Carbon dioxide dynamics in a tropical estuary over seasonal and rain-event time scales

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Tropical estuaries may release large amounts of carbon dioxide to the atmosphere, but remain understudied relative to temperate systems. Here, we investigate hydrological drivers of pCO2 along ~22 km of an aquatic continuum from the mountain river to the coastal ocean, including highly modified aquaculture and urban zones (Perancak Estuary, Ijo Gading River, Bali, Indonesia). We report seasonal spatial surveys (n = 8) and stationary time series observations (n = 4 days) during a rain event. Overall, pCO2 ranged from 330 μatm to 12,126 μatm, with the lowest values observed near the estuary mouth and in the river upstream of the urban zone, and the highest values in the upper estuary where radon (222Rn, a natural groundwater tracer) revealed elevated groundwater discharge. Average atmospheric CO2 fluxes in the upper estuary (107.3 mmol m−2 d−1) were ~5-fold greater than in the lower estuary (19.1 mmol m−2 d−1), while the river was a minor atmospheric CO2 sink (−0.2 mmol m−2 d−1). Overall, the estuary was a source of CO2 to the atmosphere with area-weighted emissions of ~67 mmol m−2 d−1. Seasonal spatial surveys revealed strong correlations between antecedent rainfall and pCO2 and 222Rn in the estuary, implying pCO2 responds to delayed groundwater discharge. A detailed 4-day time series covering an episodic flooding event revealed sharp transitions in CO2 drivers, including dilution by rainfall followed by inputs of CO2-enriched groundwater. Groundwater discharge, as traced by radon, explained most of pCO2 variability in dry and wet conditions. Overall, we highlight the importance of seasonal and episodic rainfall and how delayed groundwater seepage may drive CO2 distribution in a highly modified tropical estuary.