Six-decade change in water chemistry of large freshwater Lake Taihu, China.

Taihu lake has become a hot spot internationally due to its algae bloom. However, its natural water chemistry (major ions) received little attention though it is equally important for drinking water and aquatic ecology. Using historical data (1950s-2012) we explored the drastic change of Taihu water chemistry over the past six decades and the driving factors. Results show that major ions increased around 2-7-fold and TDS increased nearly 3-fold during the last 60 years. The dominant cation has shifted from Ca(2+) to Na(+), and the current Cl(-) is dominant over HCO3(-), the predominant anion before the 2000s. Analyses show that population increase and human activities were the major driving factors responsible for the drastic change. Whereas the mechanism of increase was different for ions, i.e., Na(+) and Cl(-) increase was directly related to the population increase and sewage discharge in the basin; SO4(2-) was related to atmospheric deposition derived from increasing coal consumption and SO2 emissions; hardness (Ca and Mg) increase was closely linked to the acidic precipitation. No increase trend of HCO3(-) was attributable to frequent outbreaks of algae bloom which consumed HCO3(-). Estimation indicated that sewage discharge in the basin contributed 23% to the lake in terms of Cl(-), exceeding the contribution from rock weathering. Current water chemistry of Taihu lake has become "anthropogenic dominance" from its original rock dominance.