Temporal variation in the surface‐water chemistry of a blanket bog on Dartmoor, southwest England: analysis of 5 years' data

Abstract

SummaryBlanket‐bog peats, mapped as the Winter Hill and Crowdy associations by the Soil Survey of England and Wales, are an oceanic manifestation of the ombrotrophic ‘raised‐bog’ (Hochmoor) peats that cover large tracts in the boreal zone of the northern hemisphere. This paper examines monthly analyses from 1992 to 1997 of major ions and other variables from an upland blanket bog in southwest England in relation to seasonality, rainfall, and the chemical composition of rainwater. Average ionic composition of surface water (and peat) integrates variable atmospheric solute inputs over the years. The dominant ions in the surface water, Na+ and Cl–, showed only weak seasonality, but divalent cations a stronger seasonal pattern with a summer maximum. Mean pH ranged from c. 4.4 in February to c. 4.2 in August. Changes in concentration of different cations were closely interlinked by cation exchange. The anion deficit, accounted for by anionic groups on the dissolved organic matter, was strongly seasonal with a summer maximum, as was optical absorbance at 320 nm. Nitrate and NH4+ were both at much smaller concentrations than in rain. Nitrate exceeded 1 μmol l−1 only during cold periods in winter, mainly following drought in the summer of 1995; NH4+ reached a few μmol l−1 only in summer. There was evidence of net retention of S by the peat in wet sites and during wet periods, and of net release of SO42– (and acidity) under dry conditions. The 1995 summer drought and ensuing dry year in 1996 had marked and persistent effects on pH, apparent ion deficit (DEF), SO42–, the divalent cations and Fe.