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July 24, 2021

Coral reefs ‘to dissolve in an acid ocean’

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Professor Bradley Eyre sampling benthic chambers at Heron Island that have been acidified in situ to study the effect of ocean acidification on carbonate sediment dissolution. Photo supplied
Professor Bradley Eyre sampling benthic chambers at Heron Island that have been acidified in situ to study the effect of ocean acidification on carbonate sediment dissolution.

Coral reefs may be dissolving faster than expected under future climate change predictions, according to a new study from Southern Cross University marine scientists.

The study indicates that the increasing levels of atmospheric carbon dioxide dissolving into seawater could reduce the rate of calcium carbonate sand formation – the structural foundations of coral reefs – by up to 80 per cent by the year 2100.

The findings are contained in the paper ‘Permeable coral reef sediment dissolution driven by elevated pCO2 and pore water advection‘ which was published recently in the journal Geophysical Research Letters.

The authors are PhD student Tyler Cyronak, Professor Bradley Eyre and Associate Professor Isaac Santos, all with the University’s Centre for Coastal Biogeochemistry Research (CCBR).

‘Ocean acidification (OA) refers to the decrease in the oceanic pH due to increasing atmospheric CO2 dissolving into seawater, and coral reefs are thought to be highly susceptible to this increasing acidity,’ said Mr Cyronak.

‘The formation of coral reefs is the net result of carbonate production by corals minus the loss, or dissolution of this carbonate material, which is expected to increase due to OA.’

CCBR director Professor Eyre said understanding how fast calcium carbonate sands dissolve was important to understanding how coral reefs would be affected by ocean acidification.

‘Calcium carbonate sands make up the majority of the benthic habitat in coral reef ecosystems. But their dissolution is understudied as most scientists are interested in calcium carbonate production by corals,’ Professor Eyre said.

Yet ignoring the dissolution of carbonate sands could be a big mistake, the authors warn.

‘These sands really make up the ‘basement’ of coral reefs,’ Mr Cyronak said.

‘They are the foundation on which corals grow and they provide valuable habitat for a lot of the fish and other organisms that live in coral reefs.

‘Corals will have to grow to keep up with sea level rise, but it may be more difficult if the ‘basement’ of the reef starts to dissolve as well. It will reduce the area in which reefs can grow from both the top and bottom.’

Professor Eyre said most research tended to focus on coral reef carbonate production.

‘But that is only half the story. The dissolution of carbonate sands has been largely overlooked by the research community.

‘In addition, most previous studies on carbonate sediment dissolution have been done in the laboratory and excluded the natural variability in seawater chemistry that coral reefs already experience.’

‘Natural variability’ refers to changes in the seawater chemistry of coral reefs over the course of a day which are largely driven by the photosynthesis and respiration of benthic organisms.

‘The changes in pH can be as large as what is expected for the end of the century,’ said Mr Cyronak.

‘What is unique about our experiment is that we offset the natural variability of seawater pH to what is expected by the year 2100. Most experiments to date have kept the pH of seawater constant, ignoring this large variability over the course of a day.’

By injecting CO2 through a pressurised silicone loop, the authors were able to lower the pH in benthic chambers placed out in the coral reef lagoon (see image).

‘This novel approach allowed us to measure the dissolution of calcium carbonate sands under natural conditions while still altering the pH of the overlying water,’ said Mr Cyronak.

‘Most other experiments that examine calcium carbonate dissolution have done it in the laboratory under highly controlled conditions. However, this is not what happens out in nature.’

The project, ‘Unravelling the synergistic effect of ocean acidification and pore water advection on carbonate sediment dissolution: a global sink’ was funded by an Australian Research Council Discovery grant (DP110103638) worth $310,000.

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