Never before have so many scientists conducted research on what impacts the declining pH value of seawater has on animals and plants in the ocean. Please find a summary of their major research results from the past years here.
The oceans have absorbed more than a fourth of the anthropogenically generated atmospheric carbon dioxide over the past 200 years. Without this natural store the greenhouse gas concentration in the atmosphere would be much higher and the temperature on the Earth quite a bit warmer. However, this storage function has a high price: the oceans have become nearly 30 percent more acidic since the beginning of the Industrial Revolution.
With an average pH value of 8.2 seawater is typically slightly alkaline. This figure has dropped to 8.1 over the past 200 years. Since pH values are logarithmically compressed, this corresponds to a decline of nearly 30 percent. By 2100 the pH value of the oceans will presumably drop by another 0.3 to 0.4 units and seawater will thus become 100 to 150 percent more acidic. That does not mean the oceans are actually acidic because even at values around 7.7 they remain alkaline, but are – in relative terms – more acidic than before.
The pH value of seawater is subject to natural fluctuations. Depending on season and region, the pH value may change. At so-called “champagne sites”, for instance, large amounts of carbon dioxide escape from natural volcanic sources. These marine regions therefore serve as windows into the future because they show which ocean dwellers are able to adapt to a low pH value and which are not.
Carbon dioxide dissolves especially well in cold water. That is why ocean acidification is progressing primarily in the polar regions. Acidification of the Arctic Ocean could result in less availability of aragonite, an important building block for calcareous shells, as early as in the middle of this century.
It never rains, but it pours. In addition to ocean acidification, increasing water temperatures and declining oxygen concentrations are also forcing ocean dwellers to adapt to new living conditions. A deadly trio. After all, when the three factors have a joint impact, organisms in the ocean react extremely sensitively. Moreover, oceans as habitats are frequently polluted and overfished.
Not all marine dwellers react equally sensitively to the declining pH value of seawater. While calcifying creatures, for example, already reach their limits at low carbon dioxide concentrations, the more acidic water hardly has an effect on other living organisms.
In some cases animals and plants differ within a single species, which is why scientists presume that some parent generations have already succeeded in preparing their offspring for the challenges of ocean acidification – a so-called epigenetic effect.
Ocean acidification represents a threat particularly for the young life stages of marine animals, such as eggs or larvae. Some larvae, for instance, no longer grow and develop so well in more acidic water. In contrast to more mature specimens, they have not yet developed all internal mechanisms necessary to protect themselves successfully against external influences.
When water becomes more acidic, it means bad news especially for all ocean dwellers that build calcareous shells, such as molluscs and sea angels. This is because they then have to expend more energy to build and maintain their calcareous shells. A potential consequence: their shells get thinner and possibly disintegrate, thus offering less protection against predators.
If the shell walls of calcifying phytoplankton species become thinner and smaller in more acidic water, this may have an impact on the entire marine carbon store. The reason is that thinner shells are at the same time lighter so their weight declines. However, this additional ballast previously meant that even the shells of tiny creatures sank to the depths – and with them the carbon in their shells. The carbon could thus be stored on the seafloor for millennia. Ocean acidification might therefore result in significantly less carbon being transported to the depths.
Today the most species-rich ecosystems of the oceans, the coral reefs, are already suffering from too warm and too acidic living conditions in some regions. By the end of this century it is even possible that only 30 percent of all corals will have enough building material for their skeletons.
This also has consequences for us humans: 400 million people currently owe their food and protection against storm surges to intact coral reefs.
Marine dwellers have close contact to the water in which they live. If the pH value of seawater drops, the pH value in the body fluids of most living creatures also declines, possibly leading to an acid imbalance. More highly developed organisms like fish can regulate their acid balance within hours or days. However, that requires energy – which may then be lacking somewhere else, such as for growth and reproduction.
Fish are generally relatively insensitive to ocean acidification. Nevertheless, in more acidic water they do not swim without any effects. After all, the declining pH value may have a sensory influence on fish and thus affect their behaviour. In laboratory experiments tropical clownfish, for example, swam towards their predators instead of away from them. Scientists additionally presume that ocean acidification impairs the sight of fish. Their otoliths, by contrast, grow well in more acidic water – which could strengthen their hearing and orientation. Or, on the other hand, completely mix them up since fish may overestimate the distance of certain signals.
Not all ocean dwellers react sensitively to the declining pH value. Some even profit from an increase in the carbon dioxide concentration. They include seagrass, macroalgae and phytoplankton species that do not form a calcareous shell. On the one hand, these plants predominantly live in coastal regions that are naturally subject to pH value fluctuations. On the other hand, the additional carbon dioxide is important for their photosynthesis. Seagrasses, for instance, can even positively influence the chemistry in the surrounding waters through their primary production.
The ocean repeatedly underwent acidification in the past, too – often with severe consequences, particularly for calcifying organisms. During the last ocean acidification event 56 million years ago many of the coral species vanished from the oceans forever at that time. Scientists can learn a lot about how life in the sea has reacted to more acidic water from these past geological eras. Today, however, the pH value is declining ten times faster than in the past.
The consequences of ocean acidification for corals and molluscs alone will cost 1,000 billion US dollars. Scientists have calculated this amount with the help of forecasts.
There is only one effective way of combating ocean acidification. We humans have to reduce our carbon dioxide emissions. However, even if we could stop all emissions from one day to the next, the ocean would need thousands of years to recover completely.