Currently, global warming is a concern not only for people linked to science but for the entire world population. One of its consequences is reflected in marine fauna and flora: acidification of the oceans. This process is reflected in the increase of atmospheric carbon dioxide (CO2), which ends up dissolved in the water, altering the chemical balance present which consequently increases the ocean acidity. The increasing acidification of the oceans that has been noted is an imminent threat to coral reefs with high productivity and ecosystems where biodiversity predominates.
What causes ocean acidification?
When talking about the acidification of the oceans, this refers to the pH of the water. Alkalinity or acidity depends on the number of hydrogen ions, thus, atmospheric CO2, when dissolved in the ocean, forms carbonic acid and releases hydrogen ions. These hydrogen ions join free carbonate ions to form bicarbonate, which allows free carbonate to be depleted in the ocean (Doney et.al., 2009).
Figure 1- The reaction between dissolved carbon dioxide and ocean water gives rise to an increase in the concentration of hydrogen ions, which ultimately results in an increase in bicarbonate ions and a decrease in ions of carbonate. The carbonate ions will modify the carbonate saturation state, leading to acidification of ocean waters.
In the last 260 years, with the beginning of the industrial revolution, the pH of the oceans has decreased by about 0.1 units. Although it does not appear to be much, pH is a numerical scale that is defined by the logarithm, that is, each unit refers to a tenfold increase, in this specific case, it represents acidity. It is expected that by the end of the sec. XXI, that acidity should increase by 0.4 units if the “business as usual” continues, measures are needed to prevent the ocean from continuing to show high levels of acidification. Currently, the oceans are increasing their acidification exponentially and it is known that this increase is the largest recorded since the last 300 million years, a time interval that includes four mass extinctions (Childers, 2020).
Figure 2- pH scale
Influence of ocean acidification on marine fauna and flora
In 2011, there was still no concrete data on how many marine species occupied coral reefs, however, the diversity of most metazoan phyla reaches its peak in reefs and is estimated to be about 25% of marine biodiversity (Veron, 2011). Some marine animals and plants create skeletons and shells based on calcium and carbonate in seawater, organisms combine these two substances to form rigid shells, and skeletons are formed from calcium carbonate. The fact that the pH of the water is acidifying causes the calcium carbonate structures to slow their growth and, in some cases, dissolve those structures (EPA, 2016).
All organisms need ideal conditions to survive, the marine environment is no exception. Each species has its specific way of reacting to the increase in ocean acidification. However, maintaining the acid-base balance requires energy by altering the resources of growth, immune function, and reproduction, with consequences on the individual's health, and may even lead to the individual's death (Hardt & Safina, 2008). In the case of marine animals with shells, individuals affected by the acidity of the ocean end up compensating for this increase by channeling the energy that should be channeled for the maintenance and growth of the shell to produce fluids compatible with the decrease in pH. Therefore, an organism that can adjust to the increase in acidity, sees its general health impaired, eventually reflecting on a fragile shell that can be fatal (EPA, 2016).
Acidification and corals
Coral reefs have the function of natural barriers, their erosion and decay end up decreasing the defense of the coastal coast against extreme occurrences related to the hydrodynamics of the waves, in addition to the loss of biodiversity (Suatoni, 2013).
The coral reefs in conjunction with the mangroves make up a productive and highly biodiverse environment, they are called hotspots of ecological variety, functioning as a spawning zone and changing the pH of the oceans creates problems for this ecosystem (Sodré, et.al., 2016). The colors that the corals express are due to the symbiotic algae that inhabit the cells of these organisms and due to the environmental stress caused by the pH change, the relationship with the host is broken, which causes the exposure of the calcium carbonate skeleton. The bleaching of corals is due not only to the change in pH but also to the increase in ocean temperature (Sodré, et.al., 2016).
Acidification of the oceans and man
The acidification of the oceans harms man in several ways. From the fragile protection of the coastal zone to the economy itself. In other words, since the corals are fragile and they act as a natural barrier, the coast is much more exposed to the force of the sea. As a rule, coastal regions have mollusk industries, and these suffer the effects of lowering the pH firsthand (Childers, 2020). An example of this is the Pacific Northwest and Northeast Atlantic coast of the USA, where several oyster producers are located, where a large production decline is observed due to the dissolution of oyster larvae in the ocean. In danger, there is also wild salmon fishing, since one of the main sources of food for juvenile salmon, pteropods, sees its shell dissolved in a more acidic environment (Suatoni, 2013).
Forecast for the coming years
Several researches indicate that between 1850 and 2100, the increase in carbon dioxide dissolved in water led to lower levels of aragonite saturation in the oceans. Through proxy evidence from isotopic analysis, research has shown that the carbon dioxide levels that were in the atmosphere previously were higher than the current levels, the projections for the future maintain an identical data model. The data suggest that values up to 1000 ppm were calculated for the Paleocene-Eocene Thermal Maximum (PETM), a result of intense volcanic activity about 56 million years ago. The levels of carbon dioxide released, was estimated at approximately one-tenth of the levels of current anthropogenic emissions, which allowed a little more time for the ocean itself to decrease chemical effects, species adaptation and improve biological performance in the ecosystem. Changes in ocean chemistry and global warming have resulted in the extinction of several marine organisms (Birchenough et.al., 2017). However, the decrease in pH that is predicted for 2100 is in line with these values, and this includes the predicted decrease of 0.1 pH units that have been observed in the last 100 years (Williamson and Widdicombe 2017).
Figure 3- Projection of the change in ocean acidification based on aragonite saturation. Comparison between the projections of 1850 and 2100, under high carbon dioxide emissions. Source: UK GOV (Birchenough et.al., 2017)
Conclusion:
Water acidification is changing diverse ecosystems in the marine environment. The influence of a slightly more acidic pH is mainly reflected in animals with the high calcium content. Animals with their deformed shells, sea larvae unable to complete the natural life cycle, reefs increasingly fragile. The human being should not forget that all negative impacts are sooner or later reflected in society itself. Fragile reefs put coastal areas at risk, as they are natural barriers that protect the coastline from wave hydrodynamics. Acidification leads to the death of marine individuals and this consequently is reflected in the fishing activity, leading to the loss and scarcity of food. It is necessary to rethink the "business as usual", to reduce carbon dioxide emissions, before the damage caused to the lungs of planet Earth, the oceans, becomes irreversible.
Reference List:
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Suatoni, L., 2013. Beyond Shellfish, Ocean Acidification is Bad for People (Op-Ed). Live Science, [online] Available at: <https://www.livescience.com/41577-ocean-acidification-bad-for-economy.html> [Accessed 15 June 2020].
Veron, J. (2011). Ocean Acidification and Coral Reefs: An Emerging Big Picture. 3rd ed. [ebook] Oak Valley: Diversity, pp.263-274. Available at: https://search.proquest.com/docview/1524240485acoutid=8318&rfr_id=info%3Axri%2Fsid%3Aprimo [Accessed 10 June 2020].
Williamson, P. and Widdicombe, S. (2017) The Rise of CO2 and Ocean Acidification. Encyclopedia of the Anthropocene. Amsterdam: Elsevier.