Coral reefs more sensitive to CO2 than previously thought

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Coral reefs and their associated animals are more sensitive to global warming than previously thought, according to a new study that shows how ocean acidification wipes out zooplankton populations, which are vital for coral survival. Researchers at the Australian Institute of Marine Science (AIMS) in Queensland studied zooplankton levels in the vicinity of underwater volcanic CO2 seeps to determine how they are affected by water acidity.

“Zooplankton gives corals vital nutrients such as phosphorus and nitrogen as well as important things like trace elements and essential amino acids. Energy from the zooplankton also gets used for making proteins and to fuel sexual reproduction and other necessary biological processes,” explained Joy Smith, a marine biologist and ecologist at AIMS, who led the study.

The results, published today in the journal Nature Climate Change, found some striking effects of CO2 on zooplankton. In waters near underwater seeps, where the high levels of CO2 made water more acidic, zooplankton numbers were reduced by more than half, compared to areas with normal CO2 levels.

Deep in the ocean, underwater seeps release massive amounts of CO2, causing the nearby waters to become more acidic and more like what the oceans will be like at the end of this century, if we continue the current rate of CO2 increase.

The researchers compared the abundance of zooplankton communities in these acidic waters to areas with normal CO2 levels. “We used carbon dioxide seeps as natural laboratories to see how ocean acidification impacted zooplankton communities, the basis of the food chain for coral reefs,” said Joy.

Such significant reduction in zooplankton numbers may be due to ocean acidification changing the coral community structure from branching corals to massive rounded corals, explained Dr Janice Lough, Senior Principal Research Scientist at AIMS. “The zooplankton’s favoured homes (providing shelter in the daytime) are branching corals whose abundance is greatly reduced in more acidic waters in favour of rounded massive corals which are less attractive to the zooplankton,” she said.

Previous research has already shown how ocean acidification can make fish “drunk”. Now, these new findings provide valuable insights on how ocean acidification can also affect coral reefs and associated animals.

“This study underscores the need to drastically reduce the level of future ocean acidification for the maintenance of today’s tropical coral reefs,” said Janice.

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