Bass, A.M., O'Grady, D.O', Berkin, C., LeBlanc, M., Tweed, S., Nelson, P.N., and Bird, M.I. (2013) High diurnal variation in dissolved inorganic C, δ13C values and surface efflux of CO₂ in a seasonal tropical floodplain. Environmental Chemistry Letters, 11 (4). pp. 399-405.

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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 (δ¹³C(DIC)) 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 δ¹³C(DIC) on a tropical floodplain using acidification-interface cavity ring-down spectrometry. Water samples were analysed for δ¹³C(DIC) and other water quality parameters at 15-min intervals for 24 h. Our results show significant diurnal variation in both DIC concentration and δ¹³C(DIC). Maximum DIC concentration, recorded overnight, was approximately 100 % greater than during the day. Maximum DIC concentration coincided with minimum δ¹³C(DIC) 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.

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