Ocean Acidification So, what is it? • The ongoing decrease in the pH of the Earth's oceans, caused by the uptake of carbon dioxide (CO2) from the atmosphere. • An estimated 30–40% of the carbon dioxide released by humans into the atmosphere dissolves into oceans, rivers and lakes. • To achieve chemical equilibrium, some of it reacts with the water to form carbonic acid. • Some of these extra carbonic acid molecules react with a water molecule to give a bicarbonate ion and a hydronium ion, thus increasing ocean "acidity" (H+ ion concentration). So, what is it? • Between 1751 and 1994 surface ocean pH is estimated to have decreased from approximately 8.25 to 8.14, representing an increase of almost 30% in H+ ion concentration in the world's oceans. • Earth System Models project that within the last decade ocean acidity exceeded historical predictions • Could undermine the functioning of marine ecosystems and many ocean goods and services Estimated change in sea water pH caused by human created CO2 between the 1700s and the 1990s, from the Global Ocean Data Analysis Project (GLODAP) and the World Ocean Atlas How much CO2 can the ocean absorb? • The total amount of any gas seawater can absorb depends on temperature and salinity • Salinity is a measure of the dissolved salt content of water Remember this relationship! Temperature or Salinity Amount of gas seawater can absorb Increases in temperature and salinity can decrease the amount of gas seawater can absorb. 4 From the wikimedia free licensed media file repository Life on Earth would not be possible without water Its chemical and physical properties actually defy some fundamental laws of physics Almost all biochemical reactions require water! pH and buffers • Measure of the acidity or basicity of an aqueous solution. – Solutions with a pH less than 7 are said to be acidic – Solutions with a pH greater than 7 are basic or alkaline. – Pure water has a pH very close to 7 • • • • Acid A chemical compound that donates H+ ions to solutions. Base A compound that accepts H+ ions and removes them from solution. • Remember H3O+ and OH-? • The H3O+ donates H+ to a solution and the OH- removes H+ from a solution • If an equal number of these ions are present in a solution the pH will not change as it is said to be neutral. The pH scale • To describe the acidity of a solution, we use the pH scale. • Acids have a low pH, so they have a high concentration of H+ • Bases have a high pH, so they have a low concentration of H+ Used with permission from purewaterproducts.com • The ocean absorbs carbon dioxide from the atmosphere • Human activities release carbon dioxide into the atmosphere • Too much carbon dioxide in the ocean has the potential to harm marine organisms and ecosystems How is atmospheric CO2 responsible for ocean acidification? When CO2 dissolves in seawater, carbonic acid is produced via the reaction: This carbonic acid dissociates in the water, releasing hydrogen ions and bicarbonate: The increase in the hydrogen ion concentration causes an increase in acidity, since acidity is defined by the pH scale, where pH = -log [H+] (so as hydrogen increases, the pH decreases). This log scale means that for every unit decrease on the pH scale, the hydrogen ion concentration has increased 10-fold. One result of the release of hydrogen ions is that they combine with any carbonate ions in the water to form bicarbonate: This removes carbonate ions from the water, making it more difficult for organisms to form the CaCO3 they need for their shells. “Battle” for carbonate! • Organisms must use more energy or make less hard part material • Existing hard parts dissolve (chemical reaction goes “the wrong way”) From the wikimedia free licensed media file repository Hgm, • dgdfg Ocean acidification: Impacts on individual marine organisms Reduced fertilization of gametes in corals and other marine organisms • Deformed flagellum in sperm that impacts their swimming • Fitness effect: lower population growth Natural range in the ocean Albright et al. 2010 Ocean acidification: Impacts on individual marine organisms Reduced hearing ability in anemone fish (clown fish) larvae • Deformed morphology of CaCO3 fish ear bones (otoliths) • Disruption of acid-base balance in neuro-sensory system • Fitness effect: lower survival due to higher predation. Simpson et al. 2011 From the wikimedia free licensed media file repository Ocean acidification: Impacts on ecological communities Tropical Oceans Predictions: • Corals will become increasingly rare • Algae will become more abundant • Because coral reefs support so many animals, biodiversity will decline Hoegh-Guldberg et al. 2007 Ocean acidification: Impacts on individual marine organisms Growth Photosynthesis Non-calcifying marine algae: Increased photosynthesis and growth • Lower pH means more dissolved CO2 for photosynthesis to fuel growth • Fitness effect: higher survival and population growth Amount of dissolved carbon Chen & Durbin 1994 The Solutions What can we do about ocean acidification? A possible geoengineering solution: Add CaCO3 to the ocean. Reduce CO32- under-saturation caused by excess CO2 dissolving in ocean water. 2+ 2Shells 3 32Shells are are made made of of CaCO CaCO33 = = Ca Ca2+ ++ CO CO H+ CO32- From the wikimedia free licensed media file repository Sounds great, but…………….. To counteract 2 gigatons of carbon/yr input of CO2, would need 20 gigatons of CaCO3/yr. White Cliffs of Dover would be rapidly consumed. Limestone Rock (CaCO3) • Limestone mining would be expensive and would cause ecological damage. • All the energy needed to move massive amounts of rock into the ocean would likely add more CO2 to the atmosphere. Oh yes, Stop adding CO2 to the atmosphere! From the wikimedia free licensed media file repository A Quick Note • Buffer Capacity: the extent to which a buffer can resist changes to the pH of the solution when an acid or base is added • The higher the [buffer] in solution, the better controlled the pH will be when an acid/base is added The End! Any Questions?
