Friday, August 22, 2014

Coral reef ecosystems acidifying 3 times faster than open ocean

New research shows that the rate of ocean acidification in coral reef ecosystems is more than three times faster than in the open ocean, much more than previously thought. This has big implications for reef ecosystems around the globe suffering under multiple stressers of increased sea surface temperatures, nutrient and other pollution and commercial fishing pressures.

Biogeochemist researchers from Southern Cross University Led by recent graduate Dr Tyler Cyronak detailed this new insight in a study published in Geophysical Research Letters titled Enhanced coral reef acidification driven by regional biogeochemical feedbacks.

Ocean acidification is well documented in open waters. Cold waters like in the southern ocean can more readily absorb carbon dioxide, so ocean acidification was thought to be proceeding more rapidly in marine ecosystems in cooler waters (see my 2007 article: Scientists say Ocean Acidity Increasing at Faster Rate)

This new research highlights that coral reef waters are also rapidly acidifying. This research examined studies done in coral reefs since the 1960s and found that the rate of acidification in coral reef ecosystems was more than three times faster than in the open ocean.

To try and work out the cause of this more rapid ocean acidification the researchers developed a model based on work done at Heron Island in the Great Barrier Reef to discover that coral reef respiration may be the cause.

“Our model demonstrated that small changes in the biogeochemistry, or the way coral reefs breathe carbon dioxide, could be to blame,” said Dr Cyronak. “During the day coral reefs take up carbon dioxide because photosynthesis is dominant and at night they release carbon dioxide through respiration. Small changes to this natural daily cycle can have a big impact on the average acidity of coral reef waters.”

Professor Bradley Eyre, director of the Centre for Coastal Biogeochemistry Research and aslo a co-author of the study, outlined the global significance:

“The findings from this study are globally important because coral reefs are under a lot of pressure from climate change,” said Professor Eyre. “Ocean acidification in particular is expected to result in reef loss, which may now be worse than previously expected.”

Another study co-author Associate Professor Kai Schulz comented that increased inputs of organic material and nutrients could be responsible.

“More organic material can change the ratio of respiration to photosynthesis,” said Associate Professor Kai Schulz. “Coral reefs are generally balanced, but if respiration increases it can have a big effect on the average carbon dioxide levels of a reef.”

The good news is that if we can reduce the amount of agricultural runoff from agricultural production and nutrient pollution, this may slow the rate of acidification and the impact on coral reef ecosystems.

“I think there are some positives to these findings,” said Dr Cyronak. “Generally ocean acidification is thought of as a global problem. However, if the pH of a coral reef can be controlled by reducing runoff and terrestrial inputs there may be solutions to help offset the global problem of ocean acidification, or at least the increased threat, on a regional level.”

The abstract for the study reads in full:

Physical uptake of anthropogenic CO2 is the dominant driver of ocean acidification (OA) in the open ocean. Due to expected decreases in calcification and increased dissolution of CaCO3 framework, coral reefs are thought to be highly susceptible to OA. However, biogeochemical processes can influence the pCO2 and pH of coastal ecosystems on diel and seasonal time scales, potentially modifying the long-term effects of increasing atmospheric CO2. By compiling data from the literature and removing the effects of short-term variability, we show that the average pCO2 of coral reefs throughout the globe has increased ~3.5-fold faster than in the open ocean over the past 20 years. This rapid increase in pCO2 has the potential to enhance the acidification and predicted effects of OA on coral reef ecosystems. A simple model demonstrates that potential drivers of elevated pCO2 include additional anthropogenic disturbances beyond increasing global atmospheric CO2 such as enhanced nutrient and organic matter inputs.

In other research from 2012, Sea cucumbers have also been shown to increase the alkalinity of the reef water providing a buffer to the increasing acidity caused by ocean acidification.

Some partially positive news if we take appropriate action. The problem is the Newman National party Government in Queennsland have reduced the red tape allowing for broadscale land clearing for agriculture and removed protections of high conservation value regrowth bushland which may add to nutrient pollution in the rivers. Development of port facilities like at Gladstone is also damaging reef ecosystems due to Coal barons seeking to export coal from major new mines in the Galilee basin and export of coal seam gas to international markets. Greenpeace continues to campaign that coal exports puts the reef in danger and there is a major divestment campaign from banks funding the new coal mines and port expansion which risks stranded assets.

The long term outlook for most coral reef ecosystems is bleak. Marine heatwaves are already decimating corals through bleaching. An experiment on Heron Island shows coral reefs in peril in a high CO2 world. It has already been modelled in peer reviewed studies that atmospheric warming of more than 2 degrees celsius will decimate coral reef ecosystems. Marine scientists have repeatedly called for greenhouse gas emissions to be slashed to save the Great Barrier Reef.

There has been an unprecedented decline in coral growth since 1990 caused by a combination of rising sea surface temperatures and ocean acidification. At the current rate of decline, coral will be unable to grow by 2050, scientists warn. The Great Barrier Reef has lost half its coral cover in the last 27 years, and it could halve again by 2022 say researchers from the Australian Institute of Marine Science. In 2009 Professor Ove Hoegh-Guldberg outlined in a presentation at the Copenhagen UNFCCC talks that Extinction threatens Coral Reefs unless CO2 limited to 350ppm.

  • Southern Cross University media release, 12 August 2014 - Falling ocean pH levels means rising threats for coral reefs
  • Cyronak, T., Kai G. Schulz, Isaac R. Santos and Bradley D. Eyre (2014) Enhanced acidification of global coral reefs driven by regional biogeochemical feedbacks. Geophysical Research Letters (14) 41 DOI: 10.1002/2014GL060849 (abstract)