Corals and Starfish

I thought I would write this post today as it is about some organisms we have already covered (quite extensively, as I got a bit carried away!): in these posts: Here, Here and Here as well as Here.

Browsing the news sites while munching breakfast this morning, I came across this story in the Guardian about the Great Barrier Reef and also this one about Caribbean reefs (Full report link in further reading), and I thought it would be a good idea to write the post I forgot to before, about the threats facing coral reefs today.

The articles mention 3 things which are affecting reefs,storms, predation and bleaching.

Storms are perhaps the most obvious cause of damage, the waves during a storm hit the coral reefs (which are in shallow waters, and so take the full force of the waves as they near land), and this breaks the coral.  This video shows the aftermath of a hurricane on a Mexican reef:

Bleaching occurs when the organisms which live inside the corals are expelled by the corals, leaving the reef looking white:

Bleached coral, image from NOAA Coral Reef Watch

This bleaching has a number of causes, changes in the level of incoming solar radiation can affect particularly sensitive photosynthetic organisms, as too high light levels can be damaging to the cells which they use for photosynthesis,prolonged exposure at low tides and bacterial infections can also impact the organisms.

A number of the causes can be linked to human activity, some of which you will have heard of, and others which are less familiar, so I will briefly cover the main ones here:

Ocean acidification is the result of increasing carbon dioxide levels in the ocean, as shown by this equation:

CO2 (aq) + H2\leftrightarrow H2CO3 \leftrightarrow HCO3 + H+ \leftrightarrow CO32- + 2 H+

What this says is that carbon dioxide and water react to form a product known called carbonic acid (which you may know from sparkling water drinks). This compound forms ions (a molecule or atom which has a charge) of bicarbonate (HCO3) which you may know from bicarbonate of soda used in cooking,and hydrogen ( H+). These then react further to form carbonate (CO32- ) and two hydrogen (2 H+).  The + or – indicates whether there is a positive or negative charge, so carbonate has a negative charge of 2.

Now, the reason this makes the ocean more acid is that, as you know, the scale used for measuring acidity is the pH scale.  This scale is basically about how many hydrogen ions there are in a solution (in this case, sea water). More hydrogen ions in a solution mean that the solution is more acidic.

This is balanced usually by calcification, which is the reaction of calcium with carbonate to form calcium carbonate (chalk or limestone are the best known forms of this), which is used in the shells of many organisms, and is used in coral reef building.  The problem comes when more CO2 is entering the oceans than can be taken out by natural processes, which leads to the oceans increasing in acidity.  One of the problems with this is that, as you probably know, acid and chalk or limestone do not go so well together, and increasing acidity affects the organisms with shells, or corals.  This puts them under stress, and they can expel the microorganisms which live inside the coral tubes.

Increasing temperatures in the oceans also places corals under stress. We all have a temperature range which we can survive within, and some organisms have smaller ranges than others, especially those which live in zones with fairly constant temperatures.

Apart from concern about coral reefs and other marine organisms, one of the problems with a warming ocean is that warmer liquids can hold less gas (Like when a beer goes flat as it gets warm, this is because the carbon dioxide in the beer is released as it warms up). The image below shows the solubility of carbon dioxide in water as temperature increases.  One of the implications of a warming ocean is that it will be less able to store carbon dioxide, and so will release more to the atmosphere.

Solubility of CO2 in water. Image from Wikipedia

At present, there is both increasing acidity in the oceans, and increasing temperatures, because the temperature increase is not enough to reduce the carbon dioxide entering the ocean, because it is not saturated, which means it is still able to take dissolve gases.

This movement of carbon dioxide into the oceans is a very important part of the carbon cycle.  The diagram below illustrates this cycle, and as you can see, the largest store of carbon is within the oceans. The numbers represent gigatons of carbon (A gigaton is 1 followed by 9 zeros tons, or 1000000000 tons)

Carbon Cycle, the bold black text is the amount of carbon stored in each place, and the purple text is the flux, or amount moving between each area. Image from Nasa.

Finally, and apologies for boring you with the chemistry above, the most abstract of the relationships for today: Crown of Thorns starfish.

Very brightly coloured Crown of Thorns starfish. Image from Wikipedia

More usual colouring for a Crown of Thorns: Image from Wikipedia

This is definitely a very pretty starfish, but it is bad news for our friends the corals.

It preys on corals, and does so by climbing on top of the coral and turning its stomach inside out (extruding) to dissolve the corals tissue with digestive acids.  Whilst predation in itself is not a problem, and is part of a normal ecosystem, there are occasionally explosions in the population of these starfish, and there is a discussion about the causes of this.  At present, the likely cause appears to be an increase in nutrients in the ocean.

This can be a bit abstract, so I will try to explain it.  I wrote in THIS POST about how the high levels of phytoplankton in the Antarctic are due to the nutrients from the continents being carried to Antarctica by ocean currents.  This ties in to today’s post because there has been observed to be a link to periods of increased rainfall or floods, and an increase in the population of this starfish a few years later. (Link to article in Further Reading)

The mechanism appears to be as follows: During periods of heavy rainfall on land, a lot of nutrients are washed off with the rainwater (runoff).  These nutrients primarily come from fertilizers used for agricultural purposes.

These are carried downstream to the oceans, where they accumulate, and are carried on currents.  These added nutrients allow for an increase in the population of phytoplankton, and these are food for starfish larvae.  This increase in food allows for more of the larvae to survive to mature into starfish, which then feed on the reef.

This video shows a survey being done of the Crown of Thorns population on the Great Barrier Reef

Finally, overfishing, both for food, and for aquarium fish can affect the balance of the ecosystem, as this video briefly discusses.

What can we do about this?

Well, reduction in the amount of fertilizers used in agriculture would reduce the run-off, but this has an impact on agricultural yield for the communities in the regions affected by the monsoon flooding.  In Australia and other places, population control measures for the starfish have been implemented with varying levels of success.

The one thing we can do something about is the overfishing of reefs and other areas, by buying sustainable fish, and not having exotic fish in home aquariums.

The good news is that reefs do recover from bleaching effects, given enough time, and provided that the surrounding ecosystem is not too damaged, and that the effect is a “pulse”, that is, a one off event which causes for example a sudden surge in ocean temperature (like El Nino).  Sustained increases in temperature, predation or acidity may be harder to recover from.

Further Reading and links

Australian Institute of Marine Science page on Crown of Thorns:

Birkeland,C: Terrestrial runoff as a cause of outbreaks of Acanthaster planci (1982)

Brodie et al: Are increased nutrient inputs responsible for more outbreaks of crown-of-thorns starfish: An appraisal of the evidence (2004)

Cox et al: Acceleration of global warming due to carbon cycle feedbacks in a coupled climate model (2000)

CRC Reef Research Centre: Controlling Crown of Thorns

Graham et al: Coral reef recovery dynamics in a changing world (2001)

IPCC page on Ocean Acidification:

IUCN workshop on Caribbean reefs report:

NASA page on Ocean Carbon:

Orr et al: Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms (2005)


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