Monitoring Coral Reef Metabolism Under Changing Oceans–Novel Insights From Seawater Stable Carbon Isotopes

Mounting environmental stressors are driving Caribbean reefs from coral to sponge and macroalgae dominance, necessitating a need for more nuanced metrics of reef metabolism under trophic transitions. Drawing upon four seasonally replicated field campaigns to Curaçao, we reveal highly variable net ecosystem productivity (−243 ± 69 C m⁻² day⁻¹ to 216 ± 36 mmol C m⁻² day⁻¹) and calcification (−143 ± 42 mmol CaCO₃ m⁻² day⁻¹ to 140 ± 21 mmol CaCO₃ m⁻² day⁻¹). We observe periods of intense net ecosystem dissolution that fall significantly below previously reported global averages. We offer two additional metabolic insights by integrating measurements of the stable carbon isotope composition of dissolved inorganic carbon (δ¹³C_DIC). First, Keeling plots of water column DIC and δ¹³C_DIC measurements suggest that zooplankton grazing may be a significant source of respired carbon to the reef ecosystem, even during periods of net autotrophy. This suggests that heterotrophic feeding by corals or other consumers is a primary energy pathway that may provide the mixotrophic coral community with a stable food source and bolster its resilience to periods of symbiont stress. Second, by isolating photosynthetic fractionation factors, we reveal a potential carbonate chemistry dependence on algal community photosynthetic plasticity. This provides an important fingerprint of how the dominant primary producers physiologically respond to changing ocean conditions, which is essential for forecasting future shifts in the community and its metabolic function. These findings suggest that integrating stable isotopes into reef biogeochemical toolkits offers unprecedented insights into community interactions in a changing ocean.