Present and future seawater CO₂ chemistry across multiple coral reef habitats and scales at Dongsha Atoll and Taiping Island in the South China Sea

Abstract

Coral reefs consist of diverse benthic habitats that influence seawater CO2 chemistry variability on multiple spatial and temporal scales. Understanding the present-day seawater CO2 chemistry variability across both habitat-specific and reef-wide scales is critical to accurately predict the effects of future environmental change. Here, we utilize autonomous sensors and discrete seawater samples across diverse habitats at multiple scales ranging from habitat-specific (inner lagoon, patch reefs and seagrass beds; 0.02-0.72km(2)) to reef-wide scales at Dongsha Atoll (250km(2)) and Taiping Island (20km(2)) to characterize seawater chemistry. Across all habitats, daily mean pH ranged from 7.79-8.60 with mean diel variability ranging from 0.19-0.91. Spatially, pH variability ranged from 0.08 (patch reef) to 1.29 (inner lagoon). Biogeochemical modification of seawater chemistry was dominated by organic carbon cycling at individual habitat scales, whereas inorganic carbon cycling dominated at the scale of Dongsha Atoll. The largest alkalinity depletion (net calcification) was associated with patch reef habitats, whereas the highest alkalinity repletion was associated with a semi-enclosed lagoon. Under two climate change scenarios (linear dissolved inorganic carbon increase derived from historical observations and the CMIP6 SSP5-8.5 pathway), pH and/or aragonite saturation state (Omega(Ar)) observations across all habitats in this study are projected to be below proposed thresholds for net reef accretion (pH < 7.7: inner lagoon similar to 10-13%; seagrass beds similar to 21-44%; patch reefs similar to 0-100%; atoll-wide similar to 4-98% of observations) or net dissolution (Omega(Ar) < 2.92: inner lagoon similar to 10-18%; seagrass beds similar to 44-75%; patch reefs similar to 77-100%; atoll-wide similar to 94-100% of observations) by the year 2100. The results highlight the importance of habitat-specific and scale-conscious assessments of future coral reef environmental conditions.