The oceans absorb about 25 percent of the global carbon dioxide emissions. One effect of this is that pH in the water is decreasing – the oceans are being acidified.

“The global trend is currently a pH decrease of 0.02 units per decade”, explains Erik Gustafsson, oceanographer and researcher at Stockholm University Baltic Sea Centre.

Future acidification is directly related to the development of carbon dioxide emissions. If the goals of Paris agreement (to limit global warming to well below 2 degrees) are met, the pH decrease will level out and pH begin to increase again during this century. A more moderate mitigation of emissions would lead to a continuous decrease and in the worst-case scenario, where emissions continue to increase significantly, pH would decrease by up to 0.4 units until the end of this century.

Erik Gustafsson, Stockholm University Baltic Sea Centre. Photo: Lisa Bergqvist

Complex situation in coastal seas

In coastal seas, like the Baltic Sea, however, the development of pH is much more complex and variable compared to in the open oceans.

“In coastal seas we have strong interactions between land, and sea and there are other processes that can counteract and enhance ocean acidification, such as properties of the freshwater input, changes in run-off and salinity and changes in production and respiration patterns over time”, says Erik Gustafsson.

In the Central Baltic Sea, as well as in the Gulf of Bothnia, ocean acidification has been counteracted, but in some other parts of the Baltic Sea the pH decrease has been faster than in the open oceans.

There are also large seasonal variations when it comes to pH in the Baltic Sea. pH typically increases in spring and summer because of uptake of carbon dioxide by phytoplankton. When organic material is decomposed, carbon dioxide is released back to the water and pH decreases again.

“In general, the more eutrophic and productive an area is, the larger the seasonal variations”, says Erik Gustafsson.

Future development in the Baltic Sea

Modelling of the future development in the Baltic Sea shows a mean pH decrease similar to the one in the open oceans – about 0.1 units 2100 with moderate CO2 mitigation, and 0.3-0.4 in the worst-case scenario. But the large seasonal variations also means that the exposure time for harmfully low pH levels increases. 

In a scenario where the Baltic Sea has recovered from eutrophication, the mean pH decrease is larger, but the seasonal variations are smaller, which altogether means less exposure time for harmful pH levels. 

Erik Gustafsson emphasises that although it’s difficult on a national or regional level to set future goals for ocean acidification, it’s important to monitor the development in all parts of the Baltic Sea, and to increase the knowledge of possible consequences for ecosystems and organisms.

“It’s also becoming increasingly important to minimise the effects of other pressures in the Baltic Sea, such as eutrophication, overfishing and hazardous substances”, he says. Potentially, the Baltic Sea can become more resilient to climate and acidification effects if we can minimise these other pressures.”

Ecological effects of acidification

Sam Dupont, senior lecturer at the Department of Biological and Environmental Sciences at University of Gothenburg, has been studying the consequences of ocean acidification for species and ecosystems for many years.

He highlights that extensive ocean acidification has occurred before. At the end of the Permian period (250 million years ago), there was a change in volcanic activity and a lot of carbon dioxide was released into the atmosphere, which led to climate change and ocean acidification.

Sam Dupont, University of Gothenburg. Photo: Lisa Bergqvist

“What happened back then?” Sam Dupont asks, and then gives the answer: “The third extinction, the biggest extinction on the planet. 92 percent of all marine species went extinct. I’m not saying that’s what going to happen now, but it’s a good reason to worry.”

Another way to study the effects of ocean acidification is by simulations in the lab. Already fifteen years ago, Sam Dupont and his colleagues collected brittle stars from the Atlantic Ocean and raised them in water where pH has been decreased by 0.2 units. The brittle stars couldn’t develop in the more acidic water, and they all died within five days.

“When we published the study, we said ‘we need to cut carbon dioxide emissions or these guys might go locally instinct’. Of course, it didn’t change anything and they are gone now”, says Sam Dupont.

“Acidification has an impact already today, and will have an increasing impact in the future, on a range of different species”, he concludes.

Effect on calcifying organisms

What has been most discussed in regards of ocean acidification is the effects on marine calcifying organisms, such as corals and shells. And these organisms are indeed affected, but not for the reason that was previously thought, Sam Dupont explains.

