Ocean acidification & coral reefs

Written by George Brett

In 2019 most of us are becoming more aware of humanity’s ever-increasing carbon output and its subsequent impacts on the natural world. It seems like nowadays you cannot turn on the TV without hearing mention of climate change and all its associated nasties. But when was the last time you remember turning on the TV and hearing about ocean acidification? In my own personal experience it appears that, outside of academic circles, no one is giving this global phenomenon the respect that it’s due. But the potential ecological, sociological and economic consequences of unchecked global oceanic pH decrease (a fancy way of saying increasingly acidic waters) cannot be understated. As we shall see, this is as much a human issue as it is an environmental one.

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Photo by Izzy Brett

While impacting nearly all marine organisms in one-way or another ocean acidification is particularly deleterious to marine calcifiers such as corals. Initially this piece was an attempt to summarise the varying effects of increasing water acidity on marine organisms across the board, but given the scale to which acidification is impacting reef systems today it is worthy of a piece dedicated to it as a whole.

What is ocean acidification?

In the same way as forests do, the oceans also act as crucial carbon sinks for our planet. This means they absorb and trap the carbon dioxide present in the earth’s atmosphere. The oceans are in fact directly responsible for making earth habitable for life today. Slowly but surely (over millions and millions of years) phytoplankton transformed earth’s atmosphere by absorbing atmospheric CO2 and emitting oxygen back into it via photosynthesis. These tiny microscopic creatures essentially terraformed the planet and kick started the journey towards life as we know it today, by creating a balanced atmosphere.

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Photo by Izzy Brett

Today however we are faced with a problem. Human activity has increased atmospheric CO2 levels at a rate faster than the environment can keep up with. In the last 250 years (since the dawn of the industrial revolution) atmospheric CO2 has increased from 280 parts per million to around 384, an increase of 40%. Such a rapid increase has had a direct effect on global water chemistry.

When CO2 gets absorbed by sea water chemical reactions occur and positively charged hydrogen ions (H+) are left as a by-product. The presence of H+ ions in the water decrease its pH, making it more acidic. So as carbon emissions increase so does the concentration of H+ ions present in the water. As a result years of human driven carbon emission increase has not only disturbed the balance I was talking about earlier, but is now beginning to fundamentally change global oceanic water chemistry. Surface oceanic pH has decreased by 0.1 pH units since the industrial revolution began. This may not sound like much, but as the pH scale is measured logarithmically this shift of 0.1 units represents a 30% increase in oceanic acidity.

Impacts on coral reefs

Corals are marine calcifiers, meaning they grow their skeletons by combining a calcium ion with a carbonate ion to form calcium carbonate. The nature of their environment means reefs are being constantly being eroded naturally, be it from animals like Parrotfish or from the physical erosion of water and waves. This means that they must continually produce their skeleton at a rate greater than that of the erosion, or they will completely fade away.

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Photo by Izzy Brett

But the increased H+ ions from the acidifying ocean are now bonding with carbonate ions in the seawater instead of calcium ions. Which unfortunately means corals cannot gather the carbonate required for skeleton building and are subsequently unable to build their skeletons. Which means coral reefs are unable to maintain their growth/calcification rate and are being eroded away.

A 2006 experiment found that current levels of atmospheric carbon have stunted both coral growth and calcification by <40%. These results draw us to the worrying realisation that oceanic pH decrease is not only destroying existing coral structures, but is also inhibiting the growth of new ones. A more recent study in 2018 predicts that all corals globally will “have transitioned into a state of net dissolving” in predicted 2080 conditions. A devastating conclusion.

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Photo by Izzy Brett

Oceanic pH decrease has also been linked to an increase in bleaching events, a process by which corals expel their symbiotic photosynthetic algae. Bleaching events are detrimental for corals, as much of their energy uptake comes from these alga known as ‘zooxanthellae’. The coral provides its resident zooxanthellae with shelter, water, CO2 and sunlight required for them to photosynthesise. In return for which they receive most of the energy, known as photsynthate, that the zooxanthellae create. But as oceanic pH continues to decrease corals are continually being denied access to this integral energy source as bleaching events increase.

Between 20-45% of the gathered photosynthate goes towards coral mucus production. This mucus is essential for coral health as it acts as a barrier against physical damage and the entry of various pathogens. Studies have shown that oceanic acidification can aid and abet both the spread and effects of marine pathogens. Therefore we are also seeing a trend in which increasing levels of acidification are systematically weakening coral defences and bolstering both the presence and the virulence of marine pathogens that impact them.

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Photo by Izzy Brett

Ocean acidification has also been shown to interfere with coral recruitment. It does so by disrupting the chemical cues that coral larvae use to find suitable settlement locations. Estimated CO2 concentrations for the end of the century are predicted to reduce coral recruitment by ~45%. Once again, we see a double whammy effect here. Not only are coral larvae failing to settle and subsequently develop into a polyp state, the ones that do manage to do so have their growth severely stunted and can often fail to reach maturity. This means that generation by generation we are seeing less mature adult corals, which in turn means there are less larvae generated during spawning events, of which fewer and fewer are successfully settling. As generations pass and water acidity continues to increase a frightening future begins to present itself.

Implications for humans

Coral reefs are of great importance to human communities worldwide. They are major constituents of the ‘environmental economics’ concept outlined by Robert Costanza in his high impact paper ‘The value of the worlds ecosystem services and natural capital’. In this paper Costanza writes that corals are “… critical to the functioning of the Earth’s life support system. They contribute both directly and indirectly, and therefore represent part of the total economic value of the planet”.

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Photo by Izzy Brett

Conservative figures from NOAA (US National Oceanic and Atmospheric Administration) state that American coral reef systems pump over $3.4 billion into the United States economy each year. This number may seem inconsequential to diffuse economies like that of the United States but to tourist dependant economies, like that of the Maldives, the cost of losing major tourist attractions like coral reefs can have detrimental effects to the country. The Maldives have already seen 68% of their coral reef systems shift from vibrant coral based communities to bleak algal dominated barons. By providing other services such as wave energy buffering, and thus coastal protection, reefs also save countries millions by preventing potential expenditure . In 2019 NOAA estimated that in total half a billion humans rely on coral reefs for food and livelihood. So with increasing carbon emissions and subsequent ocean acidification, human communities are going to be drastically affected by the resulting loss of coral reefs.

To sum up

Corals are to be considered a key-stone species. This means that any impact to them can disproportionately affect other species in the surrounding eco-system. Reefs fundamentally shape eco-system dynamics and are considered the most biodiverse places on the planet. Housing myriad species, holding up eco-systems and generating both food and wealth for surrounding human communities the importance of these natural wonders cannot be understated. As atmospheric carbon levels continue to increase and water pH continues to decrease, reefs are under more pressure now than ever. The realisation of the cost that ocean acidification poses to human communities should highlight both the importance and scale of the issue, thus prompting well guided further research based in a sound understanding of the issue at both a global and local level.

 

George is an integrated masters student studying Marine Biology with Zoology at Bangor university in Wales. He has a passion for all things sharks and reefs, with a keen interest in ethology. You can follow him on Twitter @gbrett96 or contact him directly via email at g_brett_96@hotmail.com. The amazing photographs in this article were taken by George’s sister Izzy who can be found on Instagram @izzybrettphotography.

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