Stream Carbon Flux in the Northern Pacific Coastal Temperate Rainforest: Seasonal DOC Transport and CO₂ and CH₄ Emissions

Abstract: Streams in British Columbia's humid, organic-rich Pacific Coastal Temperate Rainforest (PCTR) deliver globally significant yields of soil-derived dissolved organic carbon (DOC) to the ocean, which can affect ocean acidification, provide energy to coastal food webs, and off-gas to the atmosphere as CO₂. Although there is an established relationship between discharge and DOC, the key subsurface processes controlling stream DOC in this region have not been determined, and the balance between organic carbon export and stream CO₂ and CH₄ efflux in this region is currently unknown. To determine seasonal and landscape controls on stream DOC, CO₂, and CH₄ in the PCTR, we used automated, in situ fDOM sensors in combination with field-based sampling in up to four PCTR watersheds over a two-year period. We analyzed the hysteresis between stream DOC and discharge to develop a seasonal model of subsurface hydrologic connectivity and DOC transport, and used field data to develop reach and sub-basin scale multivariate predictive models of stream CO₂ and CH₄ flux in the PCTR. Determining seasonal and spatial controls on stream DOC, CO₂, and CH₄ in this high-carbon region has critical implications for coastal marine ecology and the global carbon cycle, and will improve our ability to predict how these systems may be affected by environmental and climatic changes in the future.