Introduction

This paper will review the field and laboratory data on the chemical factors which affect fish and fishery survival in acid waters, and an attempt will be made to relate the two sets of data where this is possible. In general, the field data being considered are predominantly from Norway where the fish species is mainly brown trout, but, where relevant, data from North America will be described. Data from the UK are dealt with by Turnpenny (this volume).

The main chemical factors, other than pH (hydrogen ion concentration) that will be discussed are calcium (hardness) and aluminium concentration. The aquatic chemistry of aluminium, which is generally present in higher concentrations in more acid waters, is complex and it is necessary at this stage to summarise the details on the subject given by Freeman & Everhard (1977), Burrows (1977), Hunter et al. (1980), Spry et al. (1981) and O'Donnell et al. (1983).

Solubility of aluminium is a direct function of ambient pH, being at a minimum at around pH 5.5, and increasing towards both extremes of the pH scale. Soluble cationic species e.g. A13+, A1OH2+ and Al(OH)2+, are formed at pH levels less than 5.5, and soluble aluminate species, e.g. Al(OH)4-, predominate at pH levels greater than 5.5. In natural waters, aluminium has a strong tendency to form complexes with other anions capable of forming coordinate bonds – for example, six different fluoride complexes are known (Burrows, 1977).