Effects of wildfire on organic matter sources and macroinvertebrate assemblages in watersheds of central Idaho, USA

Wildfires are frequent disturbances in watersheds of the western US that alter hydrologic flow, sediment dynamics, solar insolation and organic matter inputs. These changes may increase primary producers (periphyton), change sources of organic matter and alter abundance and composition of benthic macroinvertebrate assemblages following fire. We compared 3 pairs of unburned vs burned sites one year after wildfire on streams of similar size (1^st-3^rd order) in the Salmon River Basin, Idaho, to evaluate how fire affects organic matter sources and macroinvertebrate assemblages. Biomass of benthic organic matter was higher in burned than unburned sites, but a large proportion of that material was charcoal. Preliminary observations suggest periphyton biomass was also greater in burned versus unburned streams. Though invertebrate abundance was similar, assemblages of burned streams had lower diversity and greater dominance of r-strategist insects (e.g., Chironomidae, Baetidae, Simuliidae) compared to unburned streams. These results are consistent with those from other studies and support the idea that there are predictable short-term responses to wildfire in stream ecosystems.