The Killing of Coral

So, time for blog number 2 in Ocean Acidification Week! Today I just want to look at a couple of articles which discuss how ocean acidification will affect living organisms. You may have noticed that this topic has been in the news fairly recently.  For example a few stories like this caught my eye a few weeks ago. The Ocean in a High CO2 World Symposium in California was also held at the end of September, and I would recommend having a look at their twitter page which gives a pretty good chronicle of what went on there.

The first article I’m going to take a look at today is 'Coralreefs under rapid climate change and ocean acidification' by Hoegh-Guldberg etal. (2007). Obviously the main argument of this article concerns coral reefs and the impact of ocean acidification on them. You may be asking why coral reefs are affected by ocean acidification, and I think the diagram below (featured on the second page of the article) should help you link up the equation I posted in my last blog to why coral is affected. As you can see the external skeleton of coral is made from CaCO3, so the reduced concentration of carbonate ions caused by ocean acidification can prevent the growth of dense coral.

 

So why is that a problem that we're considering in this blog, when I have specifically said I want it to link into fisheries? Well, it is likely that ocean acidification will reduce the skeletal density of coral. This makes it more likely for the coral to be eroded through the grazing of fish which target lower density corals, such as parrotfish, and by waves and storms. If the coral cannot accumulate at a rate which is at least equal to the rate it is being eroded at, the quality and diversity of the habitat provided for fish by coral will be reduced. Fish mortality will rise because of increased predation, due to there being fewer hiding places, and also because there will be less food for young fish.

It's important to note a few key points here, such as that some areas will be affected at a different rate to others, among them, the Great Barrier Reef, the Coral Sea and the Caribbean Sea are more likely to face dangerously low aragonite saturation levels that others. Other area are also more vulnerable to the consequences of ocean acidification, for example in Asia, fish caught in coral reefs contributes to a quarter of the annual fish catch, providing food for 1 billion people. In over-stretched fisheries like these, the impacts of ocean acidification are only going to make the problems worse.

Orr et al.’s 2005 paper from Nature, entitled ‘Anthropogenic oceanacidification over the twenty-first century and its impact on calcifyingorganisms’ focuses on corals at high latitudes, where because CO2 is more soluble at cold temperatures, the ocean is becoming more acidic. These cold water corals which are essential to fish habitats are likely to be affected by ocean acidification. The impacts are likely to affect many, if not all trophic levels within polar ecosystems. However it is also noted that assessment of these impacts is difficult due to a lack of data.

The article also mentions how plankton will be affected by ocean acidification. Some plankton, like corals, also have external calcium carbonate skeletons. Pteropod species are predicted to shift to lower latitudes (where the temperatures are warmer, so the ocean is less acidic). If Pteropods do disappear from polar regions, zooplankton which feed on these species will be affected, as will fish species such as North Pacific salmon, cod, mackerel and herring.

So, there we have it, a just a few ways in which ocean acidification is going to affect fish stocks. Both the articles discussed above present the issues and impacts as being fairly certain and definitely detrimental. However the response of organisms will vary from region to region, and between species. Later in this blog I hope to look at how this area of science has to be careful to make sure it keeps an open mind about its predictions for the future. Scientists are now discovering that ocean acidification may have impacts far beyond those we first thought of. A good example is in this article, which discusses how ocean acidification may be making hermit crabs act more boldly. It is accompanied by a fairly spooky looking video, very appropriate since today is Halloween!