High diurnal variation in dissolved inorganic C, δ13C values and surface efflux of CO2 in a seasonal tropical floodplain

Abstract

Diurnal variations in aquatic systems may be a major factor influencing carbon cycling. However, few studies have examined diurnal variation on floodplains and wetlands, especially in the tropics. Stable isotope analysis of dissolved inorganic carbon (δ13CDIC) provides insight into the driving factors behind diurnal physio-chemical variability, but to date, the manual collection of large sample numbers at high temporal frequency has been prohibitive. Here, we report one of the first, high-resolution isotopic studies of δ13CDIC on a tropical floodplain using acidification-interface cavity ring-down spectrometry. Water samples were analysed for δ13CDIC and other water quality parameters at 15-min intervals for 24 h. Our results show significant diurnal variation in both DIC concentration and δ13CDIC. Maximum DIC concentration, recorded overnight, was approximately 100 % greater than during the day. Maximum DIC concentration coincided with minimum δ13CDIC as a result of shifting autotrophic/heterotrophic balance. Changes were significant over small time scales and showed CO2 gas evasion estimates could vary by as much as 50 % based on measurements taken less than 5 h apart. These data show that to accurately evaluate the role of tropical floodplains in global carbon dynamics, a comprehensive understanding of diurnal variation will be essential.