index (SI). On the sediment profiles, significant difference in Ca and Mg species existed between the surface sediment (0-10 cm) and deeper sediments (>10 cm). The interstitial water Ca2+ and Mg2+, ion-exchangeable Ca and Mg in the surface sediment were higher than those in the deeper sediment. In the spring, when the acid deposition is more intensive, the acid-extracted Ca and Mg in the surface sediment were lower than that in the deeper sediment in the northwest lake, due to carbonate dissolution caused by the regional acid deposition. Spatially, the higher concentration of acid-extracted Ca and Mg in the northwest surface sediment than that in the east lake was observed, indicating the pronounced carbonate biomineralization by algae bloom in the northwest lake. Statistical analysis showed that acid deposition exerted a stronger impact on the variation of acid-extracted Ca and Mg in the surface sediment than the biomineralization in Taihu Lake. For the total Ca and Mg concentration in the spring, however, no significant change between the surface and deeper sediment in the northwest lake was observed, indicating that the carbonate precipitation via biomineralization and the carbonate dissolution due to acidic deposition were in a dynamic balance. These features are of major importance for the understanding of combined effects of acid deposition and eutrophication on freshwater lakes.