Ocean acidification threat to sea life

By absorbing CO₂ the ocean is becoming more acidic, and this is happening at a faster rate than during any other period in the past 300 million years.

The acidity of sea surface water has increased by almost 30 per cent since the year 1900. Increased temperatures and acidification will amplify the impacts on biodiversity at large, overfishing, pollution and habitat destruction. Evaluation of the scale of these threats suggests that ocean acidification is a driver for substantial change in ocean ecosystems, potentially leading to long-term shifts in species composition. It has been estimated that since the start of the industrial era the oceans have absorbed some 525 billion tons of CO₂ from the atmosphere, currently some 22 million tons per day. Thus, increases in carbon dioxide in the atmosphere have made the oceans more acidic, leading to major shifts in global climate and mass destruction of species. Ocean acidification is a global stressor that constitutes a rapidly emerging problem for marine organisms, ecosystem functioning and services.

Basically, the issue of ocean chemistry is quite straightforward. Two important things happen when carbon dioxide dissolves in seawater. First, the pH drops as the water becomes more acidic. Second, this process binds up carbonate ions and makes them less abundant. This process reduces the ability of many aquatic organisms to build their shells and skeletons.

Most life on earth, both terrestrial and aquatic, requires carbon dioxide; plants need it to grow and animals exhale it when they breathe. Thanks to our burning of fossil fuels there is now an increasing amount of CO2 in the atmosphere, and most of the carbon dioxide is retained, creating a blanket around the Earth. Because the atmosphere absorbs heat from the sun this leads to increasing temperatures. Some 30 per cent of this CO2 is dissolved in seawater, where chemical changes break down the CO2 molecules and recombine them. When water and carbon dioxide mix, they form carbonic acid, which is a weak acid, but like all acids it releases hydrogen ions which bond with other molecules. By definition, seawater that contains more hydrogen ions is more acidic, i.e. it has a lower pH. pH is the scale used to measure the concentration of H+ ions in a solution. The lower the pH the more acidic the solution. So far, ocean pH has dropped from 8.2 to 8.1 since the start of the industrial era, and it is expected to fall by another 0.3–0.4 pH units by the end of the 21st century. A drop in pH of 0.1 pH units might not seem so large, but the pH scale is logarithmic. A pH value of 4 is thus 100 times more acidic than pH 6. An increase in emissions of carbon dioxide at current rates would, by the end of this century, make the ocean more acidic than it has been for at least the past 20 million years.

Such a rapid change in ocean chemistry will not give marine life much time to evolve and adapt. The shells of some animals are already dissolving in the more acidic seawater, and it is expected that acidification will have mostly negative impacts on ocean ecosystems.

Many plants and algae may thrive under more acidic conditions. Some species of algae will actually grow better when faced with increasing levels of carbon dioxide, but the algae responsible for building coral reefs will fare less well. In acidifying conditions it was found that coralline algae covered 92 per cent less area than normal, making space for non-calcifying algae. Also, acidification may limit coral growth by corroding existing coral skeletons, and the weaker reefs that result will be more vulnerable to erosion. However, some species of coral can use bicarbonate instead of carbonate ions to build their skeletons. Other species can handle a wide pH range. In the next century some common species of coral will shift, even though we do not know for sure what the change will look like.

However, these changes will also affect thousands of species that live on the reefs, in unpredictable ways.

Sea grasses that serve as shallow-water nurseries for many species of fish in coastal ecosystems and support thousands of different species may reproduce better and grow taller under acidic lab conditions. On the other hand this ecosystem is in decline due to factors such as pollution, and it is unlikely that increased acidification will compensate for other stressors.

Two major types of zooplankton (and shelled phytoplankton like coccolithophores) build shells made of calcium carbonate – foraminifera and pteropods. They are extremely important in marine food webs, as almost all larger marine animals eat these zooplankton directly or indirectly. These zooplankton are also critical to the global carbon cycle, which  describes how carbon moves between air, land and sea.  

Foraminifera are sensitive to increased acidity, as it causes their shells to dissolve, and some species from tropical waters may become extinct by the end of this century.

The same fate seems to affect certain pteropods, the shells of which are already starting to dissolve in the Southern Ocean. Like the situation with foraminifera some species may actually become extinct by the end of this century.

Like corals, numerous benthic shelled animals, like mussels, clams, starfish and urchins, may have trouble building shells in more acidic water. Shell growth for some species of mussel and oyster will be reduced by 25 per cent and 10 per cent respectively by the end of this century. We know less about the fate of urchins and starfish, but they build their skeletons from a type of calcium carbonate that dissolves even more quickly in acidic water than the type corals use. Oyster larvae may fail to grow their shells in more acidic water, which has already caused a very high death rate among oysters off the Pacific Northwest of the USA.

The problems referred to above are not universal in all benthic invertebrates, however, because species groups such as shrimps, crabs and lobsters may grow even stronger shells under higher acidity.

Fish have no shells, but they feel the effects of acidification. Although fish are in balance with their environment, when the water surrounding fish has a lower pH, some chemical reactions change the pH of fish blood. This is called acidosis. Even though most marine fish are in harmony with their environment, certain chemical reactions that normally take place in their bodies are altered, and even small changes in pH can make a huge difference in survival. In humans a drop in blood pH of 0.2 to 0.3 can even cause death.

Likewise, fish are also sensitive to pH and need to burn more energy to bring their bodies back to equilibrium. Even a slight change in pH reduces the energy a fish has to digest food, escape predators, reproduce and grow. Clownfish were found to have impaired hearing in water that was slightly more acidic than normal. This can seriously impact survival in the long run because it decreases their ability to react to the presence of prey and predators.

The worst problem for fish in future is however indirect, as ecosystem changes affect the availability of prey, and new plankton and benthic organisms may replace those present today with incalculable consequences.

Lennart Nyman

Lennart Nyman is a scientist and environmentalist from Sweden who has worked for some 50 years studying various aspects of marine, freshwater and terrestrial ecosystems worldwide.