What is the problem?

Background of Coral Reefs

Figure 1: Healthy coral reef

Coral reefs are underwater ecosystem that can be found in a lot of coastal regions between Tropics of Cancer and Capricorn[1]. They are made of thin layers of calcium carbonate (CaCO3) by reacting free calcium ions (Ca2+) and carbonate ions (CO32-) in ocean. Their shells can be damaged by many environmental factors. For instance, increase of temperature due to global warming leads to coral bleaching; grazing from herbivores results in erosion[2]. Under normal conditions, coral reefs’ accretion rate of CaCO3 is sufficient to repair the damages. However, ocean acidification inhibits this process and threatens their survivability.

Ocean Acidification and its effect on Coral Reefs

Ocean acidification occurs as more carbon dioxide in the atmosphere is dissolved into ocean and forms carbonic acid. As it decreases the pH of seawater, less amount of CO32- can be stored in ocean. The availability of free CO32- for coral reefs is further lowered since hydrogen ions from acid react with carbonate ions in water to form bicarbonate ions[3]. These two factors directly decrease the rate of production of CaCO3 skeleton. Moreover, reduction in carbonate ions lowers the saturation state, meaning calcium carbonates are more likely to be dissolved in shallow depths[4]. It is suggested that coral reefs’ skeletal density is sensitive and somewhat proportional to the amount of carbonate ions present in surroundings[5]. This would indicate that coral reefs become weaker and more prone to environmental stressors such as pollution, waves and predators as the rate of calcification falls behind the rate of consistent damages[6]. With this in mind, ocean acidification can lead to destruction or even distinction of coral reefs habitat.

Figure 2: Coral bleaching in Hawaii

Human actions that cause Ocean Acidification

Ocean acidification is a result of two main human actions that raise the atmospheric carbon dioxide level – combustion and deforestation[7]. Combustion of organic matter such as biomass and fossil fuels can release useful energy, and societies have been capitalizing on this chemical reaction to support their basis and infrastructure for many years[8]However, as this process also produce carbon dioxide, the heavy reliance has resulted in skyrocketing atmospheric carbon dioxide concentration. Meanwhile, deforestation lowers the amount of photosynthesis in vegetations, less CO2 can be absorbed and stored[9].In addition, it promotes the release of greenhouse gases (including CO2) from plants[9]. As combustion and deforestation produce net increase in CO2, they intensify ocean acidification and in turns threaten many marine communities including coral reefs.

Pervasiveness of Ocean Acidification on Coral Reefs

Coral reefs provide direct shelter, food and substrates to 25% of marine organisms[10]. Currently, ocean acidification is rapidly destroying many coral reef habitats and their associated ecosystems, this can potentially cause cascade effects that are harmful to marine environments. In addition, coral reefs can play a role in our economy. They can be used as a food, medicinal[11] and tourism source. Since coral reefs have tremendous significance on many organisms, effects from ocean acidification will bring many pervasive negative impacts.

How does this problem impact marine ecosystems?

Effects on Individuals

Figure 3: A transparent mollusc shell that has been exposed to acidification (top view).

Ocean acidification may negatively impact many marine organisms, including species that have proven to be integral components of coral reefs. Amongst the most affected by decreasing ocean pH are calcifying organisms such as corals and molluscs, that use carbonate ions found in seawater to form skeletons and shells of calcium carbonate (CaCO3) [12]. As greater quantities of carbon dioxide (CO2) dissolve into the ocean however, carbonate ions are converted to bicarbonate, becoming less available. This slows the calcification processes making it more difficult for calcifying organisms to form their skeletons, or even causing their dissolution [13]. The decrease in seawater pH places calcifiers under further physiological stress as greater energy is required to maintain suitable cellular conditions for calcification [12]. Similar CaCO3 skeletons can be found in the larvae of marine echinoderms. During this sensitive stage, exposure to high levels of CO2 may cause malformation of the skeleton and reduce swimming speed, negatively impacting larval survival and recruitment [13].

