Physical, chemical and biological (microfossil) analyses of lake sediment cores enable lake, catchment and atmospheric histories to be reconstructed with high resolution over a range of timescales. In the SWAP Palaeolimnology Programme these techniques have been used to trace the recent (post 1800 A.D.) history of a number of specially selected study sites in Norway, Sweden and the U.K. For this purpose a large calibration data-set of diatoms and water chemistry has been assembled and new statistical techniques of pH reconstruction have been developed. In addition, by comparing temporal trends and spatial patterns within and between sites, various explanations for recent lake acidification have been evaluated.

Studies of complete post-glacial (10000 years) sediment sequences at a number of sites showed that long-term acidification has occurred at some sites but the rate of change before 1800 A.D. at all sites was zero or less than 0.1 pH unit per 1000 years. Tests of the ‘land-use change’ hypothesis involving studies of past analogues, sites with no decrease in grazing intensity, sites above the treeline and sites with minimal catchments all failed and tended instead to support the acid deposition hypothesis. The only observed land-use effect was an acceleration in the rate of acidification followed afforestation at a site in the Scottish Trossachs.