Zooxanthallae and Bleaching

In the past blog posts we have looked at changes to the corals in the Red Sea and the Barrier Reef, but now it is time to go down in scale, and take a look at something much smaller. Zooxanthallae, as mentioned in previous posts, are photosynthetic bacteria that allows corals to feed. We will take a closer look at the relationship between the two, and how changing environments is resulting in bleaching. 

An enlarged view of the microscopic Zooxanthallae.

The relationship between coral and zooxanthallae is mutually beneficial. The coral provides a safe environment for the zooxanthallae. The bacteria can then produce oxygen and glucose whilst removing waste (this is photosynthesis). This allows the coral to feed and can remain sessile. This is a very important relationship as otherwise the coral would struggle to feed itself, as they cannot photosynthesise themselves. From a more commercial viewpoint, the bacteria is important as it is the zooxanthallae itself that provides the colour for the coral. The bright vibrant colours attract various marine life such as fish, which in turn attracts tourists to destinations all over the world. Thus the relationship helps to bring in an estimated $9.6 Billion per year through coral reef generated tourism. As the bacteria requires sunlight, corals need to exist in shallow, clear oceans. However due to anthropogenic activity, the pristine environment which allows this relationship to flourish is being put under incredible strain.

Corals and zooxanthallae are being put under extreme pressure by rising temperatures. Higher temperatures will result in the bacteria being expelled from the coral. As they bacteria determines the colour of the coral, the coral will become white. This also means that the coral has no way to obtain food. They are not dead however. They can remain almost in a state of hibernation for some time. This gives the conditions a chance to return to acceptable levels. If this happens then the bacteria will return to the coral and the relationship will be back on track. However too long without being able to 

feed will result in the death of the coral. 

The threshold at which the bacteria will leave the coral depends on different types of coral and zooxanthallae. Exposure to high temperatures (32 degrees and above) for as little as 7 hours can result in expulsion of zooxanthallae at 1000 times the normal rate. This shows just how sensitive the corals and bacteria can be, especially to storm events, where there are isolated incidents of rapid 

heating. For example there was mass expulsion of zooxanthallae in Jamaica after Hurricane Flora. Having said this, the corals examined in the study returned to normal after 17 days, so there is some hope for corals, as they are resilient. 

Coral and zooxanthallae have evolved together so they are almost thought of as one. Bleaching remains a serious threat to the coral, and they could be completely destroyed as a result of it. So much would be lost if this were to come to fruition, and the worlds oceans may never be able to recover. 

Sources

1. https://themarketmogul.com/the-economic-impact-of-coral-reefs/

2.http://www.sciencedirect.com/science/article/pii/0022098189901093

Case Study- Red Sea Coral Reef

The Red Sea Coral Reef is the second largest in the world in terms of length, at 1900km long and has an area of 438,000km2. The only coral reef that is longer is the Great Barrier Reef. The Red Sea is located in between Africa and Asia, and is a seawater inlet of the Indian Ocean. Corals in the Red Sea are aged up to 5000 years old. The majority of the coral forms around the shoreline.

The Red Sea Coral Reef

The dominant coral genera are Porites and Acropra. These are both types of stony corals, covering hundreds of species.  Acropora in particular is well suited to reef building as it is very good at taking up calcium carbonate from the water. The coral reefs in the Red Sea are some of the most robust in the entire world. They have a high tolerance to temperature and salinity. The fact that it is resistant to higher temperatures is particularly important due to predicted increases in ocean temperatures. In 2011, the temperature of the Red Sea had increased by 0.7 degrees since 1994. With temperatures set to rise at a faster rate, the durability of these corals will be invaluable.

An example of Porite Coral

The largest portions of the coral reefs can be found in the northern area of the Red Sea, near Egypt and Saudi Arabia. There are smaller amounts of coral to the south of the Red Sea due to the influx of material from the Indian Ocean. There is also a larger amount of mixing due to higher wind speeds, which will mean that coral reefs struggle to develop. This is because more sediment prevents the production of calcium carbonate, which is needed to secure the base for coral reefs.

 As far as the world is concerned, the Red Sea coral reefs are doing pretty well. The amount of coral cover ranges from 50-85% depending on the site being examined. However, not everything is rosy at the Red Sea. There has been significant damage done due to overfishing, and major damage done by irresponsible diving, driven by tourism. The Ras Mohammed National park was established off the coast of Egypt in 1983 to help combat the degradation of the reefs. More parks like this will be needed to created to help ensure the reefs safety.

Sources

1.http://www.coral-reef-info.com/red-sea-coral-reefs.html

2.http://www.natureasia.com/en/nmiddleeast/article/10.1038/nmiddleeast.2011.119

Hard Corals

This blog post will be focused on hard corals, and how these are very significant to the coastal system. Hard corals can just be healthy corals that's are naturally hard, but there is also an order of coral known as Scleractinia, which has a nickname called 'stony coral'. These occur in all the worlds oceans, and as suggested by their name, they are harder and more sturdy than other orders of coral. But why is this hard coral so important?

