Carbon dioxide dynamics in a lake and a reservoir on a tropical island (Bali, Indonesia)

Water-to-air carbon dioxide fluxes from tropical lakes and reservoirs (artificial lakes) may be an important but understudied component of global carbon fluxes. Here, we investigate the seasonal dissolved carbon dioxide (CO(2)) dynamics in a lake and a reservoir on a tropical volcanic island (Bali, Indonesia). Observations were performed over four seasonal surveys in Bali’s largest natural lake (Lake Batur) and largest reservoir (Palasari Reservoir). Average CO(2) partial pressures in the natural lake and reservoir were 263.7±12.2 μatm and 785.0±283.6 μatm respectively, with the highest area-weighted partial pressures in the wet season for both systems. The strong correlations between seasonal mean values of dissolved oxygen (DO) and pCO(2) in the natural lake (r(2) = 0.92) suggest that surface water metabolism was an important driver of CO(2) dynamics in this deep system. Radon ((222)Rn, a natural groundwater discharge tracer) explained up to 77% of the variability in pCO(2) in the shallow reservoir, suggesting that groundwater seepage was the major CO(2) driver in the reservoir. Overall, the natural lake was a sink of atmospheric CO(2) (average fluxes of -2.8 mmol m(-2) d(-1)) while the reservoir was a source of CO(2) to the atmosphere (average fluxes of 7.3 mmol m(-2) d(-1)). Reservoirs are replacing river valleys and terrestrial ecosystems, particularly throughout developing tropical regions. While the net effect of this conversion on atmospheric CO(2) fluxes remains to be resolved, we speculate that reservoir construction will partially offset the CO(2) sink provided by deep, volcanic, natural lakes and terrestrial environments.