Physical pollution: effects of gully erosion on benthic invertebrates in a tropical clear-water stream

1. Many tropical streams are situated in geologically old, weathered landscapes that are prone to erosion. Permanent or seasonally pulsing inputs of eroded material into the streams may have significant effects on benthos and habitats, even if chemical water quality remains unaltered. 2. This study presents data from the Cerrado Region, Brazil, where the semideciduous forests have been largely converted into agricultural areas with considerable erosion problems. In order to quantify impacts of erosion on the benthic community, a simple, inexpensive, standardized artificial substrate was exposed on the stream bed above and below the mouth of an erosion gully in two experiments at the beginning and at the end of the rainy season. 3. While the abundance and biomass of most of the biota in the reference sites did not change during the rainy season, it decreased significantly at the impact sites below the mouth of the erosion gully. It was the combination of rainfall-driven flood pulses and increased load of suspended particles from erosion which caused the decline of the benthic colonization in these streams rather than the hydrological disturbance alone. 4. Site-to-site comparisons revealed a highly significant reduction in density, biomass and taxon richness of the benthic invertebrates caused by siltation. All benthic insect taxa studied showed the same pattern, indicating a general impact of erosion on habitat quality and food sources. Semiaquatic insects adapted to shifting habitat conditions, terrestrial food sources and aerial respiration were the most resistant invertebrate group. 5. Restoration schemes for the stream catchments are urgently needed to reduce local population extinctions due to impassable stream sections. Copyright (c) 2006 John Wiley & Sons, Ltd.