What is the main reason that the oceans are becoming more acidic?

A.The dumping of garbage and sewage into the oceans.

B. Large crude oil spills in the oceans.

C. Absorption by the oceans of carbon dioxide generated by burning fossil fuels.

D. Fertilizer runoff in major rivers flowing into the oceans.

Infographic on ocean acidification
Infographic on ocean acidification

The correct answer is C. Absorption by the oceans of carbon dioxide generated by burning fossil fuels

The ocean environment has been largely negatively affected by human activities. Pollution of the ocean, in particular, oil spills and plastic pollutants have been very detrimental to living organisms.

Another concern to conservationists is the problem of marine eutrophication, in which concentrations of certain nutrients such as phosphorous and nitrogen become artificially elevated due to runoff from the land.

Eutrophication occurs when fertilizers and industrial effluents enter rivers, lakes, and estuaries. It often causes the development of large algal blooms of cyanobacteria (blue-green algae).

Acidification of the oceans has also occurred with the pH dropping by 30% since the industrial revolution. The main cause of this is carbon dioxide which has been increasingly emitted by industrial processes in which fossil fuels have been burnt.

The burning of fossil fuels releases carbon dioxide into the air in large quantities. This not only contributes to the greenhouse effect and global warming, but a great deal of the gas ends up in the oceans.

The carbon dioxide combines with water to form carbonic acid which thus drops the pH of the seawater. This has adverse consequences for ocean life, especially organisms that rely on calcium carbonate to form shells and skeletons.

The state of the oceans

The oceans make up a large proportion of the area on earth, and it is easy for people to forget that the health of the sea is crucial for our survival, and the survival of many other species.

Human activity has had a largely negative effect on the earth in both terrestrial and aquatic ecosystems. The oceans have been heavily polluted with plastics and other substances that have all had a harmful effect on living organisms and food chains.

Oil spills from tankers or accidents from oil platforms are also a major concern, as this causes many ocean animals to be injured and killed.

Many people and countries are now aware that pollution, particularly of plastics, has a severe impact on the oceans and on sea life.

Aside from pollutants, there are also problems with eutrophication and acidification of the oceanic environment. Living organisms require certain environmental conditions in order to survive and flourish, and if such conditions are badly altered, this can have a negative effect on species.


Eutrophication is usually thought of as affecting freshwater lakes, but it has been recorded in estuaries. The problem is that runoff from farms and industrial effluents contain too many of a few nutrients such as phosphates and nitrogen.

Too many of these substances will result in an overgrowth of some algal species, producing an algal bloom.

Scientists have noted, for instance, a substantial increase in macroalgae in Waquoit Bay off the coast of Massachusetts. They also recorded a drop in oxygen concentration which had a negative effect throughout this particular ecosystem.


Since the industrial revolution from 30 to 50% of all the carbon dioxide released into the atmosphere from the activity of humans has ended up in the oceans.

Since that time period, the pH of ocean water has dramatically decreased from 8.19 to 8.05, which is believed to be from the ever-increasing levels of carbon dioxide in the atmosphere.

Carbon dioxide is a gas that is released from the burning of fossil fuels in industry. The more energy we desire, the more fossil fuels we tend to burn, which then leads to even more gas being emitted and sent into the air.

The carbon dioxide causes problems such as global warming, but a fairly large amount also ends up in the oceans. Carbon dioxide reacts with water molecules to form carbonic acid, which thus causes a decrease in the acidity of the seawater.

Some of the carbon dioxide is taken up by primary producers such as phytoplankton, which use the carbons to form glucose by photosynthesis. The problem is that more carbon dioxide is being added than is being taken up, and thus there is an imbalance in the chemicals in the sea.

Ramifications of acidity

The pH of the environment is important since it has a crucial impact on living organisms in ecosystems. Many organisms living in seawater depend on carbonates such as calcium carbonate and aragonite, to form shells or skeletons.

The more acidic the seawater is the less of these carbonate substances there are which means less material for these organisms. Thus, there is likely to be a substantial decrease in the population of these species in the future if the oceans continue to acidify.

Species such as shellfish and corals are likely to be adversely affected in this way. Corals are particularly important in forming reefs upon which many other species of organisms in the ocean depend. This will thus have a cascading effect through coral reef ecosystems.

Laboratory experiments have shown that lowering the pH of the environment does cause a decrease in growth rates of some animals. This means that smaller individuals are formed which are more vulnerable to the effects of predation.

Acidity will also impact the physiological processes of organisms since pH is a factor that strongly affects the activity of enzymes. Each enzyme in a living cell or creature can only operate within a narrow range. Enzymes are destroyed (denatured) if the pH of the environment is too extreme.


  1. JP Rafferty (2019). Ocean acidification. Retrieved from Encyclopedia Britannica.
  2. O Hoegh-Guldberg, PJ Mumby, AJ Hooten, RS Steneck, et al. (2007). Coral reefs under rapid climate change and ocean acidification. Science.
  3. JC Orr, VJ Fabry, O Aumont, L Bopp, SC Doney, et al. (2005). Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms.  Nature.
  4. I Valiela, J McClelland, J Hauxwell, PJ Behr, D Hersh, K Foreman (1997). Macroalgal blooms in shallow estuaries: controls and ecophysiological and ecosystem consequences. Limnology and oceanography.
  5. SC Doney, VJ Fabry, RA Feely, JA Kleypas (2009). Ocean acidification: the other CO2 problem. Annual Review of Marine Science.


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