REGIONAL AND LOCAL VARIATION IN THE CHEMICAL-COMPOSITION OF OMBROGENOUS MIRE WATERS IN BRITAIN AND IRELAND

1. Chemical analyses are presented of major ions (H+, Ca2+, Mg2+, K+, Na+, Cl-, SO42-) in water samples from 39 ombrogenous bog sites and 10 basin and valley mires in Britain and Ireland. 2. The first axis in principal component analyses of the data is most strongly correlated with Na+, Mg2+ and Cl-, reflecting total ionic concentration dominated by sea-spray components; subsequent axes correlate particularly with Ca2+, H+ and SO42- and largely reflect terrestrial and pollution inputs. 3. The quotient of total to marine-derived Ca2+ shows a broad regional trend from values below 2 on the Atlantic coast of Ireland to 5 or more inland in Britain, with considerable local variation. Locally very high values may relate to, for example, nearby agriculture (liming) or quarrying. 4. Measured concentrations of K+ in ombrogenous sites range from 0.25 to 4.04 times the expected value from marine input; they vary sporadically with little sign of regional trends. Some of the variation reflects uptake and release by vegetation, but some must be due to local inputs of terrestrial origin. 5. The proportion of SO42- to Cl- on the Atlantic coasts of Ireland and Scotland is near that in seawater (0.103). Higher average SO42- concentrations farther east are broadly in line with expectation from published atmospheric SO42- deposition data. Concentrations of hydrogen ions and some heavy metals show comparable trends. 6. The average proportions of most major ions in ombrogenous mire waters are similar to those in rainfall in the same areas. Ionic concentrations in ombrogenous bog waters are higher than the mean values in rainwater by a factor of 1.25-2.50 or more. There is evidence that some ombrogenous bog surfaces receive substantial dry deposition of Ca. 7. Basin and valley mires are not sharply delimited from ombrogenous mires in their water chemistry, but they commonly show (i) rather higher concentration ratios of ions relative to rainwater, and (ii) proportions of ions differing markedly from those expected from precipitation inputs. 8. The data show broad correlations between variation in ombrogenous bog vegetation, species limits, and the major-ion composition of the mire waters. These probably arise at least in part from parallel responses to topographical and climatic factors. Many ombrogenous bog species and vegetation types show wide amplitude in relation to major ions within the field of variation examined here: the factors limiting the more restricted species require more critical study.