Aarhus Universitets segl

No. 429: The sea's pH balance

Carstensen, J., Reducha Andersen, N., Christensen, J. & Mohn, C. 2021. Havets pH balance - påvirkning fra klima og næringsstoffer. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 76 s. - Videnskabelig rapport nr. 429. http://dce2.au.dk/pub/SR429.pdf


In this report, we have described the marine carbonate system and how it is influenced by atmospheric exchange of CO2, inputs of nutrients and alkalinity, metabolic processes, temperature and salinity. In contrast to the oceans, where CO2 exchange with the atmosphere is the dominant process and pH decline can be reliably predicted, variations in pH and other components of the carbonate system are substantially more dynamic in Danish estuaries and coastal areas due to the complex interplay among many different processes.


Using data from research projects and the Danish national monitoring program for the aquatic environment and nature (NOVANA), we show large variations in pH on diurnal basis in shallow macrophyte-dominated systems, on seasonal scale in productive estuaries and on multi-annual scale in the response to changes in nutrient inputs. These variations are, to some degree, natural and controlled by metabolic processes (production and respiration), but increasing CO2 in the atmosphere will gradually lower the baseline and the range of variation in pH, causing coastal acidification. In this report, we have shown that the majority of Danish estuaries and coastal areas have experienced increasing acidification over the last 2-3 decades, which is primarily due to three factors: 1) increasing CO2 in the atmosphere, 2) increasing temperature and 3) decreasing nutrient inputs. Fortunately, the alkalinity of Danish marine waters is relatively high, compared to e.g. the Baltic Sea, and increasing over time, which has improved the buffer against acidification.


Acidification changes the marine chemical composition, which may potentially affect marine organisms. Lower pH will make it difficult or more energy demanding for certain organisms to form their skeletons and shells because calcium carbonate is more prone to dissociate and thereby reduce the availability to calcifying organisms. At present, this is not an imminent problem to Danish marine species due to high alkalinity and availability of calcium carbonate, but further acidification could potentially pose a problem to calcifiers in the future. Increasing pCO2 in marine waters poses another potential problem for heterotrophic marine species that will experience increasing difficulty with exuding CO2 as a byproduct from their respiration. Current pCO2 levels in surface waters are not considered lethal or critical, but this may potentially change towards the end of this century. Critical levels of low oxygen and high pCO2 are already observed in deeper hypoxic waters, and it is important to monitor the combination of these two parameters with the expected increasing CO2 in the atmosphere. Although there is no evidence of current pCO2 increases affecting marine organisms in Danish waters, it is possible that the existing acidification has already altered the competition among species and, therefore, the biological communities. Acidification of Danish coastal waters will increase and this will change the marine chemistry to new levels that the present marine organisms have not experienced before. It is unknown if the present marine organisms can adapt to such changes with the same speed as acidification is progressing.