Ocean acidification investigations in the Kiel Fjord

Continuous, reliable and direct pCO2 measurements since 2012

Greenhouse gases, and in particular carbon dioxide (CO2), are one of the most discussed topics in society today, influencing political debate, legal framework and business decision-making. From a pre-industrial 280ppm, atmospheric CO2 has risen to a present-day level of 390ppm, with this figure expected to rise to up to 1000ppm by the year 2100. An increase in atmospheric CO2 is related to issues such as global warming, rising sea levels and precipitation change and is therefore thought to be one of the main reasons for climate change. Climate change has become more apparent in the media recently following an increasing abundance of extreme weather events which have claimed the lives of many people and caused severe damage to property and homes. Although human-influenced climate change is a scientifically accepted phenomenon, the global debate about the practical steps individuals and governments must take to confront the problem is still unresolved.

Another major and important change caused by the increasing atmospheric CO2 concentration is a phenomenon called ocean acidification (OA). Its potentially threatening effects on marine life are still not entirely understood today and are the subject of various research endeavours around the globe. About 30-40% of anthropogenic CO2 dissolves into rivers, lakes and oceans. As water absorbs CO2 from the atmosphere, the strongly dissociating carbonic acid is formed, which leads to a decrease in the water’s pH; hence the process is referred to as “acidification”. If CO2 emissions continue to increase at the same or an even higher rate as today, the oceans will take up huge quantities of CO2 and OA will be accelerated. The speed at which the amount of dissolved CO2 increases and pH drops may make it difficult for organisms, especially for calcifying organisms, to cope.

Whereas CO2 partial pressure1 (pCO2) fluctuations in offshore ocean regions are relatively small, coastal habitats are characterised by much higher variability. These habitats provide insights into the tolerances and thresholds of marine life to the conditions in a future acidified ocean. The Kiel Fjord, Germany, located in the western Baltic Sea, is such a habitat with such a strong pCO2 amplitude and therefore well-suited for OA related studies². The Baltic is a brackish water, semi-enclosed sea which is characterised by a salinity gradient from the west to the east. The Kiel Fjord lies in the transition zone between the low saline water masses from the central Baltic and the high saline North Sea water. In order to understand the variability inhabiting organisms are exposed to, a detailed monitoring of the system is crucial. By weekly monitoring of the carbonate system in previous years, using water samples for total alkalinity and dissolved inorganic carbon determination, there is a good understanding of the processes of the system during the course of the year. However, discrete water samples for chemical analysis only provide an incomplete temporal trace and do not account for the daily or even hourly variability. There is the risk of missing short-term events of elevated pCO2. In order to obtain the complete temporal picture of pCO2 variability on a scale of minutes to months, in addition to discrete reference sampling, a fast responding HydroC-CO2 sensor has been installed since July 2012. Due to its small size this autonomous and versatile underwater sensor3 was easily installed off the side of a pontoon in front of the GEOMAR/ Helmholtz Centre for Ocean Research, Kiel (Figures 1, 2 and 3). The pCO2 data can be either looked at in real-time and stored on a connected computer or the internal data logger of the sensor can be used. Additional parameters measured next to the sensor are salinity and temperature. The floating pontoon guarantees an installation at a constant water depth of approximately 0.5metres.

The first data obtained by the HydroC-CO2 in summer of 2012 fully matches earlier observations. The data set of the first two months is shown in Figure 4. Fjord pCO2 is highly variable on an hourly basis. These fluctuations result from wind driven upwelling of near bottom water masses. Together with a steep salinity gradient, a stable thermocline stratifies the water body from late April to November and inhibits the gas exchange of the water masses below the pycnocline with the atmosphere. Due to the high productivity in the eutrophicated Baltic, large amounts of organic matter sink to the bottom and their remineralisation depletes the water oxygen content. As oxygen consumption is coupled to the release of metabolically produced CO2, hypoxic water masses are necessarily supersatured with CO2. These hypoxic and CO2 enriched water masses are then welled up to the surface during periods with strong offshore winds and increase seawater pCO2 up to eightfold above atmospheric levels. Depending on wind direction and strength, the high CO2 periods can last from a few days up to several weeks. Within a few minutes pCO2 may change by more than 100µatm which is considered to be a major stress for inhabiting benthic organisms. Annual mean pCO2 in the Kiel Fjord is 700µatm, but during summer mean levels exceed 1000µatm and observed peaks even reach up to 3000µatm. Due to the non-linear characteristics of the carbonate system, increasing atmospheric CO2 concentrations will lead to even higher levels in future. Therefore, coastal habitats with strong present day CO2 variability will be much more impacted by future CO2 levels than the much more stable open ocean. The projected pCO2 may reach peaks higher than 5000µatm and altogether levels which might have detrimental effects on sensitive marine organisms.

Studies like the one outlined here help us to understand the complex interactions between highly variable pCO2 levels and their potential impact on benthic organisms, especially against the backdrop of accelerating ocean acidification. Therefore autonomous pCO2 sensors are useful and important tools for the continuous monitoring within – but not limited to – coastal water bodies. In times where climate change and ocean acidification are of a major concern, it seems unquestionable that reliable and high quality data are indispensable. Even if measurements do not directly help reduce carbon dioxide emissions, they still help scientists understand the complex set of facts and form the basis for the required mitigating actions.


1. Directly measurable quantity related to dissolved CO2

2. Thomsen, J., Casties, I., Pansch, C., Körtzinger, A., and Melzner, F. (2012). Food availability outweighs ocean acidification effects in juvenile Mytilus edulis: laboratory and field experiments, Global Change Biology in press.

3. Fiedler, B., Fietzek, P., Vieira, N., Silva, P., Bittig, H.C. and Körtzinger, A. (2013). In situ CO² and O² measurements on a profiling float, Journal of Atmospheric and Oceanic Technology, 30, pp112-126.

Fietzek P., Thomsen J. & Esser D., 2013. International Ocean Systems 17(2), March/April 2013. Article.

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