Ocean Acidification
Ocean acidification refers to the gradual decrease in the pH of ocean water, caused by the uptake of carbon dioxide (CO₂) from the atmosphere. As more CO₂ is absorbed by seawater, it reacts with water to form carbonic acid (H₂CO₃), which then releases hydrogen ions (H⁺) into the water. This leads to:
- An increase in hydrogen ion concentration
- A decrease in carbonate ion concentration
- A drop in ocean pH
- Oceans becoming less alkaline
What is pH and acidity? pH is the measure of the acidity or basicity of a liquid solution. A solution’s pH represents the concentration of hydrogen ions (H⁺) and hydroxyl ions (OH⁻) on a scale of 0 to 14. Pure water has a pH of 7 and is neutral – neither acidic nor basic – with equal concentrations of H⁺ and OH⁻. A solution with a pH lower than 7 is acidic, while a solution with a pH greater than 7 is basic. The pH scale is logarithmic, so a decrease of one pH unit is a ten-fold increase in acidity. |
Influence of Other Factors on Ocean Acidification
Apart from global CO₂ absorption, several localized factors can also impact the chemical interaction between CO₂ and seawater, thereby contributing to ocean acidification.
i. Acid Rain
- Acid rain, with a pH ranging between 1 and 6, can significantly affect surface ocean chemistry. While its overall impact on global ocean acidification is minimal, it can cause notable local and regional effects by increasing the acidity of nearby marine environments.
ii. Eutrophication
- Coastal regions often experience nutrient overload, primarily from nitrogen-rich sources such as agriculture, fertilizers, and sewage discharge. This nutrient input leads to plankton overgrowth (blooms). When these blooms die and sink, bacteria decompose the organic matter, a process that consumes oxygen and releases carbon dioxide. This results in lower oxygen levels and higher CO₂ concentrations in the seawater, ultimately leading to decreased pH and increased ocean acidity.
Impact of Ocean Acidification
When seawater absorbs carbon dioxide (CO₂), it leads to the formation of carbonic acid (H₂CO₃), bicarbonate (HCO₃⁻), and carbonate ions (CO₃²⁻). These carbonate ions are vital for marine organisms that construct calcium carbonate-based shells and skeletons, such as tropical and cold-water corals, molluscs, crustaceans, sea urchins, lobsters, and some plankton.
- However, rising atmospheric CO₂ levels result in lower ocean pH and an increase in carbonic acid and bicarbonate ion concentrations. This process significantly reduces the availability of carbonate ions in seawater. As a result, organisms find it difficult to undergo calcification, and in some cases, it becomes impossible—akin to building a house while the essential bricks keep disappearing. This disruption poses a serious threat to ocean biodiversity and economically significant marine species.
Mitigation Strategies
To counteract ocean acidification, the following measures are essential:
- Reduction of CO₂ emissions
- Implementation of government policies to cap carbon output
- Banning offshore drilling activities
- Promotion of energy efficiency
- Adoption of alternative energy sources such as wind and solar power
Understanding Saturation Horizons
In the deep and cold layers of the ocean, water is naturally undersaturated with carbonate ions, which leads to the dissolution of shells of calcifying organisms. On the other hand, surface waters are oversaturated with carbonate ions and typically support shell formation.
- The saturation horizon refers to the ocean depth below which calcium carbonate minerals begin to dissolve. Some marine species can survive below this horizon due to specialized mechanisms that protect their calcium carbonate structures.
- As ocean acidification intensifies, the saturation horizon rises closer to the ocean surface. This exposes a larger number of calcifying organisms to undersaturated waters, increasing the risk of shell and skeleton dissolution.
- It’s also important to note that the saturation horizon for calcite lies deeper than that for aragonite. However, due to ongoing acidification, both horizons are now moving upward compared to earlier observations.