Characterization of the CO 2 system in a coral reef, a seagrass meadow and a mangrove forest in the central Red Sea

The Red Sea is characterized by its high seawater temperature and salinity, and the resilience of its coastal ecosystems to global warming is of growing interest. This high salinity and temperature might also render the Red Sea a very favorable ecosystem for calcification, and therefore resistant to ocean acidification. However, there is a lack of survey data on the CO2 system of Red Sea coastal ecosystems. A one-year survey of the CO2 system was performed in a seagrass lagoon, a mangrove and a coral reef in the central Red Sea, including fortnight seawater sampling and high-frequency pHT monitoring. In the coral reef, the CO2 system means and variability over the measurement period are within the range of other world's reefs with pHT, DIC, TA, pCO2, and Ωarag of 8.016±0.077, 2061±58 μmol kg-1, 2415±34 μmol kg-1, 461±39 μatm and 3.9±0.4. Comparisons with offshore site highlights dominance of calcification and photosynthesis in summer-autumn, and dissolution and heterotrophy in winter-spring. In the seagrass meadow, the pHT, DIC, TA, pCO2 and Ωarag were 8.00±0.09, 1986±68 μmol kg-1, 2352±49 μmol kg-1, 411±66 μatm and 4.0±0.3. The meadow TA and DIC were consistently lower than offshore water. The mangrove showed the highest amplitudes of variation, with pHT, DIC, TA, pCO2, and Ωarag were 7.95±0.26, 2069±132 μmol kg-1, 2438±91 μmol kg-1, 493±178 μatm and 4.1±0.6. We highlight the need for more research on sources and sinks of DIC and TA in coastal ecosystems.