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In simple terms, it was thought that the sea ice cover restricts the exchange of gases between the water and the atmosphere, and that this also limits primary production in the area – which also influences gas exchange. Recent research has shown that this is not quite correct, since there are a number of processes associated with sea ice that increase the gas exchange. Researchers are working to quantify the effects of all the different processes in order to produce better estimates of the development of ocean acidification in the Arctic.
More CO2 can be dissolved in cold than in warmer water, so, in respect of ocean acidification, there is a special focus on the Arctic. In addition, the light conditions (24-hour daylight) mean that phytoplankton are active more continuously around the clock, which also results in less pH variation over each 24-hour period. In these areas, it is more interesting to study the annual fluctuations in the phytoplankton’s effect on pH in the upper water layer. Large spatial variation of pH in the Barents Sea has also been documented, which may be due to the oxidation of organic material transported from the land and rivers.
The Arctic is the part of the Earth where the first studies of the effects of ocean acidification are expected to take place. It is at higher latitudes that the sea is expected to first become undersaturated in calcium carbonate. In fact, seasonal undersaturation of aragonite has already been measured at the surface in northern parts of the Arctic Ocean, and models show expectations of constant undersaturation by the middle of this century.
The volume of the summer ice in the Arctic has been dramatically reduced over a few decades, which has led to accelerated absorption of CO2 in the Arctic Ocean, while the brackish water from melting is poor in calcium ions (Ca2 +). Increasing runoff from rivers, where the river water also has a low calcium content, is also contributing to the undersaturation of aragonite in the surface water. Researchers believe we are now heading at full speed towards a tipping point for aragonite in the Arctic Ocean.
Global warming may potentially destabilise large amounts of methane clathrates (frozen methane) stored in the seabed sediments, especially on the continental slopes. This will cause the release of methane into the water column and the atmosphere, which increases vulnerability to ocean acidification in the Arctic. In the East Siberian Sea, large quantities of methane have already been observed leaking from the seabed.
What is ocean acidification?
The term ocean acidification is used for the ongoing reduction in the pH of sea water, caused by increased concentrations of CO2 in the atmosphere. Ocean acidification is caused by emissions of fossil CO2 into the atmosphere. Large amounts of the gas are dissolved in the seawater as carbonic acid and turn the oceans acidic.
Over the last 200 years, the oceans have taken up around a quarter of man-made CO2, and, worldwide, the average acidity of surface ocean waters has risen by 26 per cent.
Undersaturation of calcium carbonate
Calcium carbonates are important building blocks for shell-building marine organisms. This substance comes in two forms – calcite and aragonite – each with their own saturation horizon.
When seawater is oversaturated with these two forms, it facilitates shell-building by marine organisms. Conversely, if the seawater is undersaturated, the water will dissolve shells of aragonite or calcite and shell-building becomes more difficult or impossible.
Undersaturation of calcium carbonate
Calcium carbonates are important building blocks for shell-building marine organisms. This substance comes in two forms – calcite and aragonite – each with their own saturation horizon.
When seawater is oversaturated with these two forms, it facilitates shell-building by marine organisms. Conversely, if the seawater is undersaturated, the water will dissolve shells of aragonite or calcite, and shell-building becomes more difficult or impossible.
Calcium carbonate is more readily soluble at lower temperatures and increasing pressure, so the saturation horizon is created at a specific depth that varies by sea area. Below this horizon, the calcium carbonate will dissolve, and above it, it can be formed. Aragonite and calcite have different saturation horizons, whereby aragonite is the most easily soluble and will consequently have a saturation horizon in shallower water than calcite has. When increasingly more CO2 is supplied to the sea by the atmosphere, the saturation horizons of both types of calcium carbonate rise, leading to poorer conditions for shell-building organisms. Coldwater corals that live in deep water are especially affected by this.
Laboratory and field studies show that both planktonic and benthic (bottom-dwelling) species experience reduced calcification rates at lower pH. It costs them more energy to build their shells, and the shells may be less robust, which in both cases may have consequences for survival and reproduction.
The chemistry of ocean acidification
How acidic the sea becomes is determined by the concentration of hydrogen ions and carbonate ions in the water. The greater the amount of hydrogen ions and the smaller the amount of carbonate ions, the more acidic the water and the lower the pH.
CO2 molecules occur in seawater in four different forms: undissolved (CO2 (aq)), as carbonic acid (H2CO3), and in two ionic forms – bicarbonate (HCO3-) and carbonate (CO32-). The composition of these carbon compounds depends on temperature, pressure and the chemical composition of the water – which in turn is affected by latitude and depth. Consequently, ocean acidification is not uniform around the world.
The carbonate system
Carbon dioxide is dissolved if the equation is read from left to right and is formed if the equation is read in the other direction. This process allows the sea to take up large proportions of atmospheric CO2 without a dramatic reduction in its pH. But the buffer capacity has its limitations and the expectation is that when this effect gradually diminishes, acidification due to the atmospheric CO2 concentration will be amplified. Illustration: Norwegian Polar Institute (from figure by University of Maryland)
When CO2 is dissolved in the surface water, a weak carbonic acid is formed. The carbonic acid is then split into bicarbonate ions and hydrogen ions. The release of hydrogen ions reduces the pH of the water. They also combine with the carbonate ions and create more bicarbonate ions.
How acidic the sea becomes is determined by the concentration of hydrogen ions and carbonate ions in the water. The greater the amount of hydrogen ions and the smaller the amount of carbonate ions, the more acidic the water and the lower the pH.
This is what is referred to as the carbonate system, which gives seawater its buffering capacity. This means that the increase in dissolved CO2 is less than the amount of CO2 actually taken up by the sea.
Effects of ocean acidification
Ocean acidification can potentially impact much of marine life, whether through direct or indirect effects. Ocean acidification may cause problems for animals that rely on calcium to build a shell or skeleton. Plankton species, shrimps, lobsters, snails, clams, starfish, sea urchins and corals are vulnerable. In the worst case, many species may die out or be out-competed by others that are better able to tolerate acidification.
