Acidification of Lakes in Galloway, South West Scotland: A Diatom and Pollen Study of the Post-Glacial History of the Round Loch of Glenhead

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Abstract
(1) Potential acidification by chemical weathering, soil leaching and organic matter accumulation in the catchment of the Round Loch of Glenhead is evaluated using diatom and pollen analysis of a radiocarbon dated sediment core. The Round Loch is situated on granite bedrock and is likely to have been sensitive to acidification throughout the post-glacial period. (2) About 9000 years B.P. the open habitats of the late and early post-glacial were replaced by birch/hazel woodland, then oak/elm/pine woodland, and oak/hazel/alder woodland by about 5400 years B.P. After 5700 years B.P., blanket mires increasingly replaced woodland. The present vegetation on the blanket peat is a Molinia/Calluna heathland. (3) The lake was acid in the late-glacial period (pH 5.3-5.7). No evidence of long-term acidification in the early post-glacial period was found, and from 9000-4150 years B.P. the pH of the loch was probably between 5.3 and 5.6. In th mid post-glacial no indication of lake acidification that might be associated with peat development in the catchment was identified, and from 4100 years B.P. there is evidence that a slight increase in pH or nutrient availability or both occurred. (4) Despite a clear change from mineral to acid organic soils in the catchment in the mid post-glacial, feedback mechanisms operated to maintain a lake with pH stable at 5 and above for most of the post-glacial period. With the introduction of strong acid anions associated with acid deposition after A.D. 1800 the pH fell to its present value of 4.7.