There has been an exponential increase in research on the topic of ocean acidification, which broadly concerns chemical changes in the ocean in response to increased concentrations of atmospheric carbon dioxide (CO2). Uptake of CO2 by the oceans from the atmosphere can potentially alter the balance of inorganic chemicals present, in turn affecting biological processes – including photosynthesis and calcification rates at coral reefs (Gattuso & Hansson 2011; Raisman & Murphy 2013). Coral reefs are a major focus of ocean acidification research. There is concern that increased atmospheric CO2 will significantly reduce calcification and so negatively impact the overall health of these iconic ecosystems – held together by calcium carbonate that is secreted by corals.
In this chapter, we provide some background into the physical and chemical processes associated with ocean acidification, before considering the research into the effects of ocean acidification on biological organisms, with a particular focus on Australia’s Great Barrier Reef.
Ocean acidification has been described as an impending ‘ocean calamity’, and the ‘evil twin of global warming’. Public interest has fed the explosion of research on ocean acidification, considered unprecedented in the marine sciences (Browman 2016). Indeed, as shown in Figure 2.1, there were nearly 4000 articles published on ocean acidification between 2000 and 2015.
Most of the articles describe the effects of changes of pH on biological organisms; many of the claims are based exclusively on laboratory experiments (Riebesell & Gattuso 2015). However, a problem with laboratory experiments is that they cannot capture the complexities of the real world, not even the tremendous natural variability in ocean pH – which is a measure of ocean acidification.
Some basic chemistry
CO2 occurs naturally in the Earth’s atmosphere in very low concentrations, presently at about 400 ppm (parts per million). Accurate measurements of atmospheric CO2 concentration have been carried out at an observatory in Hawaii since 1959, when the concentration was 312 ppm. Pre-industrial levels are estimated to be about 280 ppm.
There is a natural exchange of gases between the atmosphere and dissolution in the ocean, with flows in both directions. CO2 dissolves in water with its solubility depending on the temperature: the lower
the water temperature, the higher the solubility of a gas such as CO2. An equilibrium may eventually be established when the flows in each direction are in balance. In turn, the strength of the flow in each direction is directly proportional to the concentration. If the concentration of atmospheric CO2 increases, this increases the rate of CO2 flow into the ocean, and eventually a new equilibrium establishes.
After dissolving in the uppermost layer of the ocean, CO2 generates a number of changes to the chemical composition of the seawater. The chemistry of inorganic carbon in seawater is relatively complex, and can be understood in terms of a set of inter-related equilibrium processes. Dissolved inorganic carbon is mainly present in three chemical forms:
- aqueous (CO2)
- carbonate ions (CO32-)
- bicarbonate ions (HCO3-).
There is also a minor amount present as carbonic acid (H2CO3).
So, carbon entering the ocean as CO2 can be transformed into these other inorganic ions and molecules. When these reactions occur, hydrogen ions (H+) are liberated into the seawater, and this in turn affects the pH of the seawater (pH is a measurement of the acidity of an aqueous solution, defined as the negative logarithm of the hydrogen ion concentration in the solution).
The term ‘ocean acidification’ refers to the decrease in pH that occurs when the concentration of CO2 in the atmosphere increases through a chain of reactions that can cause the ocean pH to drop. A neutral solution (neither acidic nor alkaline) has a pH of 7.0. Acidic solutions have a pH less than 7.0, while alkaline solutions have a pH greater than 7.0. Typically, present-day seawater varies widely in its pH, but generally it is alkaline, having an average pH in the range of 8.2 to 8.4.
Ocean acidification does not imply a scenario where the oceans become acidic (that is, the pH of the ocean falls below 7.0). The whole process could also be referred to, in perhaps less emotive terms, as ‘neutralisation’, or alternatively ‘carbonation’ (Gattuso & Hansson 2011) because there is an increase in the concentration of dissolved inorganic carbon.
Another very important component of the inorganic system in seawater is calcium, which can exist as ions in solution (Ca2+) and as solid calcium carbonate (CaCO3). Calcification is controlled by the concentrations of calcium and carbonate ions in the seawater at a particular location. How much the seawater is saturated by these is known as the calcium carbonate saturation state, and it is described by the Greek term Ω (omega). The value of Ω depends on the particular natural form of calcium carbonate considered, commonly either aragonite or calcite. When Ω falls below 1, solid calcium carbonate dissolves.
Some concerns relate to the possibility of the saturation state decreasing as the pH falls. The key issue is, that as the concentration of carbonate ions declines, thereby reducing the value of Ω, it will affect the ability of organisms to undertake calcification, which could potentially have a very detrimental impact on shells and coral reefs.
Different processes, operating on very different timescales, become important in determining the fate of CO2 in the atmosphere. Once released, it takes about a year for CO2 to mix throughout the atmosphere. The ocean consists, broadly, of three layers: the uppermost, or boundary layer; the mixed layer, affected by waves and turbulence caused from wind stress on the sea surface; and the deep-ocean layer. The very uppermost boundary layer of the ocean almost instantaneously equilibrates with the overlying atmosphere. Below that, is the mixed layer, which then absorbs the CO2 through the action of wind, waves and resulting turbulence. Once in the mixed layer, CO2 can be transported to the deep ocean, through processes that occur on millennial timescales…
For the rest of the text refer to the PDF of the chapter, here.
Image credit: Agincourt Reef, GBR, Robert Linsdell, Wikimedia Commons