High diurnal variation in dissolved inorganic C, δ¹³C values and surface efflux of CO ₂ in a seasonal tropical floodplain

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.