Arxiu d'etiquetes: global change effects

Ocean alert: Coral bleaching is massively happening!

We would like that the main picture of this post had been modified using Photoshop, but unfortunately this is not the case. Thanks to the project XL Catlin Seaview Survey, we now know that coral bleaching is massively happening. What causes coral bleaching? How does coral become bleached? Which is the importance of coral in the ocean ecosystems? These questions and more are answered in this post. 


Coral bleaching is the result of the expulsion of symbiotic algae living in the coral tissues (zooxanthellae), producing them to become completely white.

Coral before and after a bleaching event (Picture: Kendall Kritzik, Creative Commons).
Coral before and after a bleaching event (Picture: Kendall Kritzik, Creative Commons).

The presence of zooxanthellae is frequent in marine cnidarians, especially in species that live in shallow waters, and they are the responsible of the greenish, bluish, yellowish or brownish colour of many coral species. In fact, each cubic millimetre of tissue of the host has 30,000 algae cells. These zooxanthellae are single-celled algae, usually dinoflagellates, that are able to live in mutualism with the coral. So, if zooxanthellae and coral live in mutualism, which are the benefits of this relationship? Coral gets the products of photosynthesis, organic carbon and nitrogen; while the algae receive nutrients, carbon dioxide, protection and a good position with access to sunshine.

Diagram of the location of zooxanthellae in a coral (Picture: Ocean Portal).
Diagram of the location of zooxanthellae in a coral (Picture: Ocean Portal).


Several causes of coral bleaching have been detected:

  1. Increased ocean temperature. Climate change is the foremost responsible of the increase in ocean temperature and this is the main stress causing coral bleaching, but it is not the only one. The rise of temperatures may be also produced by El Niño phenomenon. With just an increase of 1ºC of the water for only one month, corals begin to become bleached.
  2. Reduced ocean temperature. As warmer water ocean may produce coral bleaching, colder water may also produce these events. Some proofs support this idea: in January 2010, cold water temperature in Florida might have produced coral bleaching that resulted in coral death.
  3. Runoff and pollution. Near-shore corals can be bleached due to the pollution carried by precipitation’s runoffs.
  4. Freshwater inundation. Due to a low salinity produced by a freshwater inundation, corals may start bleaching.
  5. Overexposure to sunlight. High solar irradiation causes bleaching.
  6. Extreme low tides. Long exposures to the air can produce bleaching in shallow corals.
  7. Disease. Diseases cause coral to be more susceptible.

All these causes produce a stress to the coral and, as a result, corals expel the algae living in their tissues.


When corals are in a healthy state, they are home to algae, so that they are in a symbiotic relationship. But, when corals are stressed, the photosynthetic machinery of algae produce toxic molecules that cause the corals to expel the symbionts. If the stress is not severe, corals can recover, but they become bleached in severe and prolonged stresses. As a result, corals death because they loose their main source of food and are more susceptible to disease.

Coral bleaching process (Picture: Great Barrier Reef Marine Park Authority, Australian Government).
Coral bleaching process (Picture: Great Barrier Reef Marine Park Authority, Australian Government).


Two worldwide episodes of coral bleaching were detected in the 1998 (which killed 16% of the coral reefs around the world) and 2010, but a recent study carried out by the NOAA and the University of Queensland confirm a more severe coral bleaching episode this year (2015). This new episode, which is triggered by El Niño of this year (together with the global change), is predicted to affect the 38% of the worldwide coral reefs, killing 12,000 square kilometres of reefs. The more altered zones will be Australia and the Pacific and Indian oceans.

Bleaching in American Samoa. The first picture (before) was taken in December 2014 and the second (after) in February 2015 (Picture: XL Catlin Seaview Survey).
Bleaching in American Samoa. The first picture (before) was taken in December 2014 and the second (after) in February 2015 (Picture: XL Catlin Seaview Survey).