Increasing levels of carbon dioxide (CO2) in the water also increases the level of bicarbonate (HCO3-) and decreases the level of carbonate (CO3-). This was believed to make it more difficult to create shells out of Calcium carbonate (CaCO3).

“But that’s not how biocalcification works”, says Sam Dupont. “Instead, calcium is binding to two bicarbonate ions, and bicarbonate is actually increasing under acidification.

The real reason that these, and other, organisms are affected by ocean acidification, Sam Dupont explains, is because of pH regulation. Acidification means that there are more protons (H+) in the water, that reaches the blood and cells of organisms. 

Sam Dupont, University of Gothenburg. Photo: Lisa Bergqvist

“To get the pH level right, you have to get rid of that”, says Sam Dupont. “But that costs a lot of energy, and if you use energy for that you have less energy for growth and reproduction.

50 percent of all species that have been tested showed negative response to acidification, Sam Dupont tells.

“We have really good reason to worry. In particular in the Baltic Sea where the ecosystem is relatively simple; if you lose a few species that will have cascading effects on the whole ecosystem.” 

What can we do?

Sam Dupont likens the acidification problem to a mouse threatened by a cat. The solution to the problem for the mouse is to gather forces and kill the cat – which corresponds to mitigating acidification by cutting carbon dioxide emissions. But meanwhile, the mouse might have to hide and buy some time – we need adapt to ocean acidification.

“There are three lines of adaptation that we can put into action right way: we need to protect at all cost what we have because the more diverse ecosystem, the more resilient it is”, says Sam Dupont. “Let’s work on eutrophication, that will buy us some time. Unfortunately, a lot of ecosystems have already been damaged, let’s restore those. And then you can do things better; aquaculture and fisheries can be damaging, let’s try to find alternative ways of doing this.”

However, a lot of emphasis today is on other solutions: to create so-called negative emissions by binding carbon in different ways. To mitigate ocean acidification one idea is to alkalinise the oceans. This is sometimes seen as a way of speeding up the natural process of rocks weathering, by grinding rocks and spread it in the water. Doing so changes the alkalinity and make pH increase (this is similar to the liming of acidic lakes, which was a common measure a couple of decades ago).

“Companies love this, because they can buy carbon credits and keep emitting as much as they do”, says Sam Dupont. “But the problem is that no one knows if it works, and if there are side effects.”

Another idea is to restore seagrass, top capture carbon and thereby raise pH. However, Sam Dupont adds, the side effect is that variability is increased, since pH goes down during the night because of respiration and reaches a lower minimum. Seagrasses need to be restored, he emphasises, but not for the reason of solving the issue with acidification.

“There is no doubt that there will be impact of acidification, and there already is. We need both mitigation and adaptation and we need to communicate better what we know. Just be careful when people come with solutions that will fix everything. Some of these solutions should be tested, but I don’t think today that we have any magical solution.”

Text: Lisa Bergqvist

See a recording of the seminar

Answers to questions from the digital audience

What is the historic pH in the Baltic Sea? Did we not have a big problem with acidification in the 1970s?

Erik Gustafsson: Historically, pH in the Baltic Sea has varied as a result of the increasing atmospheric CO2 (i.e., ocean acidification), changes in weathering, runoff, salinity, and alkalinity, as well as changes in production and respiration patterns coupled to the eutrophication problem. The picture is however fairly complex with different and varying pH trends in different Baltic Sea sub-basins.

In particular in the 1980’s, the so called ‘acid rain’ was a high-profiled environmental problem because of the acidification of lakes, streams and soils related to emissions of sulfur- and nitrogen oxides from different combustion processes leading to atmospheric depositions of sulfuric and nitric acids. The effect on the Baltic Sea was on the other hand relatively small because of a better capacity to buffer acids compared to freshwater systems (see Omstedt et al., 2015). Substantially improved treatment of emissions resulted in decreasing depositions of acids in the 1980’s and 1990’s, which in combination with liming largely improved this particular environmental issue.

