Acidification of lakes in Galloway, south west Scotland: a diatom and pollen study of the post-glacial history of the Round Loch of Glenhead

SUMMARY (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 postglacial 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 the 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.

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