Abstract
The extent of projected ocean acidification is partly dependent on the natural variability of marine carbonate chemistry—which is higher in coastal systems than in the open ocean. However, there are limited empirical studies quantifying the rate, magnitude and drivers of coastal environmental variability, preventing accurate assessments for how species and their associated communities may respond to projected climate change. Here, we quantified the annual variability of pH, temperature and dissolved oxygen in a coralline algae reef, a globally distributed biodiverse habitat that may be one of the most sensitive to projected climate change. We found that coralline algae and their communities are exposed to pH values as low as those projected for 2100 (even under a low emission scenario) for 63% of the year, including most of autumn and all of winter. Annual fluctuations in pH ranged by 0.46 units, with identifiable patterns at diel to seasonal timescales driven by various biogeochemical factors. Biologically driven patterns in dissolved oxygen and pH were coupled at multiple periodicities, and temperature was coupled to pH during the winter. Tidal cycling additionally modulated biological forcing of pH, increasing the complexity of intra-seasonal pH variability. Forecasting this environmental variability to the future led to projections of new pH extremes well beyond all IPCC emission scenarios. However, persistent long-term exposure to low pH may increase the acclimation and adaptation potential of coralline algae and their associated communities, providing a level of optimism for the continued survival of this habitat despite sensitivity to projected climate change.
Plain Language Summary
Here, we studied how the underwater environment naturally changes during the year on a coastal reef made of coralline algae, a type of red seaweed that builds reef habitats and supports diverse marine life. These reefs are thought to be especially vulnerable to climate change, particularly ocean acidification, which lowers the pH of seawater. Unlike the open ocean, coastal areas naturally experience more variability in pH, temperature, and oxygen. Monitoring these throughout the year, we found that the coralline algae reef already experiences pH levels as low as those expected for the year 2100. In fact, for about two-thirds of the year, including all of winter, the reef was exposed to these low pH conditions. We found that pH levels also varied a lot throughout the day and between seasons, influenced by biological activity of the algae and animals living in the reef, the ebb and flow of the tide, and water temperature. With some optimism, since long-term exposure to low pH is already experienced, these algae and their ecosystems may already be somewhat adapted to future conditions. This gives hope that they will be more resilient to future climate change than previously thought.
Key Points
- Coralline algae are naturally exposed to pH at or below future climate projections, especially during autumn and winter
- This is driven by an interaction between physical factors (temperature, tidal cycling) and biological processes (community metabolism)
- Given future climate projections, these pH lows may become more extreme, but prolonged exposure may increase coralline algae resilience
Burdett H. L., Mao J., Foster G. L. & Kamenos N. A., 2025. Persistence of extreme low pH in a coralline algae habitat. Journal of Geophysical Research: Biogeosciences 130(12): e2025JG009062. doi: https://doi.org/10.1029/2025JG009062. Article.
