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 mug/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 (<100 mu m) on the streambed. The deposited sediment covered most structural elements of the streambed and thereby reduced instream habitat diversity. The fish community of the stream affected by gold mining showed low species diversity, low proportion of young fishes, high proportion of midchannel surface-feeding fishes and fishes adapted to low light, low proportion of visually orienting fishes and fishes that hide in leaf-litter banks and woody debris, and low relative biomass of food fishes. These results suggest that erosion related to small-scale gold mining reduces fish diversity and shifts community structure because of (1) increased turbidity resulting from an increased load of suspended sediment and (2) a reduction of instream habitat diversity related to sedimentation. Gold mining occurred over 7 years and is still occurring, but the sediment stored in the streambed is expected to pose a pollution problem long after the gold miners have left the area.