{"id":194,"date":"2017-01-23T16:35:50","date_gmt":"2017-01-23T16:35:50","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/introduction-to-oceanography\/chapter\/5-5-dissolved-gases-carbon-dioxide-ph-and-ocean-acidification\/"},"modified":"2021-10-25T21:52:36","modified_gmt":"2021-10-25T21:52:36","slug":"5-5-dissolved-gases-carbon-dioxide-ph-and-ocean-acidification","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/introduction-to-oceanography\/chapter\/5-5-dissolved-gases-carbon-dioxide-ph-and-ocean-acidification\/","title":{"raw":"5.5 Dissolved Gases: Carbon Dioxide, pH, and Ocean Acidification","rendered":"5.5 Dissolved Gases: Carbon Dioxide, pH, and Ocean Acidification"},"content":{"raw":"Oxygen and carbon dioxide are involved in the same biological processes in the ocean, but in opposite ways; [pb_glossary id=\"1024\"]photosynthesis [\/pb_glossary] consumes CO2<\/sub> and produces O2<\/sub>, while respiration and decomposition consume O2<\/sub> and produce CO2<\/sub>. Therefore it should not be surprising that oceanic CO2<\/sub> profiles are essentially the opposite of dissolved oxygen profiles (Figure 5.5.1). At the surface, photosynthesis consumes CO2<\/sub> so CO2<\/sub> levels remain relatively low. In addition, organisms that utilize carbonate in their shells are common near the surface, further reducing the amount of dissolved CO2<\/sub>.\r\n\r\nIn deeper water, CO2<\/sub> concentration increases as respiration exceeds photosynthesis, and decomposition of organic matter adds additional CO2<\/sub> to the water. As with oxygen, there is often more CO2<\/sub> at depth because cold bottom water holds more dissolved gases, and high pressures increase solubility. Deep water in the Pacific contains more CO2<\/sub> than the Atlantic as the Pacific water is older and has accumulated more CO2<\/sub> from the respiration of [pb_glossary id=\"582\"]benthic [\/pb_glossary] organisms.\r\n\r\n[caption id=\"attachment_160\" align=\"aligncenter\" width=\"668\"]\"figure5-5-1b\"<\/a> Figure 5.5.1<\/strong> Representative carbon dioxide profiles for the Pacific and Atlantic oceans (PW).[\/caption]\r\n\r\nBut the behavior of carbon dioxide in the ocean is more complex than the figure above would suggest. When CO2<\/sub> gas dissolves in the ocean, it interacts with the water to produce a number of different compounds according to the reaction below:\r\n

CO2<\/sub> + H2<\/sub>O \u2194 H2<\/sub>CO3<\/sub> \u2194 H+<\/sup> + HCO3<\/sub>-<\/sup> \u2194 2H+<\/sup> + CO3<\/sub>2-<\/sup><\/p>\r\nCO2<\/sub> reacts with water to produce carbonic acid (H2<\/sub>CO3<\/sub>), which then dissociates into bicarbonate (HCO3<\/sub>-<\/sup>) and hydrogen ions (H+<\/sup>). The bicarbonate ions can further dissociate into carbonate (CO3<\/sub>2-<\/sup>) and additional hydrogen ions (Figure 5.5.2).\r\n\r\n \r\n\r\n[caption id=\"attachment_190\" align=\"aligncenter\" width=\"1024\"]\"Illustration<\/a> Figure 5.5.2<\/strong> The fate of dissolved carbon dioxide in the oceans. Most of the carbon ends up in the form of bicarbonate (PW).[\/caption]\r\n\r\nMost of the CO2<\/sub> dissolving or produced in the ocean is quickly converted to bicarbonate. Bicarbonate accounts for about 92% of the CO2<\/sub> dissolved in the ocean, and carbonate represents around 7%, so only about 1% remains as CO2<\/sub>, and little gets absorbed back into the air. The rapid conversion of CO2<\/sub> into other forms prevents it from reaching equilibrium with the atmosphere, and in this way, water can hold 50-60 times as much CO2<\/sub> and its derivatives as the air.\r\n\r\nCO2<\/sub> and pH<\/strong>\r\n\r\nThe equation above also illustrates carbon dioxide's role as a [pb_glossary id=\"608\"]buffer[\/pb_glossary], regulating the pH of the ocean. Recall that pH reflects the acidity or basicity of a solution. The pH scale runs from 0-14, with 0 indicating a very strong acid, and 14 representing highly basic conditions. A solution with a pH of 7 is considered neutral, as is the case for pure water. The pH value is calculated as the negative logarithm of the hydrogen ion concentration according to the equation:\r\n

