Introduction

Lake acidification became an environmental issue of international significance in the late 1960s and early 1970s when Scandinavian scientists claimed that “acid rain” was the principal reason why fish populations had declined dramatically in Swedish and Norwegian lakes (Odén, 1968; Jensen & Snekvik, 1972; Almer et al., 1974). Similar claims were being made at about the same time in Canada (Beamish & Harvey, 1972). However, these claims were not immediately accepted by all scientists. It was argued by some that acidification was due to natural factors or to changes in catchment land use and management (Rosenqvist 1977, 1978; Krug & Frink, 1983; Pennington, 1984).

In the scientific debate that followed, diatom analysis played a pivotal role. It enabled the timing and extent of lake acidification to be reconstructed (Charles et al., 1989; Battarbee et al., 1990; Dixit et al., 1992a) and allowed the various competing hypotheses concerning the causes of lake acidification to be evaluated (Battarbee et al., 1985; Battarbee & Charles, 1994; Emmett et al., 1994). However, diatoms had been recognized and used as indicators of water pH well before the beginning of this controversy. The acid rain issue served to highlight the importance of diatoms and stimulated the advance of more robust and sophisticated techniques, especially the development of transfer functions for reconstructing lake-water pH and related hydrochemical variables.

This chapter outlines the history of diatoms as pH indicators, and describes how diatoms are currently used in studies of acid and acidified waters. It then describes how diatom-based paleolimnological methods have been used to trace the pH and acidification history of lakes and how diatoms are being used to monitor acidity trends in streams and lakes.