What happens to a coral reef when the ocean around it becomes more acidic? The answer may not be limited to slower-growing corals. Some of the smallest organisms on a reef can also play an important role in holding its structure together, and changes in their abundance could alter how the ecosystem develops over time.
At midday, CO₂ levels in the acidified mesocosms exceeded those in the control tanks by an average of about 365 microatmospheres, with considerable variation around that average. The experiment also found that coral calcification declined by 15% to 20% under acidified conditions, while linear extension fell by 14% in one experiment. They can therefore contribute to the accumulation and stabilization of reef material. Because the experiment was done in mesocosms rather than on an entire reef, it cannot be read as a precise forecast for reefs worldwide.
The experiment was conducted in continuous-flow mesocosms at the Hawaii Institute of Marine Biology on Coconut Island, using unfiltered seawater from Kāneʻohe Bay on Oahu. The tanks were kept outdoors in full sunlight, allowing them to experience daily and seasonal changes in temperature and seawater chemistry similar to those on the nearby reef. Researchers increased the partial pressure of carbon dioxide, or pCO₂, in treatment tanks. The treatment was intended to simulate elevated conditions expected during this century. Authors also warned that changes of this magnitude could affect the ability of coral reefs to grow and remain structurally stable. These findings were consistent with a separate Hawaiian mesocosm study published in Nature Geoscience , which found that elevated CO₂ strongly inhibited the recruitment and growth of crustose coralline algae. Together, the findings suggest that ocean acidification could affect both reef-building processes and the balance of organisms competing for space on reef surfaces. The importance of coralline algae goes beyond the amount of space they occupy. Unlike soft-bodied algae, these organisms incorporate calcium carbonate into their structures. A reduction in their abundance could create a ripple effect. If fewer coralline algae grows on exposed surfaces, fewer areas may be available for some coral larvae to settle. At the same time, slower calcification means less carbonate is being added to the reef framework. It does not mean that all reefs will react the same way to future ocean changes. Reefs can vary in their species, water chemical composition, temperatures, and other features of the local ecosystem. However, the experiment did show the system’s vulnerability. Higher concentrations of CO₂ affected not just the water but also the development of organisms that help build reef structure.
Geological Survey , a 10-month controlled experiment in Hawaii exposed reef communities to elevated carbon dioxide levels designed to represent conditions expected later this century. In the acidified tanks, crustose coralline algae covered just 4% of the available surface, compared with 25% in control tanks. The difference represented an 86% relative reduction in coverage.
According to the U.S.
