Calcium carbonate is important for many marine animals. This chalky substance forms the ridged spicules that scaffold some sponge species, the limy deposits that build coral reefs, the protective shells of clams and snails, and the spiny-skins of sea urchins and starfish. All these organisms are important constituents of marine ecosystems, often forming the basal trophic levels that enable food-webs to function. However, there are studies that suggest increased levels of atmospheric carbon dioxide could alter the chemistry of the ocean, which could affect how marine organisms deposit calcium carbonate into their structures. This process, known as ocean acidification, could impact marine food-webs and the human communities who depend on the sea for supplementing diets and incomes.
Atmosphere and Ocean Interactions
It has been estimated that some 500 billion tonnes of atmospheric carbon dioxide have been emitted since the industrial revolution, when fossil fuel combustion and large-scale deforestation became widespread. Oceans, however, have the ability to absorb as much as half of all atmospheric carbon dioxide into the surface waters, which is a natural process of regulating the greenhouse gas. Carbon dioxide is usually absorbed in the cooler polar seas, and is subsequently circulated deep into the abyss by oceanic currents and upwellings, before arriving in the tropics, where the carbon dioxide can be released in the warmer conditions (additionally, marine phytoplankton also uptake carbon dioxide for photosynthesis).
The pH of water lowers when it absorbs carbon dioxide, due to the subsequent formation of carbonic acid. pH measures the acidity or alkalinity of a substance: pH 1 indicates highly acidic substances, pH 14 indicates highly alkaline substances, and neutral substances are pH 7. Seawater has an average pH of 8.2 at the water’s surface, which is slightly alkaline; however, if the current rates of carbon dioxide emissions continue, it is predicted that the average pH of seawater will drop to 7.8 in the year 2100. The pH scale is logarithmic, so if the pH of a solution was to drop by 0.1, it would indicate the solution is 30% more acidic. Thus, if predictions are correct, surface seawaters will be 150% more acidic in 2100, when compared to current conditions. Seawater should be slightly alkaline to allow for the concentration of carbonate ions, which are required by many organisms to impregnate calcium carbonate into their body structures. However, an acidified ocean could reduce the availability of such ions, resulting in smaller and thinner anatomical structures.
Impacts on Marine Life and Coastal Communities
Ocean acidification could impact the functioning of marine ecosystems. As many as a billion people around the world rely on seafood and other marine ecosystem services for supplementing human diets, and for fishing, fish processing, tourism and many other human activities. For the coastal communities of many island nations and developing countries, fishing and other inshore activities are especially important for cultural traditions and regional socio-economic development. Additionally, coral reefs protect equatorial coastal regions from wave action and buffer the impacts of hurricanes and other tropical storms.
Phytoplankton (microscopic algae that drift in the upper layers of the water column) harness the energy from the sun to convert water and carbon dioxide into sugars, which zooplankton (small and juvenile planktonic animals) and many other animals feed on. Encased by calcium carbonate scales, the coccolithophores are an important group of phytoplankton. However, it is thought that ocean acidification could affect the development of these life-forms, which could destabilise ocean food-webs. Corals could also be affected. Stony corals grow by depositing limestone skeletons, which eventually form reefs. However, because carbonic acid reacts with carbonate ions, it is thought that ocean acidification could impact the process by which corals deposit calcium carbonate, which could affect coral growth and reproduction, and the survival of many other organisms that depend on the reef system.
Concerns for the Future
A concern among many scientists is the rate in which carbon dioxide is being released into the atmosphere. The earth has experienced periods of high carbon dioxide levels in the past and the subsequent increase in carbonic acid levels in the oceans, indicated by layers in geological records that lack calcium carbonate deposits. Yet these former ocean acidification events are believed to have occurred over thousands of years, which gradually altered the pH of marine systems, allowing species to adapt and evolve to suit the changing conditions. Today, in contrast, large volumes of carbon dioxide are being released over a period of centuries, accelerating ocean acidification rates, which many organisms will be unable to adapt to over such a time-frame.
There are studies that suggest fluctuations in oceanic pH are a normal phenomenon, and laboratory experiments have indicated some organisms, including one species of coccolithophore, may benefit from changes in ocean chemistry. Indeed, the marine environment is not uniform and water in certain areas, such as coastal zones or regions that surround undersea volcanic vents, are susceptible to changes in water pH. However, whether entire marine ecosystems can adapt to the predicted changes is unknown, and areas that are finally-balanced, such as coral reefs, are a key concern (anyone who has maintained a marine aquarium will be aware of the problems that arise when attempting to regulate stable water conditions).
Problems in Protection
Currently, ocean acidification is not covered by the Climate Change Convention, which was established in 1992, prior to this phenomenon gaining international concerns. In addition, agreements that focus on the oceans tend to concentrate on pollution and fishing, rather than climate change threats. Unfortunately, because it is the burning of fossil fuels that causes ocean acidification, and yet it is the marine environment and coastal human communities that are primarily affected, there are few international agreements that encompass these complex interrelationships. This year’s Rio 20 Summit did include ocean acidification, and the USA pledged to provide $1 million to help fund a global monitoring network. Even so, with our understanding of the relationships that exist between the atmosphere and the oceans, it would make sense for future treaties to encompass both environments, which is how they interact in nature.