Reference: Omstedt, A., Edman, M., Claremar, B., Rutgersson, A., 2015. Modelling the contributions to marine acidification from deposited SOx, NOx, and NHx in the Baltic Sea: Past and present situations. Continental Shelf Research, Coastal Seas in a Changing World: Anthropogenic Impact and Environmental Responses 111, 234–249. https://doi.org/10.1016/j.csr.2015.08.024

Which sectors have largest impacts on the Baltic's acidification?

Erik Gustafsson: Globally, ocean acidification is strongly coupled to anthropogenic CO2 emissions (mainly fossil fuels), and the largest sector is the energy sector: energy use in industry, energy use in buildings, and also transport. In the Baltic Sea and other coastal seas, strong interactions between land and sea means that other processes can influence pH over time, which can counteract or enhance the large-scale acidification trend. Such processes include changes in weathering, runoff, salinity, and alkalinity, as well as changes in production and respiration patterns. 

Could you explain the potential effects on Baltic Sea ecosystems based on the acidification scenarios presented?

Sam Dupont: Recent studies have shown that there is a strong local component when it comes to biological sensitivity to ocean acidification. For example, two species or even two populations of the same species may have different sensitivity depending on where they live. This is a consequence of what is called “local adaptation”. An organism tends to be adapted to where it lives and stress occurs when conditions deviate from present conditions (Vargas et al. 2017, 2022). While our understanding of what is driving biological sensitivity to ocean acidification is growing, we still need to have local studies. In other word, we cannot easily extrapolate information from one region to the other.
That being said, we have very limited information on biological sensitivity in the Baltic Sea, but from the body of evidence, we can say with high certainty that a decrease in biodiversity can be expected in the near-future. Some species will locally disappear. This can have cascading consequences in the Baltic as it is an already challenged region and with simple ecosystem with low species redundancy.

It is not possible to provide a detailed picture on how these changes will occur but a decrease in biodiversity and simplification of ecosystems is very likely within a few decades.

Reference: Vargas CA, Cuevas LA, Broitman BR, San Martin VA, Lagos NA, Gaitán-Espitia JD & Dupont S (2022) Upper environmental pCO2 drives sensitivity to ocean acidification in marine invertebrates. Nature Climate Change. 12: 200-207.
Vargas C, Lagos N, Lardies M, Duarte C, Manríquez P, Aguilera V, Broiman B, Widdicombe S & Dupont S (2017) Species-specific responses to ocean acidification should account for local adaptation and adaptive plasticity. Nature Ecology and Evolution. 1:84.

In what way does overfishing affect acidification?

Erik Gustafsson: The direct impact of overfishing on acidification should be small. However, overfishing is an example on one stressor/factor affecting fish stocks, and acidification is an emerging additional stressor. This is an example of a ‘multi-stressor’ issue, where the combined effect of several different stressors (that can also include e.g., eutrophication and hazardous substances) is worse for the organisms than the effects of individual stressors.

Sam Dupont: Overfishing is a driving force of ecosystem changes. It does have consequence on fish stocks, fish size and sustainability. While in principle, this can modulate ocean acidification (e.g. through respiration or changes in the ability of ecosystems to capture or release CO2), these are likely to be small. What is more likely is that overfishing can strongly modulate the biological response to ocean acidification. Any environmental stressor that negatively impact ecosystems also reduce its resilience to other stressors such as ocean acidification. 

Have you looked at the aspect of time scale with respect to effects on the eco systems? If I understood it correctly you saw 0,02 pH decrease per decade?

Sam Dupont: The time scale at which we will see impacts of ocean acidification on a given ecosystem is depending on several factors such as: (i) the rate of chemical change; (ii) the ecosystem sensitivity; (iii) the role of other modulating factors such as temperature, food availability, nutrients, fishing, etc. For example, in the Polar region, ecosystems are highly sensitive and impacts are expected at short time scales even under low rate of chemical changes. On the other hand, some ecosystems that are quite resilient may be able to endure under high rate of chemical changes, except if they are also challenges but other environmental pressures. In the Baltic Sea, you have a given rate of change and likely high biological sensitivity and exposure to a lot of other stressors. So, you can expect to see biological changes over short period of time. For a more detail account, check Widdicombe et al. (2023), developing these ideas in the context of what we should monitor to identify the first signs of biological response to ocean acidification.