pH = -log10<\/sub>[H+<\/sup>]<\/p>\r\nTherefore, a high concentration of H+<\/sup> ions leads to a low pH and acidic condition, while a low H+<\/sup> concentration indicates a high pH and basic conditions. It should also be noted that pH is described on a logarithmic scale, so every one point change on the pH scale actually represents an order of magnitude (10 x) change in solution strength. So a pH of 6 is 10 times more acidic than a pH of 7, and a pH of 5 is 100 times (10 x 10) more acidic than a pH of 7.\r\n\r\nCarbon dioxide and the other carbon compounds listed above play an important role in buffering the pH of the ocean. Currently, the average pH for the global ocean is about 8.1, meaning seawater is slightly basic. Because most of the inorganic carbon dissolved in the ocean exists in the form of bicarbonate, bicarbonate can respond to disturbances in pH by releasing or incorporating hydrogen ions into the various carbon compounds. If pH rises (low [H+<\/sup>]), bicarbonate may dissociate into carbonate, and release more H+<\/sup> ions, thus lowering pH. Conversely, if pH gets too low (high [H+<\/sup>]), bicarbonate and carbonate may incorporate some of those H+<\/sup> ions and produce bicarbonate, carbonic acid, or CO2<\/sub> to remove H+<\/sup> ions and raise the pH. By shuttling H+<\/sup> ions back and forth between the various compounds in this equation, the pH of the ocean is regulated and conditions remain favorable for life.\r\n\r\nCO2<\/sub> and Ocean Acidification<\/strong>\r\n\r\nIn recent years there has been rising concern about the phenomenon of [pb_glossary id=\"986\"]ocean acidification[\/pb_glossary]<\/strong>. As described in the processes above, the addition of CO2<\/sub> to seawater lowers the pH of the water. As [pb_glossary id=\"546\"]anthropogenic [\/pb_glossary] sources of atmospheric CO2<\/sub> have increased since the Industrial Revolution, the oceans have been absorbing an increasing amount of CO2<\/sub>, and researchers have documented a decline in ocean pH from about 8.2 to 8.1 in the last century. This may not appear to be much of a change, but remember that since pH is on a logarithmic scale, this decline represents a 30% increase in acidity. It should be noted that even at a pH of 8.1 the ocean is not actually acidic; the term \"acidification\" refers to the fact that the pH is becoming lower, i.e. the water is moving towards more acidic conditions.\r\n\r\nFigure 5.5.3 presents data from observation stations in and around the Hawaiian Islands. As atmospheric levels of CO2<\/sub> have increased, the CO2<\/sub> content of the ocean water has also increased, leading to a reduction in seawater pH. Some models suggest that at the current rate of CO2<\/sub> addition to the atmosphere, by 2100 ocean pH may be further reduced to around 7.8, which would represent more than a 120% increase in ocean acidity since the Industrial Revolution.\r\n\r\n \r\n\r\n[caption id=\"attachment_191\" align=\"alignnone\" width=\"1024\"]\"Changes<\/a> Figure 5.5.3<\/strong> Changes in atmospheric CO2<\/sub> (red), seawater CO2<\/sub> (green) and pH (blue) in the Hawaiian Islands (NOAA PMEL).[\/caption]\r\n\r\nWhy is this important? Declining pH can impact many biological systems. Of particular concern are organisms that secrete calcium carbonate shells or skeletons, such as corals, shellfish, and may planktonic organisms. At lower pH levels, calcium carbonate dissolves, eroding the shells and skeletons of these organisms (Figure 5.5.4).\r\n\r\n \r\n\r\n[caption id=\"attachment_192\" align=\"aligncenter\" width=\"600\"]\"he<\/a> Figure 5.5.5<\/strong> The results of an experiment placing the calcium carbonate shells of pterapods in seawater with a pH of 7.8, the projected ocean pH for the year 2100 under current rates of acidification. The top row shows the shells before the experiment, and the bottom row shows the dissolution of the shells after 45 days of exposure (NOAA).[\/caption]\r\n\r\nNot only does a declining pH lead to increased rates of dissolution of calcium carbonate, it also diminishes the amount of free carbonate ions in the water. The relative proportions of the different carbon compounds in seawater is dependent on pH (Figure 5.5.6). As pH declines, the amount of carbonate declines, so there is less available for organisms to incorporate into their shells and skeletons. So [pb_glossary id=\"986\"]ocean acidification[\/pb_glossary] both dissolves existing shells and makes it harder for shell formation to occur.\r\n\r\n[caption id=\"attachment_193\" align=\"aligncenter\" width=\"1024\"]\"A<\/a> Figure 5.5.6<\/strong> Proportions of carbon compounds in the ocean at various pH levels. As the ocean pH declines, the proportion of carbonate ions also declines, reducing rates of shell formation (NOAA).[\/caption]\r\n\r\n

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