Erik Gustafsson, Baltic Sea Centre, Stockholm University, Stockholm, Sweden.
Bo G. Gustafsson, Baltic Sea Centre, Stockholm University, Stockholm, Sweden and Tvärminne Zoological Station, University of Helsinki, Hanko, Finland
Jacob Carstensen, Aarhus University, Department of Bioscience, Roskilde, Denmark.
Gregor Rehder, Department of Marine Chemistry, Leibniz Institute for Baltic Sea Research Warnemünde, Rostock, Germany.
Vivi Fleming, Finnish Environment Institute (SYKE), Marine Research Centre, Helsinki, Finland.
Photo: catazul, Pixabay.
In marine and brackish waters, the acidity of the water is mainly controlled by the inorganic carbon system (see Fact Box 1). Anthropogenic CO2 emissions will – unless reduced – gradually move the Baltic Sea towards a state where acidification becomes harmful for some organisms. The effect is caused by the uptake of CO2 in the water, but can be further enhanced by other climate effects, such as increased water temperature and a possible freshening of the sea water. This is expected to lead to changes in species composition, both directly (competitive advantages/disadvantages) and indirectly (altered food availability), potentially influencing ecosystem functioning.
Coastal seas, such as the Baltic Sea, are highly influenced by their catchment areas, which means that pH dynamics is generally more complex than in the open ocean. The reason is that pH, in addition to the response to increasing CO2, is also influenced by changes in hydrology and changes in the supply of carbon and nutrients. High-productive waters typically experience larger seasonal pH variations than low-productive waters, with higher pH peaks in spring/summer and also a more pronounced pH decline in winter. The comparatively weak long-term acidification trend can be masked behind much larger short-term variations. Furthermore, since acidification is a slow process, organisms can to varying degrees adapt to the changes.
Model simulations performed as a part of the OMAI (Operational Marine Acidification Indicator) project indicate that the expected acidification in the Baltic Sea generally follows the same trajectory as the open oceans, with a pH decline of almost 0.4 by year 2100 and a further decline of 0.3 by year 2300 in the worst-case scenario. Due to large regional differences in the area, the annual mean pH in the Bothnian Bay might decline from present-day 7.8 to 7.4 by year 2100, whereas in the Gotland Sea and Southern Kattegat mean pH could decline from present-day 8.1 to 7.7. The degree of eutrophication has a comparatively small effect on the annual mean pH, but on the other hand a considerable impact on the seasonal amplitude and thus minimum and maximum values.
The complex situation in the Baltic Sea gives a strong incentive to improve the temporal and spatial coverage of acidification monitoring. This would broaden the understanding of current acidification trends and also improve the capacity to predict future changes. An indicator for acidification in the Baltic Sea is currently under development. Monitoring of parameters relevant for acidification, i.e., the inorganic carbon system parameters, would as an added value also provide an additional handle in terms of assessing changes in primary production and eutrophication trends.
Photo: Steffen Wachsmuth, Pixabay.
Fossil fuel burning and other industrial processes have caused a rapid increase of the CO2 level in the atmosphere since the mid-19th century. The concentration now exceeds 400 ppm (Figure 1), which can be compared to an estimated range of 180-280 ppm over the last 800000 years, prior to the onset of industrialization. A fraction of the anthropogenic CO2 emissions is absorbed by the oceans, leading to a gradual pH decline – ocean acidification.
IPCC has presented several possible scenarios (Representative Concentration Pathways, or RCPs) for the future development, corresponding to e.g. different levels of CO2 emissions. RCP 2.6, for example, is a scenario where heavily reduced CO2 emissions result in declining atmospheric CO2 and increasing surface water pH before the end of this century. RCP 8.5, on the other hand, is a worst-case scenario where the atmospheric CO2 will not start to level out until the first half of the 23rd century, at a point where the CO2 level has reached almost 2000 ppm, and surface water pH has on average declined by more than 0.6 units compared to today (Figure 1).
In 2015, the so called Paris Agreement was signed. One goal of this agreement is to reduce greenhouse gas emissions so that global warming can be kept below 2 ºC – and preferably below 1.5 ºC – compared to preindustrial levels. To realize this goal, even larger emission reductions than in the RCP 2.6 scenario will likely be required.
Photo: Ricardo Resende, Unsplash.
