About This Project

Bleaching alerts rely on one temperature threshold, yet tropical reefs also face episodic low oxygen. The Metabolic Index, which compares oxygen supply to demand, predicts habitat limits at large scales and timescales, but has not been tested at reef scales. Using existing hourly reef records and coral physiological traits, we test whether it predicts bleaching better than degree heating weeks, how much species detail is needed, and if it works on different reefs and with different species.

Ask the Scientists

What is the context of this research?

Coral reefs face increasingly frequent marine heatwaves and low-oxygen episodes. Bleaching forecasts rely on degree heating weeks (DHW), a temperature-only index (Liu et al. 2005), yet reefs can change in temperature and oxygen within hours, making low oxygen a potentially hidden threat (Altieri et al. 2017; Lucey et al. 2023). As temperatures rise, corals need more oxygen while declining oxygen limits supply, pushing them toward metabolic tipping points. The Metabolic Index captures this balance using each species’ critical oxygen threshold (Pcrit) (Deutsch et al. 2015). In Panama, hourly reef data showed bleaching tracked time below these thresholds more closely than heat alone, and only a third of bleaching events reached DHW ≥4 (Lucey et al., in press). Whether this holds elsewhere in the Caribbean, and how well traits transfer across coral species, populations, and reef histories, is unknown.

What is the significance of this project?

Managers already act on DHW because it is simple and satellite-based. Oxygen is the missing piece, and it is measurable with low-cost loggers. A threshold that identifies which reefs and species that face oxygen-driven bleaching, even when heat is low, could redirect monitoring, restoration, and watershed management. In Panama, branching Acropora cervicornis bleached after two weeks of nightly low oxygen, while Siderastrea siderea needed over a month of exposure, highlighting how species differences matter. With these datasets, we can now test how transferable this approach is in different parts of the Caribbean, including Puerto Rico, where compound heat and low oxygen extremes have been intensifying.

What are the goals of the project?

We start from DHW, the operational bleaching index. Using existing hourly temperature and oxygen records and bleaching observations from available monitoring on Panamanian and Puerto Rican reefs, we will score DHW and the Metabolic Index against observed bleaching, counting alerts that preceded bleaching, alerts that did not, and bleaching events that no alert caught. To understand how much biological variation matters, we will run the Metabolic Index at three levels: (1) a lumped average of coral traits across eight common Caribbean species (broad), (2) Species-specific Pcrit traits (species), and (3) species traits plus population variability (site). We will apply each level to the highly resolved oxygen and temperature records from reefs in both countries and compare how well it predicts bleaching across different frequencies, durations, and intensities of heat and low-oxygen events. All code and data will be openly accessible.