Deoxygenation of the eastern Indonesian waters and its variability

Long-term ocean deoxygenation could lead to decline biological productivity and alter biogeochemical cycles. Ocean warming contributions to ocean deoxygenation are reasonably understood, however, there is a challenge to reveal the gaps about other modifying factors to explain different regional patterns and predicts the condition in the coming century. This study aimed to identify the deoxygenation areas in the eastern Indonesian waters, understand the variability of physical and chemical parameters as the deoxygenation drivers, and investigate the correlation between parameters. In-situ and satellite-derived data from 1995 to 2020 were analyzed with statistical methods and remote sensing techniques to enhance deoxygenation measures in higher spatial and temporal resolutions. Our findings revealed that significant deoxygenation was detected around the Arafura Sea. The oxygen minimum zone extended at 133.5° – 136.8° E in the depth of 350 – 1,000 meters, with less than 20 mmol/m3 of dissolved oxygen concentration. Nitrate, phosphate, and temperature were identified to have a strong reversed relationship with the oxygen concentration in the study area. This study also developed multiple regression model algorithms to estimate the oxygen concentration in specified depths.

Differential susceptibility of reef-building corals to deoxygenation reveals remarkable hypoxia tolerance

Ocean deoxygenation threatens the persistence of coastal ecosystems worldwide. Despite an increasing awareness that coastal deoxygenation impacts tropical habitats, there remains a paucity of empirical data on the effects of oxygen limitation on reef-building corals. To address this knowledge gap, we conducted laboratory experiments with ecologically important Caribbean corals Acropora cervicornis and Orbicella faveolata. We tested the effects of continuous exposure to conditions ranging from extreme deoxygenation to normoxia (~ 1.0 to 6.25 mg L−1 dissolved oxygen) on coral bleaching, photophysiology, and survival. Coral species demonstrated markedly different temporal resistance to deoxygenation, and within a species there were minimal genotype-specific treatment effects. Acropora cervicornis suffered tissue loss and mortality within a day of exposure to severe deoxygenation (~ 1.0 mg L−1), whereas O. faveolata remained unaffected after 11 days of continuous exposure to 1.0 mg L−1. Intermediate deoxygenation treatments (~ 2.25 mg L−1, ~ 4.25 mg L−1) elicited minimal responses in both species, indicating a low oxygen threshold for coral mortality and coral resilience to oxygen concentrations that are lethal for other marine organisms. These findings demonstrate the potential for variability in species-specific hypoxia thresholds, which has important implications for our ability to predict how coral reefs may be affected as ocean deoxygenation intensifies. With deoxygenation emerging as a critical threat to tropical habitats, there is an urgent need to incorporate deoxygenation into coral reef research, management, and action plans to facilitate better stewardship of coral reefs in an era of rapid environmental change.