Ocean acidification is predictable in surface water of the open oceans, because pH change is largely controlled by the development of atmospheric CO2. Coastal areas are on the other hand influenced by inputs of river water, nutrients and carbon from their catchments –changes in supplies can strongly influence pH. The acidification driven by increasing atmospheric CO2 can be either enhanced or counteracted by processes such as:
In the Baltic Sea, pH trends over the last couple of decades have been shown to be heavily site-dependent. Observations indicate that acidification has been partly counteracted in the open Baltic Proper and in the Gulf of Bothnia by increased alkalinity (buffer capacity). Contrasting development has, however, been observed in Danish estuaries as well as in the Laajalahti Bay near Helsinki – at these sites pH has declined faster than what could be explained by increasing atmospheric CO2 alone.
Photo: Anna Ventura, Pixabay
Model simulations were used to investigate the differences between past, present, and possible future conditions with regards to surface water partial pressure of CO2 (pCO2), pH, and calcite saturation level (ΩCa) in three different Baltic Sea sub-basins (the Southern Kattegat, the Gotland Sea, and the Bothnian Bay, respectively). The following six cases (a–f) were considered:
Scenarios c–f indicate possible future conditions towards the end of the 21st century depending on both CO2 emission scenarios and nutrient load scenarios:
The simulations demonstrate how the rising atmospheric CO2 level is the main factor that causes changes in the baseline, or typical average values, of e.g. pH (Figure 2). The partial pressure of CO2 (pCO2) in surface water tends to follow the atmospheric pCO2. In a worst-case scenario, pH in Baltic Sea surface waters might decrease by almost 0.4 units compared to present-day conditions before the end of the 21st century. If these scenarios were extended into the 23rd century, pH could decrease by another 0.3 units according to the RCP 8.5 scenario, as shown in Figure 1.
Acidification also influences the saturation state of calcium carbonate minerals such as calcite (ΩCa). The saturation state is a measure of the tendency for the mineral to either form or dissolve, with values below 1 favoring dissolution and values above 1 favoring production of shells and skeletons. Calcifying organisms, such as mussels and corals, are particularly vulnerable to ocean acidification because of the gradually decreased saturation states of calcium carbonate minerals. In the Baltic Sea there are large regional differences; Bothnian Bay surface waters are naturally undersaturated in calcite saturation level, whereas Kattegat surface waters are oversaturated. Gotland Sea surface waters fluctuate between undersaturation in winter and oversaturation in summer, but with an increasing tendency towards undersaturation in scenarios with high CO2 emissions (Figure 2).
Changes in nutrient loads and thus productivity mainly affect the range of seasonal variations (amplitude), and accordingly also extremes towards “both ends”. In the case of a highly eutrophied Baltic Sea, the seasonal variations of pCO2 and pH are thus more pronounced than in a more oligotrophic case (see Fact Box 2).
Photo: Julia Nyström, Metsähallitus
Many marine species are threatened by the ongoing ocean acidification, while other species appear unaffected or might even benefit from the increasing CO2 in the environment.
Calcifying organism are particularly threatened by the ocean acidification, but the increasing pCO2 in the marine environment also affects a range of physiological processes. Such effects are not limited to calcifiers. There are no universal thresholds for acidification parameters that apply to all organisms. Responses can vary significantly between species and communities, and there are still large knowledge gaps in terms of ecological effects of acidification. Primary producers may benefit from the improved carbon uptake energetics, whereas negative effects are most common in macrozoobenthos and potentially fish.
Studies from the Baltic Sea indicate that many of the organisms are not sensitive to moderate-to-large changes in CO2 level and pH, partly because of the naturally very large seasonal variations, and partly because of the ability to adapt to the slowly developing acidification. However, a gradual shift in pH and the acid-base system over time will both be directly harmful to some organisms (at least in the more pessimistic CO2 emission scenarios) and favor/disfavor some species more than others, leading to altered communities.
In addition, the effects of acidification are strongly modified by interactions with other drivers, including warming, enhanced stratification, and deoxygenation. This means that acidification can become an additional pressure on top of the other ones. To minimize the impact of acidification, it is for that reason important to alleviate other pressures that organisms have to cope with.
Photo: Michal Jarmoluk, Pixabay
For historical reasons, acidification in coastal seas is rather poorly studied, although dedicated studies have emerged in recent years. In the Baltic Sea, relevant measurements have been limited to the open sea monitoring stations and a few coastal sites. To improve the understanding of gradually emerging signs of acidification – particularly concerning the potentially contrasting trends between different areas – it is necessary to expand the existing marine monitoring programs with regard to inorganic carbon system parameters, and further to ensure measurements of high quality.
