Debris flows amplify effects of wildfire on magnitude and composition of tributary subsidies to mainstem habitats

Postfire disturbances, such as debris flows, may alter the recovery of stream ecosystems from wildfire. Furthermore, changes to productivity and the dynamics of nutrients and organic matter (OM) in tributaries may affect fluxes of materials and organisms to mainstem habitats. We investigated the consequences of watershed wildfire and debris flows for exports from tributaries to mainstem habitats in the South Fork Salmon River Basin of central Idaho, USA. We compared fluxes from 15 streams (n = 5 unaffected [UA], n = 5 fire-affected [FA], and n = 5 fire + debris flow-affected [FDA]) in summer 2011 (4 y postfire, 3 y postdebris flow). In early June, FDA streams exported 650% higher concentrations of sediment to downstream habitats than did UA streams (mean, FDA: 18.7 g/m(3), UA: 1.9 g/m(3)). Overall, concentrations of dissolved organic C were 75% greater in FDA than in UA streams (FDA: 2.8, UA: 1.6 mg/L), and the quality of OM was higher (i.e., more chlorophyll a [chl a] and higher chl a: OM) in FDA than in UA streams. Community composition of invertebrates exported from FA and FDA streams was similar and dominated by r-strategist taxa (Chironomidae, Baetidae, and Simuliidae), but FDA streams exported 470% more invertebrate biomass to downstream habitats than did FA streams (FDA: 8.6 mg/m(3), FA: 1.5 mg/m(3)). Higher primary production and reduced stream retention following debris flows may have contributed to increased fluxes to mainstem habitats. Increased exports of resources from FDA tributaries to mainstem rivers may enhance productivity of the recipient habitat and benefit organisms such as drift-feeding fish.