Ocean acidification can fundamentally change the structure of the reef
Tropical coral reefs can lose up to two-thirds of their zooplankton as a result of ocean acidification. A German-Australian team of researchers came to this conclusion after studying the carbon dioxide seepage at coral reefs off the coast of Papua New Guinea.
Researchers cite the loss of suitable hiding places as the cause of the zooplankton’s decline. Instead of densely branched staghorn corals, helmet-shaped hard corals that offer zooplankton little shelter grow in their place. This change in the coral reef community has come about due to increasing acidification. Since zooplankton is an important food source for fish and coral, the subsequent impact on the food web of the coral reefs are far-reaching, as the research team reports in a study recently published in the online portal of the Nature Climate Change.
For the scientists, the volcanoes off the coast of Papua New Guinea, with their unique properties, are like an open-air laboratory. “Here, we can observe under natural conditions how the reefs change because of climate change, as they absorb more carbon dioxide from the atmosphere and the acidity of the water rises,” said coral expert and co-author Prof Dr Claudio Richter in German. He is the Head of Section Bentho-Pelagic Processes at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research.
The degree of ocean acidification is indicated by the water’s pH value. For the tropics, the ocean usually has a pH value of 8.0 or higher. The lower the value, the more acidic the water, and this may cause the disappearance of coral reef species.
Our study shows that ocean acidification can fundamentally change the structure of the reef, said lead author Joy Smith, from the Australian Institute of Marine Science. Under normal conditions, staghorn corals with their forked branches offer many places for zooplankton to hide; however, under acidified conditions, the hard corals that grow in their place offer limited hiding places, she added.
As a result, reefs lose as much as two-thirds of their zooplankton. “This decline has far-reaching consequences for the reef community. For one thing, many fish species feed on zooplankton. Corals are also dependent on them. With the oceans becoming warmer and more acidic, corals need to use up more energy to build their calcium carbonate skeletons. To satisfy this additional energy requirement, as well as their need for nitrogen and phosphorous compounds, the corals would consume more plankton – something that would be more challenging due to increased ocean acidification,” said Prof Richter.
For this study, the researchers had examined two reefs in the Milne Bay Province of Papua New Guinea on three separate occasions. They found that both reefs had portions with a pH value of 7.8 and also portions with a normal pH value. Thus, they were able to do a comparison at both reefs.
The 29 different groups of zooplankton hide in the reef during the day and then ascend to feed in the upper water column during the night. “To our surprise, up to three groups of zooplankton had been affected by ocean acidification. But none has completely disappeared,” said Prof Richter.
Lead author Joy N Smith was funded through the EU doctoral programme “MARES”. She graduated at the Universities of Bremen and Plymouth.
Link to study: www.nature.com/.../nclimate3122.html