Introduction

Although most attention has been focused on the impoverishment of fish populations in acidified lakes, numerous ecological studies have shown that phytoplankton, zooplankton and benthic invertebrates have decreased in diversity in recently acidified waters. There is ample evidence to suggest that the pH is a major variable in determining the distribution of species, although recent work includes other parameters, like the Ca2+ content and the metal concentrations into the list of distribution-limiting factors. Very elegant work in this respect has been performed by Økland and Økland (Økland, J., 1980; Økland, K.A., 1980; Økland & Økland, 1980) who studied in detail the gastropod fauna in acid-polluted and non-polluted lakes in Norway. The literature describing in situ studies of the pH influence on invertebrate distribution, mortality and diversity, being large and quite impressive, will not be quoted in detail in this text, and the reader is advised to study the reports of the SNSF Project in Norway (Leivestad et al., 1976; Overrein et al., 1980), or the review paper of Sutcliffe & Hildrew (this volume) covering the subject.

Laboratory experiments on mortality of freshwater fauna reveal clearly that there exists a critical pH value below which survival is significantly reduced. This critical value is species-dependent but it is also determined by the composition of the testwater.