Ocean acidification is relatively a new environmental concern. It’s of concern primarily for its potential of changing the chemistry of our marine water bodies, and consequently, its negative impact on all other stages of ‘ecosystem sustainability’ on the Earth.
What is ocean acidification ?
Increasing levels of carbon dioxide accumulating in the atmosphere and its subsequent coming down into surface seawater through the seawater-atmospheric interface are causing the world’s oceans to become more acidic because of lowering of pH level of the sea water. The basis of this lowering of pH is a chemical reaction between carbon dioxide (CO2) and water (H2O) forming carbonic acid in the seawater.
Acidification of ocean water or any seawater does not mean that all water converts to acid. It does not mean that when we put our hands in the sea water we will burn, neither does it imply that the water will taste sour. Then why did the scientific community coin the term ‘Acidification’? Well, we know that while measuring the pH of seawater, a number greater than 7 implies and indicates that the solution is basic or alkaline in nature, and a number lower than 7 signifies a solution is acidic, as 7 is the number being regarded as the neutral pH. Standard pure distilled water is an example of neutral pH. Since the pH of normal sea water (ocean water) is typically found in the alkaline range of 8 and above, when lowering of pH in the sea water occurs, it is said to have become acidic.
Let’s take an example. Let’s say the pH of a particular sample of sea water is 8.225 at a particular time of the year, and if acidification continues to occur, the pH is found to be lowered to 8.221 at the same seasonal time of the following year. Therefore, the proper phrase to be used for describing this phenomenon is “lowering of pH”.
Now, what has happened to lower the pH of the sea water? The increased pressure of the CO2 in the atmosphere leads to more dissolution of the gas into the sea water which leads to the formation of H2CO3 (carbonic acid) that causes the reduction of pH. Generally, the change in pH is insignificant. However, if the concentration of CO2 increases to a higher extent in the atmosphere, the increased pressure causes more CO2 to be dissolved in the sea water, lowering the pH to a significant extent.
Why does acidification occur in the oceans?
The answer seems straightforward that this happens because of the solubility of carbon dioxide. But then another question comes up which is, Why do huge quantities of carbon dioxide come into the atmosphere?
Heavy quanities of carbon dioxide are released in the atmosphere as a result of high magnitude of energy consumption by human beings in the name of development – be it industrial or war or transportation – where the burning of fossil fuels cause such release. Due to its property, the carbon dioxide comes down to the 3/4th earth surface covered with the sea water and makes the sea water acidic.
Changes in the marine ecosystem
The acidification has many direct and indirect impacts upon the natural living and non-living marine ecosystems, and also on human society.
Various research on physical-chemical properties and biological characteristics of sea water, mainly at the coastal zone, have found to be significant in respect of negative impact. A few direct impacts include:
Thinning of shells of molluscs: The lowered pH of the sea water triggers the thinning of the calcium carbonate (CaCO3) shells of the molluscs. Molluscs are known to produce major CaCO3 and are vulnerable to this sea water acidification in their various life stages. The Pteropods or the ‘sea butterfly’ get their shells dissolved in this water of lowered pH. Studies have revealed that there is thinning of shells in the oysters and gastropods in the experimental set up of water with elevated the CO2, clearly proving the hypothesis.
Lowered calcification by calcifying algae: Certain algae have the uniqueness to incorporate CaCO3 into their thalli, contributing to the carbon fixation. The lowering of pH in the sea water reduces the rate of calcification in these algae. This causes a threat of reduced efficiency of the ‘global carbon sink’.
Coral dissolution and exacerbation of bleaching: Corals are symbiotic organisms partly plants and partly animals. The increased acidification in the sea water causes the CaCO3 to dissolve, hence destroying the foundation of corals.
Experiments conducted by scientists also have shown that along with temperature increase, if there is a simultaneous increase in the CO2 concentration,, the coral bleaching is increased by a significant percentage.
Disturbance in the marine food chain: The decrease in the population of pteropods (small marine planktons) can alter the marine food chain as there can be an effect in the population in the organisms that prey on these pteropods. Threat to marine biodiversity: There is a huge threat of loss in marine biodiversity as the coral reefs become vulnerable to the acidification. The large number of organisms of different species that are provided with habitat in the reefs, also face threat to their population.
Overall misbalances in the biogeochemical cycle: The change in the ocean chemistry can lead to various alterations in the biogeochemical cycle. There has been decrease in the productivity in the coral reefs as well as in phytoplanktons. Economic and social impact: Surveys carried out in the coastal areas have revealed that there is decrease in the fish quantity today compared to a decade ago. The major occupation of people living in the coastal areas is fishing, which is considered an alternative livelihood, such as small scale agriculture.
Besides the direct and indirect effect, there exist uncertainties caused by multiplicity of various unknown threats in the coastal ecosystem absolutely dependant on the nature of seawater. Therefore, if for any cause or causes, the pH is being altered by acidification which can alter the chemistry of the ocean, no one knows how soon we will reach the point of no return.
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Vineeta Ghosh
The writer is a guest lecturer in Environmental Science department, Muralidhar Girls’ College (University of Calcutta), Kolkata. She has also been working as honorary researcher at the Asian Marine Conservation Association. She was nominated as ‘Young Scientist’ in the Section of the Environmental Science, at the 99th Indian Science Congress held at Bhubaneswar, Odisha, in the year 2011 for her outstanding research contribution in Ocean Acidification.[/column]