Calcium and pH in north and central Swedish mire waters

1 We present data on calcium concentrations and pH in mire waters collected from different mire types in central and northern Sweden, compiled from published literature or calculated from field determinations of electrical conductivity and pH. 2 Measurements of electrical conductivity (after subtracting that of H+ ions) were used to calculate the most probable Ca concentrations, but only when pH was 4.7 or higher. At lower pH the possible errors become too large. 3 The data support a continuous gradient in water chemistry, but with considerable overlap, from mineral-poor ombrotrophic bogs and minerotrophic extremely poor fens, to moderately poor fens, intermediate fens, moderately rich fens and finally to extremely rich fens, rather than a discrete division. However, for hydrological reasons, we wish to retain the separation of ombrotrophy (bog) from minerotrophy (fen). 4 The frequency histogram of pH values from central and northern Sweden shows a bimodal tendency, although a considerable number of mires still occur close to the minimum (pH 5.3). Over-sampling in species-rich areas could have contributed to the depth of this apparent minimum, but not to its position along the pH scale. 5 Changes in calcium concentrations in mire waters during the last 50 years are discussed and related to changes in pH and conductivity.