Does the decrease in the pH. of oceans affect the marine ecosystem?

Introduction (Problem statement)


One of the problems oceans are facing is ocean acidification. Ocean acidification hinders growth and reproduction of various marine organism, specifically shelled organism. This is a threat to the marine ecosystem since it eventually affects all the other organisms either directly or indirectly.

In this paper, the effect of ocean acidification on crassostrea gigas is investigated. The study focuses on C. gigas’s physiological response to OA (Ocean acidification)



    • The seawater chemistry has been changing since the onset of the industrial revolution
    • The industrial revolution increased the amount of carbon dioxide in the atmosphere
    • The sea absorbs the carbon dioxide in the atmosphere because of the action of waves, ocean currents and tides
    • Approximately 30-40% of the carbon dioxide released to the atmosphere finds its way into the oceans.
    • The carbon dioxide reacts with elements in the water to form carbonic acid as shown below;

Reaction of CO2 with water to form carbonic acid



Background cont.…


  • The reaction leads to the decrease in the pH of the ocean water.
  • The decrease in the pH. of the ocean water is referred to as ocean
  • There is a direct relationship between the amount of carbon dioxide being released into the atmosphere and the rate of ocean acidification.
  • The surface pH. of water has been found to have decreased by 0.11 between 1994 and 1751.
  • The graphic below shows the relationship carbon dioxide concentration and ocean acidification
  • The graph clearly indicates that as more carbon dioxide is released into the atmosphere, more accumulates in the ocean thus decrease in the ph.



Other causes of ocean acidification



Phosphate based chemicals find there way into the oceans. Consequently, they lead to increased growth of algae and plants. As they respire, they release carbon dioxide which is absorbed by the ocean causing ocean acidification.

Effect of ocean acidification of ocean water chemistry



  • The resultant ocean water has hydrogen ions whose source is the weak carbonic acid.
  • Hydrogen ions from the carbonic acid bonds with carbonate ions, the key component for formation of skeletons for shelled organisms, to form bicarbonates which are unavailable to shelled organisms (This hinders growth of shelled organisms).
  • Furthermore, Calcium carbonate is made by sea organism such as oysters, combining calcium ions with carbonate ions.
  • Therefore, ocean acidification results to competition for the carbonate ions between shelled organism and the free hydrogen ions from the carbonic acid.

Effects of ocean acidification on Pacific oysters (crassostrea gigas )


Pacific oysters are greatly affected by ocean acidification. Studies have found that ocean acidification has negative effects on crassostrea gigas. Such effect include;

  • Delayed development of the larvae
  • Malformations in the shell of the larvae
  • Reduced fertilization success
  • Decreased calcification rates
  • Alteration to environmental variables response.

Investigataion of impact of OA on c gigas physiology.


Timmins-schiffman et al. investigated the effect of the ocean acidification on physiological processes of C. gigas.

In the study, the biological impacts of OA on shell micromechanical structure, fatty acid content, tissue glycogen, shell deposition rates, acute heat shock, mechanical stress response,  lipid metabolisms were investigated.

This was done by exposing the C. gigas to various levels of concentration of pCO2.

At the end of a period of 30 days, the impacts of OA on the above stated propertieswere investigated.



  • Results of the experiment showed decreased shell growth.
  • To test for micromechanical properties, the posterior growing edge was used. The results showed that as the acidification increases, the microhardness of the shell decreased.
  • It was also found that fracture toughness increased with acidification. The results of the fracture hardness and the microhardness are as analyzed in the graphs below;
  • Снимок2




  • From the results, it can be seen that both the fracture toughness and microhardness are affected by change in the ocean ph.
  • The above properties are dependent on elastic element distribution and microstructure dimensions.
  • Lower pH. and high levels of carbon dioxide results to difference in the structure since it is dose-dependent. The dosage is affected by change in water pH.
  • The changes may be brought about by deposition alterations and erosion of individual microstructures due to the change in pH.
  • The study further found out that carbon dioxide partial pressure of 2800 μatm and 1000 μatm were enough to bring alterations to the micromechanical properties of the C. gigas.

Impacts on other species


  • Carbon dioxide induced acidification of body fluids of various organism
  • Reduction of metabolic rates of certain organism e.g. Squid
  • Depression of the immune system of certain organism such as blue mussels
  • Altering of acoustic properties of the ocean making it hard for animals who depend on it to survive
  • Increased accumulation of toxins in certain organism due to red tide events



Elevated concentration of carbon dioxide alters the physiology of c. gigas.

Possible solutions to ocean acidification


Ocean acidification is mainly caused by carbon dioxide originating from human activities. Therefore, the best way to combat ocean acidification is by reducing emission of carbon dioxide into the atmosphere.


Other extreme ways of mitigating ocean acidification include climate engineering whereby chemicals are added to the ocean to counter the acidity.

Iron fertilization whereby iron sulphate are added in oceans to facilitate blooming of planktons which convert the dissolved oxygen to oxygen gas and carbohydrates


Ocean acidifications projections


The current ph. Of the ocean is alkaline (a ph. Of approximately 8.2). However, with increased carbon dioxide production by human activities, its is estimated that by 2100, the ph. Could reduce to acidic ( a ph. Of about 7.8)









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