Research project OMAI – Operational Marine Acidification Indicator
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. 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.
In marine and brackish waters, the acidity of the water is mainly controlled by the inorganic carbon system. 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.
Read the project's science brief here
Key messages
- Anthropogenic CO2 emissions will, unless they are reduced, bring about a planetwide acidification of marine ecosystems.
- Ecosystem effects of acidification are currently small in the Baltic Sea, but CO2 emissions could in a worst-case scenario lead to a pH decline of more than 0.6 units and probably large changes in species composition in the coming two hundred years.
- Coastal seas are influenced by processes in their catchment areas, leading to significant regional differences and also to much larger natural variations than in the open ocean – such processes can either enhance or counteract acidification.
- To assess acidification and to broaden the understanding of drivers of acidification in different areas of the Baltic Sea, it is necessary to improve the temporal and spatial coverage of acidification monitoring.
- In addition to a broadened understanding of current acidification trends and possible future development, monitoring of acidification provides insight into changes in productivity patterns.
- An indicator for acidification in the Baltic Sea is currently under development.