Reference: Widdicombe S, Isensee K, Artioli Y, Gaitán-Espitia JD, Hauri C, Newton JA, Wells M & Dupont S (2022) Unifying biological field observations to detect and compare ocean acidification impacts across marine species and ecosystems: What to monitor and why. EGUsphere. 19: 101-119.

Sam, you mention local adaptation. In the Baltic Sea species are used to large variations in pH, are they then also better off when it comes to dealing with further decreases?

Sam Dupont: There are two different aspects that need to be considered for this question: (i) exposure to high variability can provide you with the machinery and plasticity to cope with change and then make you more resilient to future ocean acidification; (ii) all organisms have a tolerance limit and living in a highly variable and challenging environment can push you to the limit of your tolerance. This can be illustrated by what happened to the west coast of the US. This is a highly variable environment due to upwelling. However, it is also an extreme environment. A small pH change due to ocean acidification was enough to lead to mass mortality of oysters. The Baltic being both highly variable and extreme (not only because of ocean acidification but other environmental stressors), it can be expected that it is quite sensitive to ocean acidification.

Are the freshwater species in the Baltic Sea (like perch and pike) less vulnerable to acidification?

Sam Dupont: There is good literature to the sensitivity of freshwater species to low pH collected through the impact of acid rain in lakes. Freshwater species have different physiological mechanism to cope with pH changes and are better than marine species at coping with such fluctuations. So they are likely to be less vulnerable indeed.

We aim to restore eelgrass beds in part to improve habitat and mitigate eutrophication effects, yet you also warn about the impact of eelgrass beds on diurnal pH. Can you rank those pressures? Do you advocate an end to seagrass restoration?

Sam Dupont: Not at all. Seagrass ecosystems are essential and play so many different roles and should be restored at all costs. It is true that we showed that seagrass is driving increased variability that can lead to increased negative effects for marine species under ocean acidification (See Cossa et al., 2024). But this is not overshadowing the positive effects. The point is that it is not a solution to locally buffer ocean acidification and should then not be sold as such.

Reference: Cossa D, Infantes E & Dupont (2024) Hidden cost of pH variability in seagrass beds on marine calcifiers under ocean acidification. Science of The Total Environment.

pH at the sea surface is one thing, but what do we know about ph changes on deeper bottoms, at least at 5-10 metres depth or perhaps 20-20 metres? How deep down do you see the effect of the chemical changes of CO2 uptake?

Erik Gustafsson: The direct effect of CO2 emissions and oceanic uptake is a pH decrease in surface water, which in the case of the open Baltic Sea is typically well mixed down to about 60-70 metres in winter. Deeper areas are isolated from a direct atmospheric influence because of a strong stratification. The water penetrating deeper areas is a mixture of North Sea surface water and Baltic Sea surface water as well as waters from intermediate depths. Thus, acidification effects reach the deeper parts of the Baltic Sea as well, but there is a delay compared to the direct effect in surface water. However, deep water pH in the Baltic Sea is largely influenced by mineralization processes, where CO2 released from decomposing organic material leads to considerably lower pH compared to the surface water.

What’s your best concrete advise to decision makers (both in public and private sector)?

Erik Gustafsson: Reducing CO2 emissions is crucial to limit both climate change and large-scale acidification effects. However, these issues are very much dependent on emissions on a global scale, which means that we have little control over acidification in the Baltic Sea. For that reason, it is increasingly important to minimize the effects of other environmental problems (e.g., eutrophication, overfishing, hazardous substances), so that Baltic Sea ecosystems and organisms can become more resilient to climate change and acidification issues. 

Sam Dupont: On one hand, keep working to mitigate CO2 by reducing emission and explore new technologies for energy and potentially CO2 capture. On the other, work towards buying the time we need by protecting existing ecosystems, restoring what has been damaged, and change practices in the sea and on land to reduce the human impact (e.g. over-fishing, habitat destruction, pollution). And yes, be careful with over-simplistic solutions that were not scientifically evaluated.

Further reading

Policy brief: Ocean acidification poses another threat to the Baltic Sea ecosystem

Policy brief: Havsförsurning ytterligare ett hot mot Östersjöns ekosystem