Small dam impairs invertebrate and microbial assemblages as well as leaf breakdown: a study case from a tropical savanna stream

Globally, dams severely affect the hydrology of lotic ecosystems, impacting biotic assemblages and ecological processes. Although their consequences are not fully explored, especially in the tropics, hundreds of small dams have been constructed worldwide in the last decade for hydropower development and water supply. Here, we assessed the effects of a small dam (ca. 0.14 km(2)) on water quality, leaf breakdown and the associated assemblages (microbes and invertebrates) in three reaches of a tropical savanna stream (upstream and downstream and in the reservoir). Water quality measured as pH, dissolved oxygen, and electrical conductivity were similar in all reaches, but turbidity and temperature were higher and water velocity lower in the downstream reach and in the reservoir. The upstream reach had higher leaf breakdown (as leaf mass loss, 72, 41 and 53%), microbial biomass (98, 74 and 73 nmol ATP g(-1) AFDM), fungal biomass (347, 240 and 260 mu g ergosterol g(-1) AFDM) and invertebrate richness (4, 2 and 3 taxa sample(-1)) than the reservoir and downstream reach. These results suggest that damming a stream can reduce water quality, impoverish invertebrate assemblages, and reduce microbial biomass and carbon processing.