Bird contact patterns with cyanidebearing substances and associated algal communities

Abstract

Gold mining is a globally important industry, operating across all continents except Antarctica. Historically, South Africa played a dominant role in gold production. Despite a relative decline in recent decades, South Africa remains one of the world's leading gold producers. The gold extraction process from crushed ores use sodium cyanide. The resulting slurry, or tailings, contain potentially toxic elements such as heavy metals and residual cyanide. These tailings are commonly stored in tailings storage facilities (TSFs), return water dams (RWDs), and drainage ditches (DDs). The cyanide containing supernatant is drained and recycled through RWDs and DDs These industrial aquatic environments pose potential ecological threats, particularly to aquatic organisms and birds. This thesis investigates the ecological consequences of active gold mining operations in the Klerksdorp-Stilfontein area, North West province, South Africa. I used a multi-taxon approach as bioindicators; algae, Odonata nymphs and adults, and birds to evaluate the physical and chemical impacts of mining on biodiversity and ecosystem health. Green algae and diatoms were surveyed at mining-impacted and reference sites. Taxon richness and abundance were significantly reduced at mining sites. This correlated with elevated conductivity and heavy metal concentrations in sediments. Cyanide concentrations in water was found to have an inhibitory effect on algae and diatoms at concentrations exceeding 19 mg/L. This suggests that cyanide toxicity may play a greater role in phytoplankton suppression than previously acknowledged. The Odonata survey examined metal concentrations in both aquatic nymphs and terrestrial adults from 19 sites. Sediments from mining sites consistently exceeded sediment quality guidelines for metals such as arsenic, mercury, copper, and zinc. Metal concentrations in nymphs were significantly higher than in adults, confirming their role as sensitive indicators of localised aquatic contamination. Adult dragonflies, however, had metal compositions more akin to the sediments of the waterbodies than the corresponding nymphs. The reasons for this is not quite understood. Adult Odonata are therefore, also reflecting environmental metal concentrations, supporting their utility in broader-scale environmental monitoring. Bird surveys, conducted over two years, recorded 185 species (including six South African Red Data Book species), with significant differences in species richness, abundance, and community composition between mining and reference sites. TSFs and RWDs were frequently used by birds, particularly migratory waders and ducks, despite the presence of cyanide-containing supernatant. Behavioural observations showed 75 species coming in contact with cyanide matrices such as supernatant and TSF shorelines via ingestion and dermal contact. The TSFs, resembling shallow water lakes, raises concerns of ecological traps, where visual cues mislead birds into selecting poor-quality habitats. While weak-acid dissociable (WAD) cyanide concentrations remained below the 50 mg/L limit set by the International Cyanide Management Institute (ICMI), the detection of bird carcasses and frequent contact with cyanide-bearing supernatant suggest that sublethal effects and chronic exposure risks persist. This research highlights the need for species-specific risk assessments, environmental monitoring, and site-specific mitigation strategies to reduce the ecological risks associated with gold mining. It also reinforces the importance of integrating multi-taxa approaches in environmental impact assessments of mining operations.