Downstream Effects of Erosion from Small-Scale Gold Mining on the Instream Habitat and Fish Community of a Small Neotropical Rainforest Stream

Small-scale gold mining has caused widespread siltation of South American streams, but little information is available on the effects of an increased load of suspended and deposited sediment on diverse Neotropical fish communities. We used a paired watershed design to compare the instream habitat and fish community structure of an undisturbed rainforest stream and a stream affected by gold mining. In the period 1994-2001, the affected stream showed high turbidity (424-2874 nephelometric turbidity units), high concentration of suspended sediment (318-2468 mg/L), and elevated concentrations of potassium, aluminum, and iron. Mercury was detected in the low-water season (0.67 µg/L). The main fractions of P, Al, and Fe were associated with suspended sediment and were not in solution. The sediment load of the polluted stream (3.10 ± 0.77 tonnes/year/ha) was mainly (95.6%) produced by the eroding goldfields. Sedimentation was evident from the accumulation of a 23-cm-thick layer of fine sediment (