Increasing atmospheric carbon dioxide (CO2) is not only altering climate, but it is making the oceans more acidic. This process, known as ocean acidification, is predicted to lead to massive rates of extinctions particularly of corals.
Given the huge biological, ecological and economic impact of ocean acidification, a large amount of research effort has been dedicated towards understanding the process and organisms’ physiological and behavioural responses.
But a paper published recently in Marine and Freshwater Research highlighted that the experimental parameters of many experiments on ocean acidification, particularly in the laboratory, may in fact be incorrect. Therefore, our current understanding of the effects of ocean acidification on marine organisms may need to be revisited.
A large number of experiments on marine or terrestrial organisms, such as corals, insects or trees, compares the physiological, growth morphological and behavioural responses of target species in current versus future levels of atmospheric CO2. Current levels of atmospheric CO2 are usually regarded in the order 350 ppm to 400 ppm. Future levels of atmospheric CO2 are typically taken as what the Earth’s atmosphere will contain in the year 2100, or values in the order of 600 ppm to 700 ppm. There is no protocol or consensus as to what the exact CO2 value should be with some experiments even taking a pre-industrial value of 290 ppm (e.g. Ghannoum et al 2010) or going beyond 700 ppm to values in the order of 1000 ppm (e.g. Ibrahim et al 2010) and even as high as 6000 ppm (e.g. Anderson et al 1985).
ocean CO2 and pH levels
Increasing atmospheric CO2 to 700 ppm or 1000 ppm will decrease the pH, or acidity, of seawater. The process of ocean acidification is explained in detail elsewhere. Briefly, the oceans absorb approximately 30% of the atmosphere’s CO2. So increasing CO2 into the atmosphere will lead to more CO2 being absorbed by oceans. When CO2 dissolves in seawater it reacts with water to form carbonic acid. This is a weak acid that soon dissociates into biocarbonate and hydrogen ions. It is the increase in hydrogen ions that is leading to ocean acidification.
Scientists researching ocean acidification typically use pH values in their experiments as predicted by the Intergovernmental Panel on Climate Change (IPCC) – this is similar to how atmospheric CO2 values are determined in experiments. Current values of pH in oceans is approximately 8.1, down from a pre-industrial value of 8.2. By 2100, it is predicted that ocean pH could be in the order of 7.8 or 7.7 (http://ocean.si.edu/ocean-acidification ). Therefore, ocean acidification experiments in the laboratory will establish a control treatment with a pH of 8.2 and an experimental treatment with a pH of 7.7.
experimental pH levels may not match field conditions
Challener et al (2016) were interested in whether these experimental parameters actually reflected field conditions. In particular, they were interested to know the extent and magnitude of diel and seasonal fluctuations in seawater pH. Their study site was located in Florida, USA, and they collected seawater samples at various times of the day and over the year. On a daily basis, the researchers found that pH fluctuated between 7.70 and 8.06. Over several seasons, pH fluctuated between 7.36 and 8.28. Therefore, natural diel and seasonal fluctuations of marine water pH was greater than the typical pH levels set in laboratory experiments.
The study by Challener et al was on a nearshore, seagrass dominated, marine environment and may not reflect the pH environment on, say, coral reefs or deep ocean. Therefore, the vast literature on ocean acidification should not be ignored based on this one study. The more pertinent point the authors make is not to assume that a value for an experimental parameter will reflect the reality of what the experimental subject experiences in the field. Rather, characterising and understanding the field environment of the experimental subject prior to a controlled experiment in a laboratory setting is needed.
sensors to measure ocean pH and dissolved CO2
Andersen, IH., et al 1985. Growth, photosynthesis and photorespiration of Lemna gibba: response to variations in CO2 and O2 concentrations and photon flux density. Photosynthesis Research, 6: 87-96.
Challener, RC., et al 2016. Variability of the carbonate chemistry in a shallow, seagrass-dominated ecosystem: implications for ocean acidification experiments. Marine and Freshwater Research 67, 163–172.
Ghannoum, O., et al 2010. Photosynthetic responses of two eucalypts to industrial-age changes in atmospheric [CO2] and temperature. Plant, Cell & Environment, 33: 1671–1681.
Ibrahim, MH. et al 2010. Changes in growth and photosynthetic patterns of oil palm (Elaeis guineensis Jacq.) seedlings exposed to short-term CO2 enrichment in a closed top chamber. Acta Physiologiae Plantarum, 32: 305-313. http://link.springer.com/article/10.1007%2Fs11738-009-0408-y