Seasonal and shorter-term changes in surface-water chemistry on four English ombrogenous bogs

1. Analyses are presented of major ions (H+, Ca2+, Mg2+, K+, Na+, Cl-, and SO4(2-)) and some other variables in surface-water samples, at roughly two-month intervals from March 1992 to May 1993, from a blanket bog in south-west England and a blanket bog and two raised bogs in northern England. 2. Metallic cation concentrations were generally lower in winter and higher in summer, but there was much short-term variation reflecting varying weather conditions. High Na+ inputs in rain led to high concentrations of other cations, including Ca2+, by cation exchange with the peat. 3. K+ showed a peak in early or late spring, with a decline through the growing season and autumn possibly related to plant uptake. Its concentration in the surface water is probably also strongly influenced by the amount and timing of rainfall. 4. The pH was stable within c. 0.2 units for much of the year, but tended to fall in dry periods, especially in summer, and when total ionic concentration rose through large additions of Na+ in rain. At some sites there were sharp falls of pH in late summer associated with oxidation of sulphides in the peat. 5. With changing concentration, the relative proportions of univalent and divalent ions in the water approximated to theoretical expectation for cation exchange, the plot of log sigma (divalent ions) against log sigma(univalent ions) giving a slope close to 2.0. 6. The Ca2+/Mg2+ quotient, probably buffered by cation exchange with the peat, was nearly constant round the year at all sites, but varied from site to site. 7. Cl- was relatively constant for much of the year; a strong late-winter peak at the northern sites was evidently due to high rainwater input during stormy westerly weather. The summer increase in cations was mainly balanced by increased organic anions (humic substances) or SO4(2-). 8. The data indicate retention of a substantial part of the SO4(2-) input in the peat for much of the year at most sites, with episodes of SO4(2-) and acidity generation during summer periods of low water level. All the bogs were effective sinks for atmospheric NO3- inputs except at near-freezing temperatures in winter. 9. Anion deficit, optical absorbance at 320 nm and total Fe concentration were all highest in summer and lowest in winter: all are related to the concentration of dissolved organic matter. 10. Temporal changes in surface-water chemistry were least marked at the wettest and most oceanic blanket-bog site (Plym Head, Dartmoor), and most pronounced on a lowland raised bog subject to relatively low rainfall (Helwith Moss, N. Yorkshire).