An example of a Scleractinia Coral.

The answer is that they provide a natural barrier against waves, which helps to protect global urban populations, coastal communities and the beach itself against erosion and water damage (such as flooding). The corals reduce the amplitude of the incoming waves by absorbing energy. The waves then have less erosionary power to cause damage. They act as a dampener against the waves. This ability of coral reefs to act as protection against waves will become even more important due to climate change. It is thought that the frequency and severity of storm events such as hurricanes are likely to increase. This will mean that more and more protection against storms will be required.

Governments spend billions on flood defence schemes the world over. Defra spent £2.56 billion between 2007 and 2011, and this figure is set to rise.

DEFRA Spending From 2006-2015

From the figure, we can see just how much DEFRA has been spending on flood defense. Spending reached a peak in 2010-2011, where it totaled almost £700 million. This is an astronomical figure, and without corals as a form of natural defence, who knows how much it could have been, or how much it will be in the future.

 We can see that so called 'hard engineering' is incredibly expensive, and perhaps other soft engineering techniques should be considered. The question is are soft engineering methods effective enough?

The protective nature of coral reefs assumes that they are healthy. They are no longer as effective at absorbing wave energy. This can be caused by acidification or even bleaching due to high temperatures. This means they are more likely to suffer damage from the waves as their structural integrity has been compromised. This can then result in more coastal erosion as they are then not as well equipped to protect the coastline.

Here we've had a brief look at how hard corals protect the coast. It may well be wise for governments to invest in promoting coral reef growth, as this will help coral populations to soar, as well as being less expensive than large man made structures such as sea walls, groynes and dykes.

Sources

1. Coral.org/blog/hard.corals.natures.seawalls/

2. https://www.google.co.uk/amp/s/www.carbonbrief.org/factcheck-how-much-is-the-government-really-spending-on-flood-defences/amp

Changes to the Great Barrier Reef

Changes to the Great Barrier Reef

A famous example of a coral reef, the Great Barrier Reef (GBR) is the largest in the world. It covers an approximate area of 133,000 square miles, composing 2900 individual reefs and 900 islands. It is famously so big, that it can be seen from space. 1981 was a big year for the GBR, as it was named a world heritage site, due to the sheer number and diversity of the wildlife supported by these reefs.

Great Barrier Reef at its best, showcasing all its diversity.

This was a massive boost for tourism and visits to the GBR, but in the prevailing 30 years or so, the area has been going through some tough times. On the 28^th November, BBC news reported that due to high water temperatures throughout 2016, 67% of corals had ‘died’ in the northern section of the GBR, as well as 6% from the central section. Despite this doom and gloom, some positive news from the southern part of the reef where the vast majority of the reef remains in good health. This was the worst ever recorded bleaching event in the Barrier Reef’s history and with global ocean temperatures set to rise, this may not be the largest bleaching event for long….

As well as rising temperatures, the GBR faces a threat from the declining amount of coral calcification. This is the result of increased ocean acidification. The level of coral calcification has decreased by 14.2% since 1990, the sample area encompassing 328 colonies from 69 reefs that make up the GBR. This is unprecedented in at least the last 400 years. The impact of a decrease in the amount of coral calcification means coral reefs lose their ability to deposit calcium carbonate (CaCO3). This is important as thousands of coral species derive their structural integrity from creating a calcareous skeleton. The peak year was 1970, where calcification was increasing by 1.76 gcm-2 year-1. It’s been downhill ever since.

And it continues to get worse. Kroon et al, 2012, published a paper stating extensive nitrogen, phosphorous and herbicides had been found throughout the GBR. 80000 tonnes of nitrogen now enters the GBR every year, with phosphorous at 16000 tonnes a year. These inputs into the GBR have come as a result of anthropogenic activity, especially from agricultural sources, urban development and deforestation. The danger of these inputs is that it may lead to eutrophication, hypoxia and reductions in coastal biodiversity. This will affect the coral as well, as the algal blooms will prevent coral from using photosynthesis to feed and will therefore begin to bleach.

As we have seen, the GBR has experienced some testing times, and now it is under threat like never before. The outlook doesn’t look promising, however hopefully there can be success built around the presently healthy southern part of the reef. In summary, coral populations have been on the decrease ever since 1970, due to a mixture of anthropogenic factors. In order to save the Great Barrier Reef concerted effort from Australia and other world powers will be required.

 

Sources

Full BBC article: http://www.bbc.co.uk/news/world-australia-38127320

http://science.sciencemag.org/content/323/5910/116.full (Glenn De'ath et al, 2009)

http://www.sciencedirect.com/science/article/pii/S0025326X11005583 (Kroon et al, 2012)