Seasonal patterns in riverine carbon form and export from a temperate forested watershed in Southeast Alaska

Riverine export of carbon (C) is an important part of the global C cycle; however, most riverine C budgets focus on individual forms of C and fail to comprehensively measure both organic and inorganic C species in concert. To address this knowledge gap, we conducted high frequency sampling of multiple C forms, including dissolved organic C (DOC), inorganic carbon (as alkalinity), particulate organic C (POC), coarse particulate organic C (CPOC), and invertebrate biomass C across the main run-off season in a predominantly rain-fed watershed in Southeast Alaska. Streamwater concentrations were used to model daily watershed C export from May through October. Concentration and modeled yield data indicated that DOC was the primary form of riverine C export (8708 kg C/km²), except during low flow periods when alkalinity (3125 kg C/km²) was the dominant form of C export. Relative to DOC and alkalinity, export of particulate organic C (POC: 992 kg C/km²; CPOC: 313 kg C/km²) and invertebrates (40 kg C/km²) was small, but these forms of organic matter could disproportionately impact downstream food webs because of their higher quality, assessed via C to nitrogen ratios. These seasonal and flow driven changes to C form and export likely provide subsidies to downstream and nearshore ecosystems such that predicted shifts in regional hydroclimate could substantially impact C transfer and incorporation into aquatic food webs.