The world’s oceans are becoming more acidic. Over the past 200 years, the average acidity of surface waters has risen by about 30 per cent worldwide, and Arctic waters are particularly vulnerable. This is causing problems for marine organisms with calcareous shells, such as the marine snails known as sea butterflies.
Planktonic species, including the marine snails commonly known as sea butterflies, face serious problems as the oceans become more acidic. They are a vital part of the diet of many fish, seabirds and marine mammals. The whole ecosystem can be disrupted if such key species disappear.
1. State
Oceans becoming more acidic
Arctic seas are particularly vulnerable to ocean acidification. This is because cold water can absorb more CO2 than warmer water, and because fresh water from rivers and melting ice weakens the buffering capacity of the seawater, or its ability to counteract acidification.
CO2 levels in Norwegian waters rising
Monitoring of pH and dissolved CO2 shows that the CO2 content of seawater in Norwegian waters is rising. This is because CO2 emissions due to human activity have risen, resulting in higher levels of CO2 in the atmosphere.
Carbonate essential for marine life
The increase in acidity in itself can have serious effects, but it also changes seawater chemistry in ways that may have even more far-reaching impacts on marine organisms. As the CO2 concentration rises, the content of carbonate ions in seawater drops. Carbonate is an essential building block for many marine animals and algae that form calcareous shells or skeletons. Examples are sea butterflies and cold-water corals.
So far, monitoring has shown that most Norwegian sea areas have sufficient carbonate. However, the deep waters of the Norwegian Sea are undersaturated with respect to carbonate. Although a carbonate deficiency is natural in deep waters, it is worrying that the zone of carbonate deficiency is now expanding upwards through the water column.
Clear seasonal variations – long-term trend uncertain
Monitoring shows clear seasonal variations in pH in the upper 100 metres of the water column in Norwegian waters. This is closely linked to biological activity, and is natural. In spring and summer, algae grow and absorb CO2, and the seawater becomes less acidic. In autumn and winter, the algae die and decompose, releasing CO2 and making the seawater more acidic again.
Historical data from the North Sea also show wide variations in pH from year to year, almost as large as the seasonal variations. Monitoring must therefore be continued for a number of years before any long-term trend in pH can be identified in the North Sea. In the Norwegian Sea and the Barents Sea, on the other hand, scientists have been able to show a downward trend in pH by comparing recent monitoring data with the results of research cruises in the 1980s and 1990s.What about the future?
Modelling indicates that acidification will speed up during this century, resulting in seawater pH levels that are lower than they have been for millions of years. This will influence seawater chemistry and therefore have impacts on ecosystems both in coastal waters and in the open sea.
2. Impact
Species at risk from ocean acidification
As seawater becomes more acidic, less calcium is biologically available. This can cause problems for animals that need calcium in the form of carbonate to build shells or skeletons. Without sufficient carbonate, they cannot build strong shells and skeletons. Many different groups of animals are at risk, including plankton, shrimps, lobsters, gastropods, bivalves, starfish, sea urchins and corals. In the worst case, many species may die out or be outcompeted by other species that are more resistant to acidification.
Planktonic species such as sea butterflies are an important part of the diet of many fish, seabirds and marine mammals. If sea butterflies or other key species are lost, whole food chains will be disrupted and we are likely to lose marine species diversity.
Lower pH may in itself have negative impacts. Recent research has for example shown that it can mage the sense of smell in fish, making it more difficult for them to recognise predatory fish and other dangers and their own territories, and to distinguish between close and distant relatives.
Research has also shown that changes in pH can influence the availability of nutrients and trace elements for marine organisms. Their availability may increase or decrease as pH drops. In the worst case, organisms may absorb too little of essential substances and too much of substances that are toxic in excess.
Impacts expected to become more serious
The impacts of ocean acidification have only just begun to appear. To start with, CO2 dissolves in surface waters, lowering the pH. The effects gradually spread to deeper and deeper waters. This is a very slow process. pH levels in the world’s oceans will therefore continue to sink for many years.
Marine food chains in Arctic waters are relatively simple, and ecosystems are vulnerable if key species like sea butterflies are affected. It is highly probable that ocean acidification will have major impacts on ecosystems in these waters, but our knowledge is still limited.
Interactions between ocean acidification, climate change and pollution may exacerbate the negative impacts. However, the picture may not be as consistently negative as first thought. Scientists expect different groups of organisms to respond in different ways. More recent research suggests that some species with calcareous shells can compensate for increased acidity by using more energy to build their shells, provided that they have adequate food supplies.
3. Response
Monitoring and research under way
It is only very recently that the impacts of ocean acidification have been recognised. Monitoring and research focusing on this problem are therefore of recent date.
Monitoring of ocean acidification
In 2010, the Climate and Pollution Agency started to monitor ocean acidification in Norwegian waters. To obtain a clear picture of the changes that are taking place, it is not enough merely to measure changes in pH. Changes in the concentrations of different carbon compounds are also needed. In 2010, measurements were made in surface waters along two transects, Oslo–Kiel and Tromsø–Longyearbyen, and at various depths in the water column along a transect from Arendal to Hirtshals.
The monitoring programme has since been expanded, and now includes both surface waters and the whole water column in the Skagerrak, the Norwegian Sea and the Barents Sea.
Research on ocean acidification
Ocean acidification is the subject of one of the flagship research programmes at the Fram Centre in Tromsø. In one project, researchers are looking at how ocean acidification will affect cold-water corals in the Trondheimsfjord. In laboratory experiments, various organisms that live in northern waters are being studied to find out how they respond to the pH values expected during this century.
It is important to carry out research on the combined impacts of different factors, for example ocean acidification combined with rising temperatures and pollution. A project at the International Research Institute of Stavanger (IRIS) is a good example of this kind of research. Scientists are studying the effects of various combinations of higher pH, higher temperature and simulated oil spills on the development of shrimp larvae.
State of the Environment Norway, 29 May 2013. Article.
