Can alluvial landforms attenuate post-wildfire lake sedimentation rates and external phosphorus inputs?

Severe large-scale landscape disturbance in forested source water regions by wildfire can increase the transfer of fine sediment and associated phosphorus (P) to aquatic systems. Notably, the subsequent mobilization of sediment-associated P to receiving surface waters can increase primary productivity in lakes and severely degrade water quality. The goal of this research was to examine abiotic controls on the form and mobility of particulate phosphorus (PP) in benthic sediment of an oligotrophic lake in Waterton Lakes National Park (WLNP), Alberta. In 2017, the Kenow wildfire severely burned an area of 35 000 ha which increased the transfer of pyrogenic materials to receiving streams in WLNP. The temporal distribution and concentration profiles of PP forms (non-apatite inorganic P, apatite P, organic P) and P mobility (equilibrium phosphorus concentration, EPC0) and major elements in a sediment core collected from Lower Waterton Lake (LWL) were evaluated to assess the potential of post wildfire fine sediment inputs to influence P concentrations in the overlying water column, and the potential internal loading legacy effect that wildfires may have on lake P-dynamics. Despite elevated PP levels observed in fire impacted tributaries that flow into LWL, the particulate P fractionation data show that post-fire P loading from pyrogenic materials to the lake was extremely low and only very minor post-wildfire changes in TPP speciation were observed. The presence of a large alluvial fan impeding river flow, coupled with the ingress of fine sediment in gravel bed rivers and adjacent floodplains, most likely attenuated the influx of pyrogenic materials into LWL. The mobility of PP in lake bottom sediment corresponded with historical landscape disturbances (e.g., flooding), which are likely associated with the remobilization of deposited pyrogenic materials in the alluvial fan, demonstrating a fire-flood sequence. Although the data presented in this thesis suggests that wildfire will have a minimal effect on the internal loading of P in LWL, it is increasingly acknowledged that non-stationarity due to changing climate may produce flow conditions which will resuspend and deliver pyrogenic materials to the lake, potentially creating a post-wildfire legacy effect. This thesis provides new knowledge regarding the effects of alluvial landscape forms on the post-wildfire delivery and mobility of PP into critical source water areas