Are the oceans becoming more acidic?
Scientists have known for decades that carbon dioxide as a greenhouse gas is warming the climate. A relatively new discovery, however, is that increasing atmospheric CO2 concentrations are causing the world's oceans to undergo acidification. The consequences for marine life such as fish and shellfish are being studied at the Alfred Wegener Institute by a team of scientists led by Prof. Hans-Otto Pörtner, head of the Section Integrative Ecophysiology.
Why do oceans acidify when carbon dioxide in the atmosphere increases?
The CO2 doesn’t remain in the atmosphere, but enters the oceans’ surface layers. There it dissolves and combines with water to form carbonic acid, as we also know it – albeit in larger concentrations - from mineral water. Carbonic acid dissociates into protons and bicarbonate, and it is due to this process that the seawater acidifies. This acidification is measured as a shift in water pH. A shift of between 0.1 to 0.15 pH units has occurred since pre-industrial times. This figure may not appear to be large, but the acidity of the water is measured on a logarithmic scale, such that a shift by 0.1 to 0.15 pH units represents an increase by about 30 percent.
What are the current consequences for marine animals and plants?
This is a complex problem. Firstly, it will be more difficult for organisms such as mussels, whose shells consist of aragonite, to accumulate and sequester the calcium carbonate required for shell formation. Some organisms react very sensitively. They are not able to compensate for the CO2 induced disturbance of acid-base status. We find large differences between organisms: In the lower animals, the ability to regulate pH in body fluids is poorer than for instance in fish. An acidification of extracellular fluids may cause decrements in the metabolic rate of body tissues and also in the performance capacity of the animals.
Which animals and plants are particularly affected? Mussels, whose shells become thinner? Corals, which can no longer form a skeleton?
One of the more vulnerable groups includes, for example, sea urchins and sea cucumbers, but also mussels, where a decrease in calcification rates may occur. Prominent examples are tropical coral reefs, where a decrease in calcification is also noticeable. In the case of corals, it is clear that ocean acidification needs to be seen in the context of ocean warming. The organisms exist at the limits of their temperature tolerance window. When acidification becomes an additional stressor, the situation is exacerbated and leads to a further decline in their performance. Since the early 80s, coral reefs have repeatedly become bleached. The reason is that corals live in symbiosis with certain algae, which are responsible for 90 percent of their energy production. When temperatures rise beyond a limiting value, these symbionts are expelled, causing the coral to bleach. Due to global warming, such extreme temperatures are reached more and more often. As a result, coral reefs are no longer able to recover from the bleaching events, with the consequence that in some areas coverage with living coral has been significantly reduced. If one adds the acidification component, which further slows down calcification, then the capacity of the organisms is even less to compensate for processes such as erosion, a natural phenomenon in coral reefs. This could therefore force an entire ecosystem to the brink of extinction. And if you consider that around 25 percent of all fish species on this planet live in coral reef ecosystems, it could mean a devastating loss in biodiversity.
If we should succeed in considerably suppressing CO2 emission in the next few years - would the acidification problem be solved?
I have my reservations. While it would certainly be mitigated, we currently have no threshold limit above which we can call the situation critical. One threshold value, which has been put forward by the scientific advisory board of the Federal Government, is 0.2 pH units. This value is more or less derived from a gut feeling and practical considerations, and cannot yet be considered a reliable risk limit. We have already initiated changes and these changes will continue, even with reduced CO2 emissions.
What will happen if we don’t manage to rapidly reduce the CO2 emissions in the coming decades? Will the ocean turn sour?
It will certainly not turn sour, but the shift would of course be quite severe. According to some predictions, we could see a shift of 0.45 pH units by 2100 and even 0.77 pH units by 2300. The latter would correspond to a 6-fold increase in acidity, meaning that we would find oceans quite different from today, in which the equilibrium will have shifted dramatically to a higher proportion of less sensitive organisms. In some regions sensitive species such as corals might even go extinct. The polar oceans would be most severely affected. Recent research indicates that certain bivalve species (clams) have a reduced capacity to compensate for a change in their acid-base status. However, the tropics, with its coral reefs could also be strongly affected, owing to a combination of warming and acidification, which would be especially aggravating.
Are fisheries threatened?
In recent years we have observed that fish stocks are changing their range of distribution. One prominent example is the cod, which is leaving the southern North Sea zone and shifting its range to the north. However, we do not yet know, to what extent the temperature tolerance of cod is affected by acidification. Mussel aquaculture could also be affected. We already see today that large mussel cultures in the northern Mediterranean are affected by the warming trend in the summer months. It is quite apparent that these animals have already reached their thermal limits and would additionally be stressed by future ocean acidification.
