Hydrological drivers of carbon dioxide cycling from headwaters to the coastal ocean on a tropical island

This thesis investigates the hydrological drivers of carbon dioxide cycling from mountain headwaters to the coastal ocean in modified and unmodified systems on a tropical island (Bali, Indonesia). The central hypothesis is that a combination of rainfall and groundwater seepage drive aquatic CO₂ fluxes. Chapter 1 provides a general review of the topic. Chapter 2 examines atmospheric CO₂ fluxes in a reservoir and a tropical volcanic lake. The highest pCO₂ occurred in the wet season in both systems. In the natural lake, the metabolism of surface water was an important driver of CO₂ dynamics, while in the reservoir, groundwater seepage was the major CO₂ driver. As the natural lake was a sink of atmospheric CO₂ and the reservoir a source of CO₂ to the atmosphere, future increases in reservoir construction may partially offset the natural lake CO₂ sink. Chapter 3 examines carbon dynamics in a highly modified estuary. In the upper estuary atmospheric CO₂ fluxes were five-fold that of the tidally dominated lower estuary, while the mountainous river atmospheric CO₂ fluxes were negligible. The estuary source of CO₂ to the atmosphere was comparable to high emission temperate estuaries and higher than the reported averages from tropical estuaries. Rainfall caused a delayed pCO₂ response due to groundwater seepage entering the estuary. Chapter 4 examines the interconnectivity of mangrove forests, seagrasses and the coastal ocean in a tropical embayment with no river inputs. The CO₂ source was associated with delayed groundwater inputs from the mangrove-dominated upper embayment, shifting to a CO₂ sink in the lower seagrass-dominated embayment. Chapter 5 summarizes the thesis. Overall delayed groundwater seepage, as a result of episodic rainfall events, played an important role on carbon dynamics along the aquatic continuum. Modification of the tropical aquatic landscapes investigated, including drainage of mangroves, removal of seagrass beds, and construction of reservoirs has the potential to increase aquatic CO₂ emissions to the atmosphere.