Seasonality and the effect of a marine heatwave on the metabolism of Red Sea coral reefs

Coral reefs are in global decline due to climate change, shifting reef ecosystems from 24-hour net heterotrophy to autotrophy, altering carbon cycles and reducing resilience to stressors. Red Sea coral reefs experience some of the warmest waters globally, exceeding 30 °C and reaching up to 38 °C on reef flats during summer. Despite these extreme environmental conditions, the metabolism of Red Sea coral reefs remains understudied. Here, we measured day/night Net Ecosystem Calcification (NEC) and Net Ecosystem Production (NEP) on two reefs using a semi-Lagrangian approach across different seasons, including the 2023 summer marine heatwave and bleaching event (11–12 °C-degree heating weeks). The heatwave caused a shift toward net heterotrophy and dissolution. Sampled Red Sea reefs exhibited high NEP relative to NEC compared to global averages, suggesting Red Sea reefs may be short-term CO 2 sinks.