The response of macroinvertebrates to experimental episodes of low pH with different forms of aluminium,during a natural spate

24 h experimental episodes were created in a soft-water stream in upland Wales, by the simultaneous addition at separate points of sulphuric acid, aluminium sulphate and citric acid. In an upstream reference zone (A) the pH remained above 7, while in the treatment zones, B, C and D successively downstream, it was reduced to c. pH 4.9. Concentrations of filterable aluminium were 0.05 mg l^-1 in Zone B, 0.27 mg l^-1 (0.11 mg l^-1 ‘labile’) in Zone C, and 0.23 mg l^-1 (0.00 mg l^-1 ‘labile’) in Zone D. Chemical manipulation coincided with a spate, during which flow increased from 0.02 m³s ^-1 to a maximum of 0.07 m³s^-1. Only the ephemeropteran B. rhodani showed a drift response: drift density was not affected by flow (Zone A) or by organically bound aluminium (Zone D), but increased approximately 6-fold in both the acid (B) and ‘labile’ aluminium (C) zones; its benthic density declined significantly in Zones C and D, by 78% and 89% respectively. We relate these results to the biological importance of aluminium chemistry during natural acidic episodes.