Understanding the Science of Ocean and Coastal Acidification

Dissolved Carbon Dioxide: Gases in Liquid?

Just as solids like sugar can dissolve in water, gases like carbon dioxide do as well. This idea is easily demonstrated in a bottle of soda. The manufacturer dissolves carbon dioxide in the beverage. The dissolved carbon dioxide is invisible to the naked eye, but once the bottle is opened carbon dioxide escapes as bubbles that tickle your nose. The extra carbon dioxide in soda water adds acidity to the liquid. Similarly, about one third of the carbon dioxide gas in Earth’s atmosphere dissolves into the oceans. 

Carbon Dioxide Imparts Acidity: Transformations of Carbon Dioxide in Water

Once carbon dioxide dissolves in water, it reacts with water molecules to form carbonic acidcarbonic acid . Carbonic acid can be further transformed to bicarbonatebicarbonate  and carbonatecarbonate  ions. These four different forms of carbon (dissolved carbon dioxide, carbonic acid, bicarbonate, and carbonate) exist in balanced proportions in seawater. As more carbon dioxide is added to seawater,  the balance shifts and the carbonate ion concentration decreases as it is transformed to bicarbonate due to increasing acidity. 

How Acidity is Measured: pH

The acidity of a liquid is reported as pHpHA representation of hydrogen ion concentration (molar hydrogen ion concentration to the negative base 10 logarithm) . The lower the pH value, the higher the acidity of a liquid. Solutions with low pH are acidic and solutions with high pH are basic (also known as alkaline).

Prior to the Industrial Revolution, average ocean pH was about 8.2. Today, average ocean pH is about 8.1. This might not seem like much of a difference, but the relationship between pH and acidity is not direct. Each decrease of one pH unit is a ten-fold increase in acidity. This means that the acidity of the ocean today, on average, is about 25% greater than it was during preindustrial times. 

Acidity and Availability of Shell-forming Calcium

Carbonate

Marine life uses carbonate from the water to build shells and skeletons. As seawater becomes more acidic, carbonate is less available for animals to build shells and skeletons. Under conditions of severe acidification, shells and skeletons can dissolve.

Coastal Acidification

Closer to Home: Coastal Acidification

Human activity also contributes to acidification in coastal waters.  Acid-forming compounds (including carbon dioxide) are released

to the atmosphere when fossil fuels are burned, and excess nutrients contribute to acidification in coastal waters when algal bloomsalgal blooms The rapid and often excessive growth of one or more species of algae, typically in a lake or coastal waters peak and die.   

Acid Rain

Burning fossil fuels for energy releases water and carbon dioxide as the main byproducts, but nitrogen oxides and sulfur dioxide are also released in smaller amounts. These two acid-forming compounds fall back to Earth’s surface. They can land in coastal waters directly, or more often mix with water in the atmosphere before falling as acid rain. Acid rain typically has a pH between 4.2 and 4.4. 

Excess Nutrients Delivered Via Streams

The elements nitrogen and phosphorus are essential nutrients for living things. For this reason ,farmers homeowners, and gardeners supply nitrogen and phosphorus to crops, lawns, and gardens to stimulate plant growth. However, water can carry excess nutrients down streams and into coastal waters. Agricultural activities are a major source of nutrients to coastal waters, but other sources include

sewage, wastewater treatment plant effluent, and nitrogen oxide air pollution. In coastal waters excess nutrients stimulate the growth of algae. Algae multiply rapidly under ideal growing conditions and algal blooms can impair water quality by causing hypoxia, foul odors and even toxins. A less well-known fact is that algal blooms can contribute to acidification. When algae die their decomposing tissue releases carbon dioxide directly into the water, resulting in acidification.