Effects of stream ecosystem metabolisms on CO2 emissions in two headwater catchments, Southeastern China

Abstract

Headwater streams, as critical zones due to their proximity to terrestrial sources, emit large quantities of CO2 owing to their higher drainage density of catchments, substantial total areas, and stream lengths. Stream ecosystem metabolism (EM, i.e. photosynthesis and respiration) can significantly affect the in-stream carbon cycle. However, how stream CO2 emissions respond to ecosystem respiration (ER) is still not well known. Here we report in-situ daily observations of headwater stream gross primary production (GPP), ecosystem respiration (ER), isotopic composition of dissolved inorganic carbon (δ13CDIC) and CO2 emissions conducted in five streams in two headwater catchments of the Changjiang River in Southeastern China. The results show that δ13CDIC, pCO2, and CO2 emission rates have significant diel variations. Both the day and night δ13CDIC values were more depleted than the theoretical values in the two catchments, but the nocturnal depletion was greater. The headwater streams have high EM, with an average ER of −1.99 ± 0.82 g C m−2 d−1and GPP of 1.52 ± 0.84 g C m−2 d−1 in the two catchments. ER increases with DOC concentration but decreases with water discharge.The nocturnal pCO2 (3885 ± 2959 μatm) was significantly higher than in daytime (2525 ± 2221 μatm). The higher nocturnal CO2 emission rate (225 ± 165 mg C m−2h−1) was about 42 % higher than in daytime. DOC uptake velocity (Vf) was 0.46 ± 0.14 m d−1, and increased with lower water discharge and lower catchment-scale DOC yield. DOC loss rate (ζ) was 0.96 ± 1.25 d−1 and declined exponentially with higher water discharge and catchment-scale DOC yield. Moreover, the removal fraction of DOC via ER was about 0.32 ± 0.14, contributing to 31 ± 16 % of the CO2 flux in streams. The re-estimated annual CO2 flux was about 14.62 ± 4.99 Tg C yr−1 from the Changjiang headwater streams network. This study emphasizes the necessity of incorporating the effects of EM into the river network's CO2 emissions and the importance of the headwater streams to river carbon cycling.