Interplay of Seasonal Hydrology and Landscape Topography Drives Black Carbon Export in the Fraser River

Abstract Fire is an important driver of carbon cycling across terrestrial and aquatic ecosystems, but global fire regimes are changing. Black carbon (BC), a product of biomass burning, is more environmentally persistent than its parent biomass carbon and cycles differently than bulk organic carbon. This study aims to refine understanding of the environmental drivers of BC flux from land to ocean through year‐long measurement of BC in the Fraser River in British Columbia, Canada. The Fraser River’s environmental context is distinct from systems that currently form the basis of understanding of BC export, characterized by highly seasonally variable hydrology, and with its basin spanning diverse ecosystems from glaciated mountainous regions to dry flatlands. We found that the Fraser River exported 18,765 ± 2,734 Mg yr −1 of BC, with dissolved black carbon (DBC) comprising 3.3 ± 0.9% of annual dissolved organic carbon (DOC) flux, both lower than previous estimates would suggest. Strong seasonal variation in the DBC content of DOC and BC aromaticity were measured in the Fraser River. This reveals the importance of seasonal hydrology in the export of different pools of BC and indicates that seasonality and hydrologic regime should be given more consideration in future estimations of global riverine BC flux. These findings bring to light the importance of seasonality, hydrology, and basin topography in BC transport, with implications for global carbon cycles in a changing climate.