The dynamics and export of dissolved organic carbon from subtropical small mountainous rivers during typhoon and non-typhoon periods

Abstract. Small mountainous rivers (SMRs) are important conveyors of the land-to-ocean organic carbon export. However, relatively few studies have focused on dissolved organic carbon (DOC) compared to particulate organic carbon. In a long-term project (2002 to 2014), stream DOC was monitored in three neighboring subtropical small mountainous rivers of Taiwan. The objective was to relate DOC concentrations to water discharge and to quantify DOC flux during typhoon and non-typhoon periods. Seasonal fluctuations of DOC concentrations were closely correlated with air temperature at all sampling stations. During non-typhoon periods, increasing water discharge led to decreasing DOC concentrations due to a dilution effect. However, during typhoon periods, DOC concentrations increased with some lead time along the hydrograph and reached the annual maximum which likely sources from a significant input of litter and upper soil layers. The mean DOC concentration of the studied systems (< 1.0 mg L⁻¹), is ranked in the lowest 1 % among the world rivers. However, mean DOC yield (~ 30 kg ha⁻¹ y⁻¹), is ranked in the top 30 %, which is attributed to high rainfall and substantial organic carbon stocks in the watersheds. Up to 25 ± 5.6 % of the annual DOC flux was contributed by typhoon events, which occupied ~ 3 % of the monitoring period. We conclude that typhoon events are important drivers for the land-to-ocean export of dissolved organic matter. Predicted future increases in frequency and magnitude of typhoon events will likely accelerate the release of terrestrial carbon and enhance its land-to-ocean transfer via dissolved organic matter.