[Temporal and spatial variations of major ions in Nam Co Lake water, Tibetan Plateau].

In order to investigate the temporal and spatial variations, sources, and major controlling factors of the major ions in Nam Co Lake water, inshore surface water samples were collected at a fixed site (30 degrees 47.27'N, 90 degrees 58.53'E, 4718 m a. s. l.) from 2006 to 2010, at the vertical profiles in the center of the lake in August 2009, and at both the vertical profiles in the center of the lake and at the surface layers of different sites in the Nam Co Lake in October 2010. The results indicated that Na+ was the dominant cation and HCO3- was the dominant anion in the lake water. The concentrations of most ions were higher in monsoon seasons (June - September) and lower in non-monsoon seasons, especially when the lake was frozen (January -April). However, the Ca2+ concentration showed a reverse trend of seasonal variations, namely, higher values in the frozen period and lower in monsoon seasons. Analysis of water samples collected from the vertical profiles indicated that the concentrations of all ions except Ca2+ increased with the depth in nonmonsoon seasons (e. g. October). The major ions in Nam Co Lake were mainly contributed by river input. There were a variety of factors that influenced the temporal and spatial variations of the major ions in the Nam Co Lake, such as evaporation, precipitation, pH values, etc., among which, evaporation was the most important controlling factor, causing the increasing Na+ concentration and decreasing Ca2+ concentration in the lake water.