Ocean Acidification

The impact of groundwater on pCO₂ variability was assessed in two coral reef lagoons with distinct drivers of submarine groundwater discharge (SGD). Diel variability of pCO₂ in the two ecosystems was explained by a combination of biological drivers and SGD inputs. In Rarotonga, a South Pacific volcanic island, SGD was driven primarily by a steep terrestrial hydraulic gradient, and the water column was influenced by the high pCO₂ (5,501 µatm) of the fresh groundwater. In Heron Island, a Great Barrier Reef coral cay, SGD was dominated by seawater recirculation in permeable sediments (i.e. tidal pumping) and pCO₂ was mainly impacted through the stimulation of biological processes. The Rarotonga water column had a relatively higher average pCO₂ (549 µatm) than Heron Island (471 µatm), however, pCO₂ exhibited a greater diel range in Heron Island (778 µatm) than in Rarotonga (507 µatm). SGD flux rates were quantified using a radon (²²²Rn) mass balance. The Rarotonga water column received 29.0 ± 8.2 mmol free-CO₂ m⁻² d⁻¹ from SGD, while the Heron Island water column received 12.1 ± 4.2 mmol free-CO₂ m⁻² d⁻¹. Both systems were sources of carbon dioxide to the atmosphere (averaging 8.8 ± 3.4 and 2.5 ± 2.1 mmol CO₂ m⁻² d⁻¹ in Rarotonga and Heron Island, respectively), with SGD-derived free-CO₂ most likely contributing to the outgassing of CO₂. Studies measuring the metabolism of coral reefs via changes in carbonate chemistry (e.g. photosynthesis, respiration, calcification, and calcium carbonate (CaCO₃) dissolution rates) may need to consider the effects of groundwater seepage on water column carbonate chemistry and greenhouse gas evasion. Local drivers of coral reef carbonate chemistry such as SGD may offer more approachable management solutions to mitigating the effects of ocean acidification (OA) on coral reefs.

Cyronak T., Santos I. R., Erler D. V., Maher D. T. & Eyre B. D., in press. Drivers of pCO2 variability in two contrasting coral reef lagoons: The influence of submarine groundwater discharge. Global Biogeochemical Cycles. Article (subscription required).