The Acidic Devil

Continuing from my last post, this entry will deal with the impact of Ocean Acidification (OA) on coral reefs. But first, how are OA and climate change related and what is the process behind it?

Here are some worrying facts from Hoegh-Guldberg et al. (2007) that show the link:

• The concentration of CO₂ now exceeds 390 ppm
• 25% of anthropoenic CO₂ emissions currently enter the ocean
• Seawater carbonate concentrations have reduced by ∼30 μmol kg^–1 seawater  
• The ocean has become more acidic by 0.1 pH unit 
• Since 1900 the average ocean temperature has increased by 0.74°C

This image from Hoegh-Guldberg et al. (2007) clearly indicates the relationship of CO₂ and pH (through the carbonate on the left hand side). Therefore as CO₂ concentration in the atmosphere increases, the pH decreases becoming more acidic as indicated by the decrease in carbonate concentration. 

How does this process work? 

Well, from the equation, it is not too difficult to follow. As CO₂ is highly soluble in seawater, it reacts to form an intermediary state of carbonic acid. The carbonic acid then dissociates to bicarbonate (HCO3-), carbonate (CO32-) and hydrogen (H+) ions. With increasing CO₂ concentrations and subsequent increased dissolved CO₂, it results in more H+ ions being buffered by carbonate and consequently, there is a reduced availability of carbonate ions to combine with calcium ions. Crucially, this inhibits the process of calcification and therefore reduces the ability of marine calcifiers, such as coral to build.

The Calcium Carbonate Saturation State (Ω)

This saturation state is highly variable but is greatest in the tropics. Calcium carbonate is deposited by organisms as calcite, aragonite or high magnesium calcite. The crystalline form we are interested in as aragonite as aragonite is incorporated into corals' skeletal growth (Coles, 2008). Therefore the aragonite saturation state is a significant limiting factor in the ability of coral to maintain its structure. 

Coral Response to Reduced Calcification as a Result of Reduced Aragonite

Hoegh-Guldberg et al. (2007) highlights three possible responses:
1. Reduced linear extension and skeletal density of corals.
2. Physical extension or growth rate will be maintained but at the cost of skeletal density. 
3. Maintain their skeletal growth and density by investing more energy in calcification.

These responses do come at a cost as they promote side effects that affect the ability of coral reefs to perform some of their benefits. Please read this paper for more information. 

Future Projections

For ease of viewing I have decided to tabulate the future projections that I have found. Furthermore, it will give you the key elements of information to be able to understand the impacts of OA. Please note that the first three scenarios do not take into account physiological acclimation or evolutionary mechanisms that could alter the arrival of these scenarios. The bottom two scenarios are from a paper by Orr et al. (2005) that focuses on colder environments. 

Projection Limitations

A paper by Pandolfi et al. (2011) highlights that these future projections are complicated by physical acclimation and the role of evolution in terms of phenotypic plasticity. Furthermore, the paper states that there is insufficient knowledge surrounding the impact of OA on other environmental variables. Whilst projections are useful, Pandolfi et al. (2011) argue that a more well rounded approach be adopted that includes all issues. Please have a further read as this truly is interesting!

I hope this has given you an idea of OA and what  the future holds!

Stay tuned, 

Seb