Ocean acidification increases the agony of the Baltic Sea

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 when CO₂ dissolves in 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 decades. 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 among 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 more. 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. Highly productive coastal habitats that are suffering from hypoxia are already experiencing lower pH values and CaCO₃ saturation states than projected for the coming centuries. Kiel Bay in the western Baltic Sea is an example of such a habitat. At the moment, these areas can be used as model systems when testing the responses of adapted marine ecosystems to 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. At present, there are a few studies reporting significant changes in the Gulf of Finland. Wintertime surface and deep-water pH has decreased there significantly between 1972 and 2009 and between 1979 and 2015, respectively. The decrease has been sharper in deep water than in surface water, 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 conditions. However, the main driver controlling the magnitude and direction of the future pH trends is the atmospheric CO₂ concentration.

Information on the effect of OA on the 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 the experiments using bladderwrack populations from the Kiel Fjord and the Gulf of Finland, 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 the bladderwrack, as summer heatwaves have proven especially detrimental. Climate warming, decreasing salinity, and coastal eutrophication all favour fast-growing filamentous green algae, epiphytes and phytoplankton over bladderwrack. Local or regional actions, such as alleviation of overfishing and eutrophication may mitigate the ongoing loss of the bladderwrack.

Bivalves are dependent on their protective shells and, as calcifying organisms, they are especially vulnerable to OA. According to the recent studies, the benthic life stages are able to compensate for the costs of acidification when food is abundant. 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 salinity. Projected desalination and OA, and the resulting decrease in CaCO₃ saturation state could thus set a severe constraint for the future of blue mussels.

What can a mussel do in an unfavourable environment? It has been suggested that dense macroalgal or seagrass habitat could offer a temporal refuge from acidification stress. In experimental conditions, blue mussel has been able to maintain most of its calcification activity by shifting it into the daytime, when the photosynthetic activity of the 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 adaptation potential against the effects of OA because fluctuating environments facilitate the maintenance of high genetic diversity.

The complexity of the Baltic Sea CO₂ system complicates meaningful monitoring of OA. Monitoring must be both spatially and temporally frequent, and be based on highly accurate and precise measurements. Monitoring of Baltic Sea water pH started in 1979 and it is coordinated by the HELCOM COMBINE programme. pH is still the only parameter of the four carbonate buffer system variables (total alkalinity, pH, dissolved organic carbon, pCO₂) that is measured on a regular basis. 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 of fossil fuels. Promoting ecosystem resilience against OA could be achieved through conservation actions that maintain biodiversity by freeing the ecosystems from other environmental stressors, such as eutrophication and overfishing. Our knowledge of the effects of OA 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.

Anu Vehmaa

This article is based on a report from the BALSAM-project, funded by the Swedish Institute. The report will be will be available under the section “Recent Publications" in Acid News No. 2/2021. A longer version of this article, including references, can be found under the heading “Ocean Acidification Working Group” on  https://www.airclim.org/ (link to the article: https://airclim.org/ocean-acidification-increases-agony-baltic-sea). Please note also information on the Ocean Acidification Action Week (May 3–9, 2021) on page 13 in this issue.