Marine monitoring should include inorganic carbon system parameters for assessing acidification in the Baltic Sea. The technology required for precise measurements of pH and the other inorganic carbon system parameters (i.e., AT, CT, and pCO2, see Fact Box 1) is in place. To assess acidification, at least two but preferably three of the inorganic carbon system parameters should be measured. Historically, pH and AT have been measured most regularly. pCO2-measurements are nowadays automated in a way that they can be operated continuously on commercial vessels, allowing to trace productivity with a unique spatiotemporal resolution.
Measurements of inorganic carbon system parameters are necessary to monitor acidification in the Baltic Sea. An added value of such measurements is that they can also be used to quantify primary production directly based on CO2 uptake and release, providing a robust estimate of eutrophication trends and in extension insight into the linkage between primary production and deep water oxygen demand.
Development of an indicator for acidification in the Baltic Sea is currently under the auspice of HELCOM EN Eutrophication.
Photo: JØNΛS, Unsplash
Carstensen, J., Duarte, C.M., 2019. Drivers of pH Variability in Coastal Ecosystems. Environ. Sci. Technol. 53, 4020–4029. https://doi.org/10.1021/acs.est.8b03655
Dickson, A.G., Sabine, C.L., Christian, J.R., Bargeron, C.P., North Pacific Marine Science Organization (Eds.), 2007. Guide to best practices for ocean CO2 measurements, PICES special publication. North Pacific Marine Science Organization, Sidney, BC.
Gustafsson, E., Gustafsson, B.G., 2020. Future acidification of the Baltic Sea – A sensitivity study. Journal of Marine Systems 211, 103397. https://doi.org/10.1016/j.jmarsys.2020.103397
Havenhand, J.N., Filipsson, H.L., Niiranen, S., Troell, M., Crépin, A.-S., Jagers, S., Langlet, D., Matti, S., Turner, D., Winder, M., de Wit, P., Anderson, L.G., 2019. Ecological and functional consequences of coastal ocean acidification: Perspectives from the Baltic-Skagerrak System. Ambio 48, 831–854. https://doi.org/10.1007/s13280-018-1110-3
Müller, J.D., Schneider, B., Rehder, G., 2016. Long-term alkalinity trends in the Baltic Sea and their implications for CO2-induced acidification. Limnol. Oceanogr. 61, 1984–2002. https://doi.org/10.1002/lno.10349
Schneider, B. and Müller, J.D., 2018, Biogeochemical Transformations in the Baltic Sea - Observations Through Carbon Dioxide Glasses, 110p., Springer Oceanography, ISBN 978-3-319-61698-8, https://doi.org/10.1007/978-3-319-61699-5
The oceans play a central role in the global carbon cycle because of processes in seawater that influence air-sea CO2 exchange. When gaseous CO2 dissolves in seawater, it reacts with water and forms carbonic acid (H2CO3). The protons (H+) of the carbonic acid may partly dissociate, changing the concentrations of bicarbonate (HCO3-) and carbonate (CO32-) through chemical equilibrium reactions.
When added CO2 equilibrates with the large pool of dissolved inorganic carbon (CT) in seawater (i.e., the sum of aqueous CO2, carbonic acid, bicarbonate, and carbonate), the bicarbonate concentration increases while on the other hand pH, carbonate, and also the saturation levels of calcium carbonate minerals (CaCO3) decrease. Carbonate (mobilized by dissolution of calcium carbonate minerals) can form bicarbonate by reacting with either CO2 or H+ – added carbonate thus functions as a buffer by increasing the seawater’s capacity to neutralize acids.
The chemical reactions that follow additions of e.g. CO2 or carbonate continue until new equilibria between the dissolved inorganic carbon species are established (proportions of the different CT species as a function of pH are indicated in Figure 3). This complex chemical system is often referred to as the marine inorganic carbon system. In marine and brackish waters, the inorganic carbon system exerts the main control on pH.
Analytically, the inorganic carbon system is determined by four measurable parameters;
The inorganic carbon system parameters undergo changes on different time-scales. There are processes that mainly affect short-term variations of e.g. pH, including daily and seasonal changes of production/respiration patterns (Figure 4–5), and there are processes that drive long-term changes, resulting in a gradually shifting baseline of pH, or in other words a change of the normal operation space experienced by marine organisms. The main process driving such changes is the uptake of anthropogenic CO2, but changes in AT concentration/buffer capacity can also produce long-term pH changes (Figure 6).
Monitoring and scientific basis
Erik Gustafsson, Bo G. Gustafsson, Jacob Carstensen, Gregor Rehder and Vivi Fleming
ISBN 978-92-893-6980-0 (PDF)
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