Water chemical changes along a latitudinal gradient in relation to climate and atmospheric deposition

Evaluating trends over time (nonparametric Mann–Kendall test) for 18 water chemical variables from 79 reference lakes, distributed all over Sweden, during spring since 1984 showed most significant trends for atmospheric deposition driven sulfate (SO₄) concentrations. The decrease in SO₄ concentrations was on average 2.7 times higher at lower (56°N to 59°N) than at higher latitudes (60°N to 68°N). This large difference in the rate of change between lower and higher latitudes could not solely be explained by atmospheric deposition as the rates of change in SO₄ wet deposition differed by a factor of only 1.5 between lower and higher latitudes. Significantly higher rates of change at lower than at higher latitudes are known from the timing of lake ice breakup, a typical climate change indicator. The rates of change in the timing of lake ice breakup differed by a factor of 2.3 between lower and higher latitudes. Other water chemical variables showing significantly higher rates of change at lower than at higher latitudes were water color (a factor of 3.5), calcium (a factor of 2.9), magnesium (a factor of 5.5) and conductivity (a factor of 5.9). The rates of change of all these variables were strongly related to the rates of change in the timing of lake ice breakup along a latitudinal gradient (R ² = 0.41–0.78, p < 0.05), suggesting that climatic changes can accelerate atmospheric driven changes at especially lower latitudes. This acceleration will result in more heterogeneous lake ecosystems along a latitudinal gradient.