Reef Structure

Figure 4: Low complexity reef covered with turf algae.

Ocean acidification has the potential to completely restructure a coral reef, and perhaps irreversibly damage it. This ecosystem is particularly vulnerable as it depends on the calcifying organisms that are the most negatively affected by a decreasing pH[14]. As ecosystem engineers, corals provide structural complexity to reefs which serves as important habitat for diverse communities of organisms [12]. Linked with reductions in ocean pH however, are declines in reef complexity and coral diversity, shifting an ecosystem from one dominated by calcifying organisms to one dominated by non-calcifying organisms like algae. As a result, organisms dependent upon corals for food, shelter, and settlement are also threatened [15]. Furthermore, the population of molluscs and echinoderms that also use CaCO3 skeletons can be forced to decline [16]. Without this major part of the food web, other species that may be able to tolerate the pH change are unable to find resources and are ultimately squeezed out of the area [14].

Effect on Ecosystems

Examining the naturally acidified ecosystems around volcanic CO2 vents, useful analogues of future environmental conditions, can provide insight into the changes that occur due to ocean acidification. At Shikine Island, Japan, there was a reduced fish biodiversity in the high CO2 regions around the vents, where corals were largely unable to live (Figure 3)[17].

Instead, these areas are dominated by a high abundance of low-profile ‘turf’ algae. Increased levels of CO2 increase the productivity of such species, possibly by supplying a greater concentration of CO2 required for photosynthesis [15]. Rising CO2 may also reduce the abundance of calcifying grazers that would otherwise control algal populations, such as urchins and molluscs. Low-profile algae do not provide the structure required for many coral reef species to survive, leaving them more exposed to predators and without substrate for settlement [15]. This leads to a drastic decline in biodiversity and community restructuring. High CO2 sites have been associated with a greater abundance of generalist species that are able to tolerate the increased CO2 and feed on the abundant algae [17]. However, most tropical fish are not able to tolerate the physiological stress that higher CO2 provides, inhibiting them from inhabiting the area [17].

What is the extent of the problem?

Measurable Ecosystem Changes & Present Status

Figure 5: This image has contains Porites and Acropora corals. This picture was taken at National Park of American Samoa.

The increase of carbon uptake into the oceans is changing the conditions of the oceans, making them more acidic. Ocean acidification correlates to negative skeletal growth and the destruction of corals[16]. The effects of ocean acidification is occurring globally. It is reported that coral reef framework disappears when pH=7.7 in the Papua New Guinea reefs and when pH=8.0 in the Galápagos[18]. Porites is a keystone coral species of the Great Barrier Reef that has experienced negative skeletal growth due to ocean acidification. From 1871-2000 the skeletal density of Porites in the Great Barrier Reef has decreased by 11%, declines are most significant in recent years[16].  The skeletal growth of Porites was also measured in the  South China Sea and the Central Pacific Ocean. Ocean acidification in the South China Sea is responsible for a 7%±3% decrease in the skeletal density of Porites’ between the years 1901-2000. Measurements and analysis of Porites in the Central Pacific Ocean indicate that ocean acidification effects have not yet impacted Porites in this region. Notably, the pH in the Central Pacific Ocean is higher than both the Great Barrier Reef and the South China Sea[16]. Guo et al. explain this finding by the fact that ocean acidification has increased by 2.5-3.5 times faster in coral communities than in open ocean waters[16]. Figure 4 in the Guo et al. paper illustrates the decrease in density of Porites over time due to ocean acidification.

