Ocean Acidification


Photo Credit: David Liittschwager/National Geographic Image Collection


The anthropogenic production of carbon dioxide will change the world’s climate over the next 100 years.  The ocean plays a vital role in absorbing atmospheric carbon dioxide but the rate it is generated is resulting in a decrease of ocean pH with a subsequent alteration of ocean chemistry; a process called ocean acidification.

What is Ocean Acidification?

Carbon dioxide in the atmosphere is absorbed by seawater, resulting in a series of chemical reactions that decrease seawater pH, carbonate ion concentrations, and the saturation states of important calcium carbonate minerals.  These chemical changes are known as Ocean Acidification.  Another way of explaining this is to say that as more carbon dioxide is generated in the atmosphere, the oceans are going to become more acidic as they will absorb more carbon dioxide.  The oceans pH has dropped from 8.2 to 8.1 between pre-industrial times and may get as low as 7.8 by 2100.

The Technical Part for Those that are Interested!

The carbonate system consists of three main inorganic forms: CO2 (aq), bicarbonate (HCO3) and carbonate (CO32-) ions.  A forth form, carbonic acid (H2CO3) also exists but is normally represented by Co2 (aq) as its concentrations are minimal (~0.3%).  The carbonate system is related via a series of chemical equilibrium reactions that can shift according to changes in temperature, pressure and salinity.  As atmospheric levels of CO2 increase it causes changes in the carbonate system of the oceans as the system re-establishes equilibrium.  Atmospheric CO2 is absorbed by surface seawater and is in thermodynamic equilibrium according to Henry’s Law.  Dissolved CO2 forms a weak acid (H2CO3) which rapidly dissociates to form bicarbonate ions (HCO3) and a proton (H+).  The HCO3 also dissociates to form carbonate ions (CO32-) and another H+.  The H+ formation is what lowers the pH and makes the water more acidic.

What are the Impacts of Ocean Acidification?

The calcium carbonate minerals form the building blocks for the skeletons and shells of many marine organisms.  Where most life is found in the ocean at present seawater is typically supersaturates with carbonate minerals.  However, ocean acidification is resulting in parts of the ocean becoming undersaturated with these minerals, which threatens the growth and structural integrity of marine calcifies. Ocean Acidification can also interfere with the extracellular and intercellular pH of organisms, enzyme activity and recruitment and settlement of coral and fish larvae.  It also has the potential to alter biological processes like photosynthesis that use inorganic carbon.  A decrease in seawater carbonate ion concentrations is also reducing the oceans naturally buffering ability against future Ocean Acidification.

Besides these impacts, scientists are still investigating the potential impacts of Ocean Acidification on different species and habitats.  Importantly, species-specific responses are being identified suggesting that some marine species may prove resilient to future level of ocean acidification.

  A Visual Representation of the Potential Impacts of Ocean Acidification

Photo Credit: David Liittschwager/National Geographic Image Collection

Photo Credit: David Liittschwager/National Geographic Image Collection

Photographer David Liittschwager with the help of Dr. Victoria Fabry used the shell of a Limacina helicina antarctica, which is a Pteropod to demonstrate the potential impacts of Ocean Acidification on calcifying marine life.  The Pteropod shell was exposed to conditions simulating the saturation state of seawater, with respect to aragonite, that is projected by 2100 in the Southern Ocean (Under the IS92a business as-usual CO2 emissions scenario).

Within 6 days the gloss of the shell had gone and at 11 days the leading edge of the Pteropod’s shell growth had dissolved.  By the end of the experiment at day 45, only the soft material of the shell remained.

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