Fluvial dynamics of dissolved and particulate organic carbon during periodic discharge events in a steep tropical rainforest catchment

Abstract

In small catchments with rapid flood pulses, detailed temporal data are essential because high‐discharge events can be measured in hours and days, rather than weeks and months. Using high‐resolution (15 min) sampling, we studied the dynamics of aquatic dissolved and particulate organic carbon (DOC and POC) export through episodic discharge events in a small pristine rainforest catchment in northeast Australia between November 2009 and March 2010. High temporal resolution using this instrumentation requires extensive calibration with concurrent field sampling. The concentration of DOC and POC peaked during times of high stream discharge, reflecting an increased mobilization of soil‐water carbon stocks. DOC was the major form of organic carbon in the stream (< 70% of the total carbon export). The majority of total organic carbon exported from the catchment (84%) occurred during significant discharge events (discharge > 50 L s−1), which occurred only 9% of the time. Export of DOC and POC totaled 195 and 68 kg km−2 month−1, respectively, with a DOC : POC ratio of 2.9 ± 0.9. If this subcatchment was sampled at weekly intervals the lateral export of carbon would have been underestimated by between 49% and 78% for DOC and POC, respectively. Preliminary δ13C and molar C : N values of the dissolved and particulate matter suggest that during discharge events, less microbially processed material from the upper soil layers dominated organic matter export, with the opposite being true in nonflood conditions. Not only will the quantities of organic matter exported change in different discharge conditions, but the source and quality may also shift. This study reveals that a field‐portable instrument for DOC and POC quantification can yield robust, high‐temporal‐resolution carbon budget estimates, though detailed, site‐specific calibration is essential.