Because of low alkalinity and high primary production, the daily fluctuation of pH in the surface water is already high, and ocean acidification is projected to increase this variation further.
The ever-increasing carbon dioxide (CO2) concentrations in the atmosphere result in global warming. Yet a significant share of the CO2 is also taken up by surface oceans. This buffering effect mitigates climate change, but at the cost of causing ocean acidification (OA), or shifts in the acid-base equilibria of seawater. OA means that the pH of the ocean is decreasing.
This is bad news for marine organisms. The reduced calcium carbonate (CaCO3) saturation state impairs calcification rates of plants and animals that use carbonate to build their shells and skeletons. OA can also induce other physiological maintenance costs, which can in particular be reflected in growth and survival at early life stages. While OA can have negative effects on some species, others may benefit from it. Possible benefiters include macroalgae, due to improved carbon availability
OA is controlled by alkalinity – the buffering capacity – of the water. The higher the alkalinity of the water, the smaller the changes in pH are when CO2 dissolves into it, and the lower the alkalinity the larger the changes in pH.
The Baltic Sea is considered to be especially vulnerable to OA because its alkalinity is considerably lower than that of the oceans, although total alkalinity of the surface water has increased over recent decades1. The change in alkalinity has been highest in the low-saline northern parts, with the effect decreasing gradually as salinity increases towards the south. So far, the increase in alkalinity has balanced a notable share of the CO2-induced acidification. Nevertheless, researchers warn that the increasing alkalinity should not be interpreted as protection against future OA.
In the Baltic Sea, OA is yet another encumbrance on a long list of burdens, eutrophication still being the largest. Due to the combination of low total alkalinity and high primary production, the daily fluctuation of pH in the surface water of the Baltic Sea is already substantial, and OA is projected to increase this variation even more2. The largest differences in sea water pH between day and night occur near macroalgal and seagrass beds as well as in phytoplankton blooms. Moreover, progressive eutrophication amplifies the seasonal fluctuation of seawater pH by increasing production and mineralisation. Mineralisation of organic carbon increases acidification in coastal areas. Furthermore, in coastal areas suffering from upwellings of CO2-enriched deep water, the OA effect is intensified by hypoxia3.
Highly productive coastal habitats that are suffering from hypoxia are already experiencing lower pH values and CaCO3 saturation states than projected for the coming centuries3. Kiel Bay in the western Baltic Sea is an example of such a habitat. The future OA will be amplified in these areas, which could put their communities beyond their tolerance limits. At the moment, these areas can be used as model systems when testing responses of adapted marine ecosystems toward high levels of acidification.
It is not easy to detect significant OA trends and draw conclusions about the present situation in the Baltic Sea. Even though the widest monitoring data sets include over a thousand pH observations, the quality of the historical data is partly questionable because the monitoring schedules, equipment used, and accuracy of the measurements have changed over the years. At present, there are a few studies reporting significant changes in the Gulf of Finland4,5. Wintertime surface and deep-water pH has decreased there significantly between 1972 and 2009, and between 1979 and 20155. The decrease has been sharper in deep water (0.008 units/year) than in surface water (0.003–0.006 units/year), possibly because of increased decomposition and CO2 production caused by eutrophication. Recent modelling studies have projected the same phenomena: climate change and increasing nutrient loads will affect acidification mainly by modifying seasonal cycles (summer maximum and winter minimum), and deep water conditions2,6. However, the main driver controlling the magnitude and direction of the future pH trends is atmospheric CO2 concentration.
Information on the effect of OA on Baltic Sea organisms is slowly accumulating. Most studies have been done at the species level, although evidence on community responses has begun to appear in recent years. In the coastal zone, the species studied include bladderwrack (Fucus vesiculosus, F. radicans) and blue mussel (Mytilus edulis trossulus complex). They are both keystone or foundation species in their habitat, and other organisms thus depend on their success. According to experiments using bladderwrack populations from the southern Baltic (Kiel Fjord)7,8,9 and the northern Baltic (Gulf of Finland)10,11, OA may have slightly positive effects on growth in the form of increased carbon availability and storage, but the response is small in comparison, for example, to the effect of warming, and varies between seasons. Regardless, the future seems challenging for bladderwrack, especially as summer heatwaves have proven detrimental. Climate warming, decreasing salinity and coastal eutrophication all favour fast-growing filamentous green algae, epiphytes and phytoplankton over bladderwrack. Thus, the ecosystem functions that these perennial species provide are threatened. Local or regional actions, such as alleviation of overfishing and eutrophication may mitigate the ongoing loss of bladderwrack12.
Bivalves are dependent on their protective shells and, as calcifying organisms, they are especially vulnerable to OA. According to recent studies using Baltic blue mussel populations, the benthic life stages are able to compensate for the costs of acidification when food is abundant13. The larval stages are less fortunate, due to high calcification rates during the formation of the first larval shell and the limited energy provided by the egg. Calcification is energetically costly for the Baltic blue mussel, and the costs increase with decreasing salinity14. Projected desalination and OA, and the resulting decrease in CaCO3 saturation state could thus set a severe constraint on the future of blue mussels.
What can a mussel do in an unfavorable environment? It has been suggested that dense macroalgal or seagrass habitat could offer a temporal refuge from acidification stress15. In experimental conditions, Baltic Sea blue mussel has been able to maintain most of its calcification activity by shifting it into the daytime, when the photosynthetic activity of bladderwrack increases the mean pH of the habitat. Studies using the mussel population from the Kiel Fjord suggest that the Baltic Sea blue mussels might have more adaptation potential against the effects of OA than populations originating from more stable environments, because fluctuating environments facilitate the maintenance of high genetic diversity16.
The complexity of the Baltic Sea CO2 system complicates meaningful monitoring of OA. To detect slowly progressing changes in seawater pH, monitoring must be both spatially and temporally frequent, and be based on highly accurate and precise measurements. The coordinated monitoring of Baltic Sea water pH started in 1979. Unfortunately, the existing data does not fully meet the quality requirements. Of the four variables that describe carbonate buffering of sea water (total alkalinity, pH, dissolved organic carbon, pCO2) only pH levels are measured on a regular basis. In the case of total alkalinity and pCO2 there is currently only partial national monitoring. The monitoring is coordinated by the HELCOM COMBINE program, and responsibilities for the Baltic Sea sub-areas are divided between the coastal states and their governmental institutes. According to HELCOM, development of more comprehensive OA monitoring is in progress, and will be in place by 2024.
In essence, OA and global warming have the same cause, so the main remedy for both environmental problems is to cut down the release of CO2 from burning fossil fuels. This is addressed by the same global agreements that concern greenhouse gases in general, such as the Paris Agreement. However, OA does not treat all areas equally and some areas, like the Baltic Sea, are more vulnerable than others. In order to target the most effective adaptation measures correctly, from a governance perspective it is therefore also important to address the OA problem independently. In the Baltic Sea, promoting ecosystem resilience against OA could be conducted through conservation actions, which maintain biodiversity by releasing the ecosystems from other environmental stressors, such as eutrophication, overfishing and underwater construction. This can be implemented, for example, through the HELCOM Marine Protected Areas network. Our knowledge of the OA effects on the Baltic Sea ecosystem is still in its infancy. So more research is needed before tackling of the acidification issue can be effectively included in conservation and management plans.
Acknowledgement: This article is based on a report from the BALSAM-project, which is funded by the Swedish Institute. The report will be available under “recent publications” in Acid News No. 2/2021.
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