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)


Splashing around

Apologies for the gap in posting, but it has been one of those crazy weeks.   It has been suggested that maybe, when I don’t have the time to research and write an evolution post (They usually take 4-6 hours), that I should write general science and  nature posts.

I have not been sure about this, because I try not to write about my opinions, but I have figured that I can write about things I am passionate about, whilst not having to research for quite so long for each post, and also save some of my fellow bloggers from the email spam I send them with links and stuff.

So, I hope you like this new section of posts, and that I can still manage to show you things you did not know.

Today I would like to introduce you to the Southern Ocean.

This is where the Southern Ocean is located:

Southern Ocean (marked in blue). Image from wikipedia

The Southern Ocean is (in)famous as being a wild place, full of mountainous waves, and generally a very scary place.

The waves are so big, and the weather is so fierce there because basically, there is no land mass for the winds to run into, and lose their strength, and there is no shallower water as we find on the continental shelves, or islands, or land masses to break up storms and waves.

The gap between Antarctica and South America is known as the Drake Passage, and it is notorious for having the worst storms in the Southern Ocean, which is saying something!  The storms which occur here are stronger than the others in the Southern Ocean because the current which circles Antarctica (the Antarctic Circumpolar Current) is one of the largest currents on the planet, moving 125*106 cubic meters of water per second.  This cold water current basically just zips around and around Antarctica, like a car stuck on a roundabout, and keeps the warmer waters from the other oceans away from Antarctica.  This current is squeezed through a gap of 800 km between the tip of South America and Antarctica, and, as a result, the current speeds up, and creates rougher seas (Like when a river is narrower in some parts than others, the speed increases through the narrow parts).  Add this to the winds which circle Antarctica butting up against the warmer air from South America, and you have a recipe for some very very big storms!

The waters are much deeper than average ocean waters, with the depths of 4000-5000m, and the depth of the continental shelf being 400m or so, compared to the “normal” continental shelf depth of approx 130m.

Despite all this (or perhaps because of this), the Southern Ocean is one of the most productive oceans on Earth.  Everyone has seen the penguins in the Antarctic on TV, but they are merely near the top of a huge food web, as this picture shows:

Antarctic Food Web, image from Discovering Antarctica

The reason the Southern Ocean is so productive can seem a bit complicated, but it is really quite straightforward, and I will try to explain it as easily as I can:

One of the cool things about water is that frozen water is less dense than liquid water (hence ice floats), and there is a lot of ice around near the Antarctic.  This frozen water does not contain salt, so the salt is deposited into the surrounding water as it freezes, making that water more salty, and denser.

This increase in saltiness (salinity) makes the surface water denser than the water below it, and because salty water has a higher freezing point than fresh water, it can become colder than the water just below it.

As we know, warm air rises, and cold air sinks, and the same applies to water, so this surface water sinks, and is replaced by warmer water from below, which then cools, and sinks, and so the cycle repeats.

This is crucial, because the water from the bottom of the ocean has a whole load of nutrients in it.  The reasons for this are also fairly straightforward:

In the other oceans, the Atlantic, Pacific, Indian etc, dust blows off of the continents which contains nutrients from the soil.  This falls into the oceans, and drifts down to the bottom (along with any “marine dust” from animals between the surface and the bottom), and is then carried along in ocean currents until it reaches Antarctica, where it rises to the surface. (Sciency link showing the Antarctic thermocline is in “Further Reading”)

These nutrients are used by organisms such as phytoplankton (we met them in this blog post HERE ), which are at the very base of the food web.  Because of this abundance of nutrients, the ocean is able to support a very large number of these photosynthesizing organisms (also known as primary producers).

As an indicator of how many of these organisms there are, in the food web just above them is an animal which looks like a shrimp, called Krill.  This animal is extremely small, 1-2cm long on average, and yet, the 2011 harvest from Antarctica was 180,000 tons, so thats a LOT of Krill!  The estimates of the overall population are hard to come by, due to the difficulties in accessing the region, and the different methods used for calculating the population, however, current estimates are around 350 million tons.

Antarctic Krill. Image from Wikipedia

This seems like a large amount, but it is not just us using the Krill (We do not really eat krill, we use it for farmed fish, something my fellow blogger Argylesock has been writing about a bit lately).  The krill are what is known as a keystone species, that is, they underpin the ecosystem.   They keep the phytoplankton numbers down, and are food for pretty much every other species which feeds in the ocean, including the enormous Blue Whale, which eats nothing but krill, with each adult eating around 6 million krill per day!

Blue whale filtering sea water out of its mouth after dining on some krill. Image from Blue Whale info

So, why am I telling you things you already know instead of writing about more weird evolution stuff?

Well, this morning there was an article in the papers, about a presentation made at the Science Museum in London by the crew of an Antarctic research ship which has been sailing round the Antarctic for a couple of years, gathering lots of really cool data for scientists to spend the next 5 years or so going over (in 6 months, they have analysed 1% of data), and during this trip they made a not so cool discovery:

In the 1980s, a paper was published stating that plastics which were dumped were ending up as small pellets in the open ocean, many hundreds of kilometers from land.  This has led to a lot of scary stories, especially over the last 5 years or so about the “Atlantic Garbage Patch” or the “Pacific Garbage Patch”, with claims that there was more plastic in the sea than plankton (There is not), or that there is a garbage island the size of America (there also is not). What there is however, is many small pellets of plastic floating on, or near the surface of the ocean, which get ingested by marine animals.  There is also a lot of plastic debris washing up on beaches around the world, as we have all noticed.

The Southern Ocean, being so remote, and with its own currents and wind systems, was assumed to be a fairly pristine ocean.  That is until these researchers came back from their trip down there.

Southern Ocean, looking calm for once! Image from NOAA

They found 50,000 pieces of plastic per square kilometre, mostly consisting of microscopic pellets, and weirdly, fibres thought to be clothes residues from washing machines.

For me, as an environmental biologist, and generally someone crazy about nature, this is not good news, but as always, I try to focus on the “What can we do?” rather than yell about how bad we all are.

We cannot at present, remove this plastic from the oceans easily, so we are stuck with what is already there, so we should look at what we can do to not add any more to it.

Firstly, if you use plastic bags from the supermarket, re-use them, or, get a re-useable cloth bag.

Secondly, find out if there is any recycling in your area.  Here in Denmark, we can drop our bottles back to the shop for recycling (and a little extra cash, which is always good!).

There is increasing pressure on corporations, and governments to work to find biodegradable materials for plastics.  I got hold of some rulers which were made of corn starch of all things! (This means it is edible apparently, but it doesn’t look too tasty!)

Finally, try to not buy products which have a ton of plastic wrapping on them.   I saw a single pepper in the supermarket a few days ago which was shrink-wrapped…..what the hell is that about?

The oceans are not just there to look awesome and scary for us, the organisms in them play a vital role in the global food supply, and with the problems that are surrounding global fish stocks, I am not sure we want to mess up the oceans anymore than we already have.   We may not have known before, but we do now, so we should start acting on what we know.

Further Reading:

Guardian Article about the plastic:

UN FAO: Krill fact-sheet:

Sarmiento et al: High latitude controls of thermocline nutrients and low latitude biological activity (Figure 4):

Moore, Lattin & Zellers: Density of Plastic Particles found in zooplankton trawls from Coastal Waters of California to the North Pacific Central Gyre:

Morris: Plastic debris in the surface waters of the South Atlantic (Marine Pollution Bulletin 1980):

Tara Ocean Interview with Chris Bowler: News “Ecologists Fear Antarctic Krill Crisis”: