Lethal and sub-lethal effects of copper upon fish: a role for ammonia toxicity?

Summary

Copper in the aquatic environment

Although traces of copper are essential constituents of some enzymes, copper is toxic to both animals and plants at levels only just in excess of those found in many unpolluted aquatic environments. Levels of dissolved copper are often increased from anthropogenic origins such as mine washings and direct application as an algicide, molluscicide or anti-fouling agent. Natural rock weathering adds about 2 χ 10⁵ tonnes yr⁻¹ of copper into the river systems of the world. In comparison Bowen (1979) estimated an annual addition of 6.19 x 10⁶ tonnes of copper from mining sources. Global copper emissions tripled between 1950 and 1980 (Moore and Ramamoothy, 1984) and this order of magnitude difference between anthropogenic and natural inputs has led copper to be classified as one of the more potentially hazardous heavy metals (Sposito, 1986). While copper concentration in surface waters in general rarely rises above 5μg1⁻¹ or 0.1 μmol 1⁻¹ (Spear & Pierce, 1979), in common with other heavy metals, copper is mobilized during acid episodes. Thus, particularly in those soft, poorly buffered waters that are prone to acidification, copper concentrations can be significantly elevated by acid rain. Turnpenny et al. (1987) found that in mildly acidic streams in Wales, copper and zinc were more important determinants of fisheries status than the acidity itself.

Fortunately, copper can exist in natural waters in a wide variety of physico-chemical forms many of which are of low bioavailability and/or toxicity. Of the divalent ions in the first transition series, irrespective of the ligand type or concentration, copper forms the most stable organic complexes (Irving & Williams, 1953).