Cumulative effects of fire in the Fraser River basin on freshwater quality and implications for the Salish Sea

The movement of water from land to the ocean serves as a major biogeochemical link between terrestrial and marine systems, through which terrestrial disturbances can impact freshwater quality and coastal oceans. Wildfire is a major terrestrial disturbance, however its influence on water quality in large freshwater systems and the ocean is understudied. As anthropogenic pressures change fire regimes globally, it is important that this connection is better understood. The Fraser River's basin has significant wildfire history, and the Fraser River has major influence on its receiving waters of the Salish Sea, making this an ideal system in which to investigate the influence of fire on water quality from freshwater to the ocean. This study assessed cumulative impacts of wildfire on Fraser River water quality using historical water quality and fire data. Wildfire in the Fraser River basin explained up to 16.3 % of variance in water quality, and fires burning closer to major waterways had immediate influence on water quality, while farther away wildfires had a delayed influence on water quality. For water quality variables of particular importance in the Salish Sea, wildfire was linked to short term decreases in the riverine concentrations of some constituents, and delayed increases in the concentrations of nearly all assessed water quality variables. These findings have implications for coastal ocean productivity and contamination, and identify fire as an important agent of biogeochemical cycling between land and ocean.