Dynamics of ocean acidity, CO2 fluxes and metabolic rates on a shallow reef of Weizhou Island: a buoy-based observational study

Abstract

The metabolic processes of calcification and production serve as crucial indicators of how environmental changes impact reef health. Previous studies suggest that Net Ecosystem Production (NEP) primarily drives Net Ecosystem Calcification (NEC) in the short-term. However, the functional relationship between these two carbon metabolisms remains poorly understood. We employed a mooring buoy approach to obtain simultaneous, high-frequency data of seawater pH, aragonite saturation state, CO₂ fluxes, and carbon metabolic rates over a coral reef on Weizhou Island for 37 consecutive days. Our findings revealed a strong linear correlation between NEC and NEP across both diel cycles and day-to-day timescales—this relationship held even when analyzing nighttime periods alone. This indicates an intrinsic link between carbon metabolisms that can operate independently of light. Furthermore, we observed predominantly negative daily NEC and NEP values, indicating persistent net CaCO₃ dissolution and net heterotrophy across the studied reef for over weeks. Our results suggest that CaCO₃ dissolution is more likely to occur in waters with heterotrophic conditions, implying that heterotrophy contributes to CaCO₃ dissolution. This tight coupling could be explained by reef sediment dissolution through the Carbonate Critical Threshold (CCT) mechanism. Our study highlights the significance of ambient respiration in driving reef ecosystem-scale CaCO₃ dissolution, especially in reefs with low live hard coral coverage. This process releases alkalinity into the seawater, helping to neutralize respiration-induced acidification. Additionally, we identified a higher rate of respiratory CO₂ release as the primary driver of CO₂ emissions from the studied reef.