With the rise of carbon dioxide gas in the atmosphere, more carbon dioxide is being dissolved into the ocean. The rise of carbon has led the ocean’s pH to decline, making the ocean more acidic, affecting marine life, specifically marine calcifiers like coral polyps.

 

The simple definition of ocean acidification is that this is a chemical process where the pH of seawater declines over time (decades or longer), which is due to the ocean’s carbon dioxide intake in the atmosphere. Compared to pre-industrial times, atmospheric carbon dioxide has increased immensely due to many human-driven factors. Higher concentrations of atmospheric carbon dioxide have led to increases in carbonic acid, free hydrogen ions, and bicarbonate. It is projected that in the future when the ocean is even more acidic, there will be fewer and smaller marine calcifiers due to these reasons in addition to the decrease in carbonate ions. Stony corals build hard skeletons of calcium carbonate by combining calcium and carbonate ions from seawater. These minerals are the building blocks for skeletons and shells of marine calcifiers and as a result of ocean acidification, these minerals have become undersaturated and affected the ability of marine calcifiers to produce and maintain their shells and skeletons. Ocean acidification alters ocean chemistry by reducing the pH and the number of carbonate ions in seawater. Corals need to expend more energy to raise the pH of seawater in their isolated, extracellular compartment for calcifying fluid necessary for calcification of their skeleton. During calcification, bicarbonate ions are converted to carbonate ions so that calcification can occur at a much faster rate to build coral reefs. However, marine life like these calcifiers becomes too stressed for their physiological mechanisms to function with the increasing rate of ocean acidification factors in this oceanic environment. 

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Declining Seawater pH Alters Chemistry of Seawater that Creates Stressed Marine Environment

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The decline in seawater pH is a major concern to ocean water chemistry especially since this will have detrimental effects on marine calcifiers’ environments. Carbon dioxide reacts with seawater that forms carbonic acid that releases hydrogen ions. The concentration of hydrogen ions and pH are inversely related meaning that when there is a higher concentration of hydrogen ions, there will be a lower pH, and vice versa. In the past decades, pH has decreased by 0.0017 to 0.0024 units per year. If the pH value were to go under 0.2 units compared to the pre-industrial average, there will be many high-risk changes to the marine food web (Gattuso and Hansson, 2020). The lowering in pH affects many essential nutrients in the ocean that are vital to the environment and wellbeing of marine organisms like coral polyps and mollusks. There becomes an undersaturation of nutrients like calcium carbonate and aragonite. In low pH, aragonite and calcium carbonate will dissolve. Aragonite is grown by pumping protons to raise saturation state within its calcifying compartment. As ocean acidification continues to decrease the ocean’s pH at an increasing rate, it has been more difficult for marine calcifiers to adapt to the changes in ocean chemistry, creating a stressed environment for these organisms. Acidifed conditions will eventually lead to the dissolving of shells and skeletons that will affect the abundance of marine calcifiers, as well as the shelter other marine organisms like fish, live in since reef structure will begin to break down.

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Absorption of Carbon Dioxide Enhances Stratification

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Each day, a fourth of all carbon dioxide produced by humans is absorbed by the oceans. Carbon dioxide is a weak acid that when absorbed by seawater, causes changes in seawater chemistry. For example, as global warming and carbon dioxide levels continue to increase, the concentration of inorganic nutrients will decrease. Global warming has enhanced stratification that has reduced vertical mixing, nutrients, and oxygen within layers. As carbon dioxide concentration increases, more heat becomes trapped. Stratification, the division of water column, becomes enhanced which negatively affects marine productivity. Nutrients in seawater are not being mixed as thoroughly so less oxygen and nutrients are being distributed to various marine life. Enhanced stratification leads to larger mixing barriers to exchange heat, carbon, oxygen, and essential nutrients. The layers of ocean water closer to the surface will have higher temperatures and will have less dissolved oxygen. Stratification will trap inorganic nutrients that typically move to the surface layers, creating an oligotrophic environment for marine calcifiers like coral reefs.

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