Sediment‐phosphorus dynamics can shift aquatic ecology and cause downstream legacy effects after wildfire in large river systems

Abstract

Global increases in the occurrence of large, severe wildfires in forested watersheds threaten drinking water supplies and aquatic ecology. Wildfire effects on water quality, particularly nutrient levels and forms, can be significant. The longevity and downstream propagation of these effects as well as the geochemical mechanisms regulating them remain largely undocumented at larger river basin scales. Here, phosphorus (P) speciation and sorption behavior of suspended sediment were examined in two river basins impacted by a severe wildfire in southern Alberta, Canada. Fine‐grained suspended sediments ( TPP ) and particulate phosphorus forms (nonapatite inorganic P, apatite P, organic P), and the equilibrium phosphorus concentration ( EPC 0 ) of suspended sediment were assessed. Concentrations of TPP and the EPC 0 were significantly higher downstream of wildfire‐impacted areas compared to reference (unburned) upstream river reaches. Sediments from the burned tributary inputs contained higher levels of bioavailable particulate P ( NAIP ) – these effects were also observed downstream at larger river basin scales. The release of bioavailable P from postfire, P‐enriched fine sediment is a key mechanism causing these effects in gravel‐bed rivers at larger basin scales. Wildfire‐associated increases in NAIP and the EPC 0 persisted 6 and 7 years after wildfire. Accordingly, this work demonstrated that fine sediment in gravel‐bed rivers is a significant, long‐term source of in‐stream bioavailable P that contributes to a legacy of wildfire impacts on downstream water quality, aquatic ecology, and drinking water treatability.

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