Reversal of ocean acidification enhances net coral reef calcification
Key finding. Raising the alkalinity of seawater flowing over a One Tree Reef flat, which lifted the aragonite saturation state by about 0.4 units toward pre-industrial values, increased net community calcification by 6.9 ± 0.9 per cent — the first seawater chemistry manipulation of a natural coral reef community, and evidence that reef growth is already depressed by acidification.
What question did this research address?
Coral reef calcification has declined over recent decades, but attributing that decline to ocean acidification has been close to impossible. In the field, saturation state co-varies with temperature, light, nutrients, and productivity, so a correlation with reef growth identifies nothing on its own. Laboratory and mesocosm work isolates the chemistry but not in a natural community.
This paper asked whether the chemistry could be manipulated in the field instead — pushing a real reef's seawater back toward pre-industrial conditions and measuring what its calcifiers did in response.
What did we find?
The experiment exploited a natural flow. At One Tree Reef in the southern Great Barrier Reef, First Lagoon sits about 30 centimetres above Third Lagoon, so for roughly 60 minutes after each low tide water runs one way across the reef flat between them.
On 15 of 22 days, sodium hydroxide and the inert dye Rhodamine WT were pumped onto that flat together; on the other 7 days dye alone was added as a control. Comparing the ratio of alkalinity to dye upstream and downstream separates biological uptake from mixing and dilution, since only alkalinity is consumed by calcification.
An average of 17.3 ± 2.3 per cent of the added alkalinity was taken up by the reef community. Upstream and downstream alkalinity-to-dye slopes differed significantly on experiment days and not on control days.
Converting that uptake to a rate gives a 6.9 ± 0.9 per cent increase in net community calcification, significantly greater than on control days.
The size of the response is what the chemistry predicts. Aragonite saturation state was raised by an average of 0.4 units across the study area, and the roughly 15 per cent calcification response per saturation unit found in laboratory work implies a 6 per cent increase — close to the 6.9 per cent observed, though the comparison between single-species studies and a mixed reef community should be treated cautiously.
Tropical surface waters have fallen from an aragonite saturation state of about 4.5 before industrialization to about 3.8 by 1995, and are projected to reach roughly 3.0 by mid-century and 2.3 by 2100.
Why does it matter?
It converts a correlation into an experiment. Because alkalinity was added directly, its effect is uncoupled from temperature, light, and the other drivers that move with it in nature — so the reef's response can be attributed to carbonate chemistry rather than merely associated with it.
The direction of the manipulation is what makes the inference work. Restoring chemistry toward pre-industrial values increased calcification, which means present-day calcification is below its pre-industrial value. Acidification is not only a future risk to reef growth; it is already subtracting from it.
Deliberate alkalinity addition has been proposed as a way to shield reefs from acidification. These results show it could work in principle, but the volumes involved make it plausible only in highly confined settings such as protected bays and lagoons. Protecting reefs at ocean scale still requires deep and rapid cuts in carbon dioxide emissions.
Citation
Rebecca Albright, Lilian Caldeira, Jessica Hosfelt, Lester Kwiatkowski, Jana K. Maclaren, Benjamin M. Mason, Yana Nebuchina, Aaron Ninokawa, Julia Pongratz, Katharine L. Ricke, Tanya Rivlin, Kenneth Schneider, Marine Sesboüé, Kathryn Shamberger, Jacob Silverman, Kennedy Wolfe, Kai Zhu, and Ken Caldeira (2016). Reversal of ocean acidification enhances net coral reef calcification. Nature 531, 362-365.