International Journal of Marine Science, 2024, Vol.14, No.3, 231-244 http://www.aquapublisher.com/index.php/ijms 232 2 Causes and Mechanisms of Ocean Acidification 2.1 Carbon dioxide absorption Ocean acidification is a multifaceted phenomenon resulting primarily from human activities, specifically the emission of carbon dioxide (CO2) into the atmosphere. The mechanisms through which ocean acidification occurs involve the absorption of CO2 by seawater and subsequent chemical reactions, leading to changes in the ocean's carbonate chemistry. This section details the primary causes and chemical processes involved, as well as historical trends and future projections. The primary cause of ocean acidification is the absorption of atmospheric CO2 by the world's oceans. Approximately one-quarter of the CO2 released by human activities, such as the burning of fossil fuels, deforestation, and cement production, is absorbed by seawater. When CO2 dissolves in seawater, it reacts with water molecules to form carbonic acid. This weak acid rapidly dissociates into bicarbonate ions and hydrogen ions. The increase in hydrogen ions leads to a reduction in pH, making the water more acidic (Das and Mangwani, 2015). This increase in acidity has far-reaching implications for marine life, particularly those species that rely on calcium carbonate to form their shells and skeletons. The reduction in carbonate ions is particularly detrimental to calcifying organisms, such as corals, mollusks, and some plankton species, which need carbonate ions to build their calcium carbonate structures (Tagliarolo, 2019). Understanding this fundamental process is crucial for grasping the broader impacts of ocean acidification. 2.2 Chemical processes in seawater Once CO2 is absorbed by seawater, a series of chemical reactions occur that alter the carbonate chemistry of the ocean. The carbonic acid formed from dissolved CO2 dissociates into bicarbonate (HCO3-) and hydrogen ions (H+). This increase in H+ ions lowers the pH of seawater, making it more acidic. Additionally, the formation of bicarbonate reduces the concentration of carbonate ions (CO3 2-), which are crucial for the formation of calcium carbonate (CaCO3) used by marine organisms to build shells and skeletons. These chemical processes have significant implications for marine life, particularly calcifying organisms such as corals, mollusks, and some plankton species, which rely on carbonate ions to form their calcium carbonate structures (Doney et al., 2009). The reduction in carbonate ion availability makes it more difficult for these organisms to build and maintain their shells and skeletons, leading to potential declines in their populations and broader impacts on marine ecosystems. 2.3 Historical trends and future projections Historically, the pH of the world's oceans has remained relatively stable, providing a consistent environment for marine life. However, since the onset of the industrial revolution, there has been a significant increase in atmospheric CO2 levels due to human activities such as fossil fuel combustion, deforestation, and industrial processes. This rise in CO2 levels has led to a corresponding increase in the amount of CO2 absorbed by the oceans, resulting in a measurable decrease in oceanic pH. Data indicate that the average pH of surface ocean waters has decreased by about 0.1 units since the pre-industrial era, which corresponds to a 30% increase in acidity (Mostofa et al., 2015). Projections based on current CO2 emission trends suggest that the pH of the oceans could drop by an additional 0.3 to 0.4 units by the end of the 21st century. Such changes would significantly alter marine ecosystems and the organisms that inhabit them. For instance, reduced pH levels and decreased carbonate ion concentrations would severely impact calcifying organisms, leading to weaker shells and skeletons and affecting the overall biodiversity and functionality of marine ecosystems (Zeng et al., 2015). These future projections underscore the urgent need for strategies to mitigate CO2 emissions and protect marine environments from the adverse effects of ocean acidification. 3 Impacts on Marine Organisms 3.1 Effects on calcifying organisms Ocean acidification has profound effects on marine organisms, impacting their physiology, behavior, and ecological interactions. This section examines the specific impacts on calcifying organisms, fish and invertebrates, and marine microbes. Calcifying organisms, such as corals, mollusks, and some plankton species, are particularly
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