Prognosis for the Future

Due to ocean acidification, it is estimated that corals and macroalgae will decrease calcification by 10-50% compared to pre-industrial rates by 2050[19].  With decreasing calcification, corals are becoming less dense which is associated with corals becoming more fragile. Climate change increases the frequency and severity of storms[20]. The repercussions of climate change will induce more stress on coral reef ecosystems which are already less dense and more fragile due to less calcification. The 2019 IPCC report indicates that each decade since the late 1980’s the open ocean surface waters have decreased in pH within the range of 0.017-0.027 pH units[21]. Panel C from Figure 07 SPM of the IPCC report shows projected decreased pH levels in consideration to the different Representation Concentration Pathways (RPC). The continuous decline in ocean pH is concerning for the health of the world’s coral reef ecosystems. The IPCC project that by 2100, 92% of the global coral cover will be lost due to the combined effects of ocean acidification and carbon emissions based off of the different carbon emission scenarios reported[22]. In the future, if global carbon emissions do not decrease ocean acidification will continue to increase. As the pH of the oceans continues to decrease, less calcification occurs in coral reef ecosystems and organisms decrease in density and increases the chemical dissolution of the corals[23]. Coral ecosystems not only face the threat of ocean acidification, but they are also under the stress of coral bleaching, overfishing, pollution, and other stressors. Although ocean acidification plays a role in the disappearance of global coral reefs the combined effects of all stressors might be considerably greater.

Given the impact, what are the solutions?

Ocean acidification is a major cause of coral reef bleaching. Currently though, there are no effective mass scale solutions for defending coral reefs from ocean acidification. Many potential solutions have been explored to slow the deaths of corals though. An issue with helping coral reefs deal with ocean acidification is political. Due to a lack of borders in the ocean, some countries don't want to be responsible for providing resources to conservation efforts[24]. Not to mention, since the coral reefs are in the open ocean, there are no barriers to acidic ocean water or other potential anthropogenic stressors[25].

Decreasing CO2 Emissions

Since a cause of ocean acidification is carbon dioxide emissions leading water to decrease in pH, the first and most obvious solution would be to decrease CO2 emissions. This would begin with major countries recognizing climate change as a threat and to cut down their CO2 production. Recently, proposals such as the Paris Climate Agreement have been put in place by countries around the world promising to lower their greenhouse gas emissions through efforts such as carbon taxes and investing in renewable energy sources[26]. To effectively reduce ocean acidification to a manageable level, the CO2 concentration in the atmosphere would need to be below 450 parts per million which would translate to approximately increasing the pH of ocean water by 0.16 In other words, this would decrease the acidity by over 30%[27][28].

Phytoplankton and Iron

Phytoplankton can also help decrease the amount of CO2 in ocean water and thus reduce acidification. Phytoplankton perform photosynthesis that consumes CO2 and produces O2 as a product. They usually bloom with the introduction of iron from whale feces and thus increasing whale populations would increase iron concentrations that would help fertilize phytoplankton leading to the removal of CO2 [29]. Alternatively, simply adding iron to the water could also be viable[30].

Figure 6: Coral reefs have been bleached due to an increase in ocean water acidity.

Adding Chemicals to Change pH

Adding chemicals such as limestone powder (CaCO3) to the ocean water is also a possible solution that is being tested. These compounds could either act as a buffer or instead, change the pH to a less acidic level.  This would only be a local solution as adding solution to the entire ocean would be logistically unviable as well as expensive. This solution though, might have adverse effects on corals and other marine life. There are no long-term studies to show that these compounds are safe for marine life and thus need further experimentation before further implementation[31].

Cost Benefit Analysis of Potential Solutions

The goal of protecting corals from ocean acidification is one that is debated. Many argue that it would be too expensive to protect coral reefs and there would be little to gain from doing so. Both tasks of mitigating ocean acidification and conserving coral reefs are very difficult logistically and some scientists argue that efforts should be directed at conserving other species instead.  Others argue the opposite saying that there are many benefits of coral reefs that provide such as shelter for fish, sponges, and crustaceans, as well as production of chemical compounds that can be used in medicine[32][33][34]. A report put out by the Convention of Biological Diversity stated that the benefits far outweigh the costs and thus protecting corals is a worthwhile endeavor[35].


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