In a surprising discovery, researchers studying spikes in seawater acidity off Bermuda reveal that the coral reef itself can change the local pH. When climate events usher in phytoplankton blooms – which are basically feasts for growing corals – reef productivity goes up, which in turn leads to seawater acidification. The findings were published in Proceedings of the National Academy of Sciences this week.
Rising levels of carbon dioxide in the atmosphere have led to ocean acidification, declining seawater pH, and reductions in calcium carbonate (CaCO3), which corals use to build their skeletons. Since pre-industrial times, oceanic uptake of anthropogenic CO2 has acidified open-ocean surface waters by 0.1 pH units, though less is known about acidification in near-shore and coral reef environments.
Scripps Institution of Oceanography’s Andreas Andersson and colleagues studied the Bermuda coral reef in the Atlantic Ocean from June 2007 to May 2012. They took temperature measurements and collected seawater samples at four separate locations every month to track CO2, pH, and the "saturation state" of the mineral aragonite – a measurement of CaCO3. A reduction in aragonite saturation state means lower CaCO3 production and higher CaCO3 erosion and dissolution.
During the summers of 2010 and 2011, the team observed rapid acidification events driven by enhanced coral growth and calcification, which were likely caused by an increase in food supply thanks to phytoplankton blooms.
Not only does the formation of CaCO3 produce CO2, well-fed corals respire more CO2 than their symbiotic algae can take up through photosynthesis – making the water around them more acidic. Those events coincided with a climate phenomenon known as the North Atlantic Oscillation (NAO). A negative winter NAO state has been linked to higher winds and extensive offshore mixing – which brings up nutrients and triggers productivity blooms in Bermuda during the springtime.
It’s hard to say how this local drop in pH and elevation of CO2 impact the corals – except that it certainly complicates our understanding of how reef ecosystems respond to climate change and ocean acidification. "Many lab experiments suggest that coral calcification is negatively affected by local acidity but we do not really know how important this is if corals have plenty of food, light, and energy," Andersson explains to IFLScience. They may be better prepared to cope than we think.