This coming Monday we will talk about ocean pollution. A current focus on plastic pollution in our oceans has – to some extent – caused us to forget about another major threat to our oceans, that of acidification.
Ocean absorb about 30% of carbon dioxide (CO2) that enters the atmosphere. As emissions of CO2 to the atmosphere increase, then the amount taken up by oceans (the size of the ocean CO2 sink, in other words) also increases. Oceans are therefore an important sink for anthropogenic (i.e. human) CO2 emissions. But what happens to that additional CO2 in oceans? Basically CO2 dissolves in sea water forming carbonic acid (H2CO3). Increased concentrations of carbonic acid reduce the pH (increase the acidity) of seawater. This increased acidity impacts other compounds and ecological processes, including the availability of carbonate ions (the building block for calcareous marine life, such as corals). For example the exoskeletons of corals are largely formed from aragonite, a form of calcium carbonate (CaCO3). We have know for some time that consumption of carbonate ions starves calcareous marine life of the material they need to grow (and, e.g., to keep pace with rising sea level) and to protect themselves from potential predators. Moreover the increased acidity can result in the dissolving of existing calcareous shells. So calcareous marine life faces two problems as a result of acidification – a shortage of carbonate to produce new shell material, and a weakening of their existing shells as a result of the increased acidity (reduced pH) of ocean water. Marine life faces other problems of course, including over exploitation,oceanic warming and pollution …. And not just pollution by plastic, pollution by toxins such as heavy metals is also becoming an issue, as we will discuss in this coming week’s tutorials!
Early last year (2018) the Woods Hole Oceanographic Institution (WHOI), US, summarised the threat of ocean acidification on corals, focusing in particular on the link between ocean acifidication and the weakening of coral skeletons. Click here to view the article. Note that researchers at WHOI found that coral growth was much more complex than previously had been thought, and this complexity perhaps explains why some corals in some parts of the world are more vulnerable to ocean acidification than others. Basically what the researchers found was that the polyps – the tiny animals that form coral – were still able to produce aragonite at lower pH, but the aragonite was thinner, or weaker, because of an abundance of carbonic acid (HCO3–) ions relative to carbonate (CO32-) ions in seawater. This results in corals located in exposed areas, where wave strength is greater, for example, being more vulnerable to damage than those corals in less exposed locations, even though the pH of the seawater might be similar.
Below you can watch an excellent, relatively short, documentary on the subject on ocean acidification, produced by the BBC and released in 2018. Australia gets a bit of a hammering in the documentary. The Great Barrier Reef is one of the world’s great coral ecosystems and is off the northeastern coast of Australia. Australia is one of the world’s largest exporters of CO2 emitting coal – in fact coal is Australia’s most valuable export ….
https://www.youtube.com/watch?v=Bxcq9QemIP0