Carbon cycling and exports over diel and flood-recovery timescales in a subtropical rainforest headwater stream

摘要

Catchment headwaters comprise the majority of all stream length globally, however, carbon (C) dynamics in these systems remains poorly understood. We combined continuous measurements of pCO_2 and radon (~(222)Rn, a natural groundwater tracer) with discrete sampling for paniculate organic, dissolved organic and inorganic carbon (POC, DOC, and DIC) to assess the short-term carbon dynamics of a pristine subtropical headwater stream in Australia, over contrasting hydrologic regimes of drought, flash-flooding and recovery. Observations over 23 days revealed a shift from carbon losses dominated by CO_2 outgassing under conditions of low flow (66.4 ± 0.4% of carbon export) to downstream exports of carbon during the flood (87.8 ± 9.7% of carbon export). DOC was the dominant form of downstream exports throughout the study (DOC:DIC:POC = 0.82:0.05:0.13).The broadest diel variability among variables occurred during the drought phase, with diel variability up to 662 μatm d~(-1) (or 27 μM [CO_2~*] d~(-1)), 17 μM d~(-1) and 268 Bq m~(-3) d~(-1) for pCO_2, dissolved oxygen and ~(222)Rn, respectively. Diel dynamics indicated multiple interrelated drivers of stream water chemistry including groundwater seepage and in-stream metabolism. The catchment exported terrestrial carbon throughout the field campaign, with a mean net stream flux of 4.7 ± 7.8 mmol C m~(-2) (catchment area) d~(-1) which is equivalent to 1.4 ± 2.3% of the estimated local terrestrial net primary production. Our observations highlight the importance of accounting for hydrological extremes when assessing the carbon budgets and ecosystem metabolism of headwater streams, and provide a first estimate of aquatic carbon exports from a pristine Australian subtropical rainforest.