Simpson, G; (2000) Palaeolimnological analogues in defining target assemblages for the recovery of acidified surface waters: a desk study. (ECRC Research Reports 74 ). UCL Environmental Change Research Centre: London, UK.

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Abstract

1) Palaeolimnological techniques have been widely employed to study lake acidification. This approach has been central in testing the cause-effect relationship between acid deposition and lake acidification, and in assessing the magnitude and extent of surface water acidification across the UK. 2) Most of these palaeolimnological applications have been based on diatom analysis, and the use of diatom-pH transfer functions to make reconstructions of hydrochemical change in upland lakes associated with acidification. . 3) Following the signing of the Second Sulphur Protocol, attention is now focusing on emissions reductions and the reversibility of surface waters acidification. There is a clear need for criteria against which to evaluate the recovery process. 4) In order to evaluate future recovery, Flower et al. (1997) have proposed a palaeolimnological technique for defining targets for the recovery of acidified surface waters. This is based on the technique of analogue matching of lake sediment diatom assemblages. Multivariate statistical methods are used to identify modern analogues for the pre-acidification diatom assemblages of acidified lakes. The chemical and biological status of modern analogue lakes can then potentially provide recovery targets for acidified systems. 5) This approach has been successfully applied to several acidified lakes, and modern analogue systems defined for the pre-impact (pre-acidification) status of these impacted sites. An advantage of the approach is that it can provide recovery targets for both chemical and biological status of acidified lakes. 6) Modern analogue matching as currently applied makes several key assumptions: a) that analogue matches based on a single biological group (diatoms) effectively represent the hydrochemical and biological variation of low alkalinity systems; b) that the modern data set used to identify modem analogues contains the range of hydrochemical conditions represented by the fossil assemblages; c) that a suitable stable 'baseline' (pre-impact) status can be defined. 7) Prior to more comprehensive application of the modem analogue approach to acidified lakes in Britain, these assumptions require evaluation. Three studies are proposed: a) Extension of the current modem lake dataset used for analogue matching by the inclusion of minimally impacted low alkalinity sites from northern Scotland. b) Development of the current technique by including two more fossil groups (chironomids and cladocera) in the modern surface sediment dataset used in the matching procedure. This will allow the assumption that diatoms represent wider ecosystem variation to be tested, and should result in more robust analogue matches. c) A study of hydrochemical and biological variation in the pre-acidification conditions of acidified lakes through high-resolution palaeolirnnological study of selected Acid Waters Monitoring Network lakes. This will allow the stability of baseline (preacidification) conditions to be evaluated.

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