Nevertheless, coral bleaching doesn’t only occur in massive episodes. Each year, during summer months, some limited coral bleaching is reported all over the globe.


Despite the fact that coral reefs comprise less than 1% of the underwater ecosystems, they play a major role in the ocean. One quarter of marine life depends on coral because they are the nursery of the sea, so they are an important protein source for animals and humans. Moreover, they protect shorelines from waves and tsunamis. In addition, from an economical point of view, they are one of the most important places of tourist interest and support fishing industries. In fact, they provide food and livelihoods for more than 500 million people around the world.


All the activities you do to lessen your carbon dioxide production are good to prevent the Earth from global change and, therefore, are good to avoid coral bleaching. Keep doing like that! Share with us: which are the actions that you take to prevent global change? 



Why are beaches disappearing?

Probably you have listened that our beaches are disappearing. Why do beaches disappear during storms? Why do beaches not regenerate naturally? There are several causes that explain the regression of line coast, having all of them a human origin. In this article, I want to explain which are the reasons of the regression of beaches and which are the possible solutions to this problem. 


Beaches are zones placed between land and sea where sediments accumulates. Not only are they a place where people can enjoy, but also a habitat for many animal and plant species and with a defensive function.


On beaches, there is a sediment accumulation from fluvial flows. Swell, on the one hand, causes a displacement of these sediments through the coast (with more or less intensity), what is known as longitudinal littoral transport. To maintain a beach, the amount of sediments that disappear from a beach have to be the same that those that are added. On the contrary, the beach reduces (prevails the erosion) or increase. It has to be added the transversal littoral transport, which consists on the swell moving sediments from emerged beach to underwater beach, or backwards. Wind, at the same time, can produce an accumulation of sediments in the more interior part of the beach, creating dunes.

Funcionament dinàmic de la costa (Imatge: Directrices sobre Actuaciones en Playas)
Dinamic functioning of coast  (Picture: Directrices sobre Actuaciones en Playas)

So, a beach is working correctly if:

  • There is a stable source that contribute with the necessary sediments to make a beach.
  • There is a free movement of the sediments through the coast and in the transversal way.

Therefore, the main problems of regression of beaches are due to a modification of one or both factors.


Now, it’s time to talk about the reasons why beaches in our littoral are being reduced or in regression. As mentioned above, these can be classified in two types: causes that reduce the source of sediments and causes that impede their movement.


Construction of dams, with the aim of regulating the flow of rivers, is one of the main causes of the regression of beaches. The construction of dams produce a retention of sediments in the reservoir of water, what impedes their movement river downhill and, for this reason, their arrival in coast. It is this accumulation that explain that the useful life of dams is just 55-60 years. To give an example, in the Ebro Delta (Catalonia) arrives every year 200,000 tonnes of sediments, 10 times less than what is necessary to maintain the delta constant and 100 times less than what arrived in 20th Century.

Les preses produeixen l'acumulació de sediments i, per tant, la regressió de les platges (Foto extreta de Adasa).
Dams produce an accumulation of sediments and, for this reason, the regression of beaches (Picture from Adasa).

The urbanization of littoral zone close to beaches impedes the mobilization of sediments. Without going any further, due to the increase of the interest in the last decades of the population to coast, there have been a massive construction of promenades behind beaches, followed by important flat blocks. This has supposed the destruction of dunes and their plants mostly in all the littoral. Dunes are important zones of accumulation of sediments, so they constitute a sediment reservoir, it is that after a storm, wind can sweep along sand form this ecosystem and, thus, it can naturally regenerate the beach. Moreover, dune’s vegetation give stability to the beach since it affixes the sand and impede its erosion.

La construcció de passeigs marítims sovint va acompanyada de la destrucció de les dunes, el que té un efecte negatiu sobre les platges (Foto de
Construction of promenades is usually accompanied by the destruction of dunes, that has a negative effect on beaches (Picture from

There are other explanations, like the occupation of the surface of rivers by urbanizations or the removal of sand and gravel, among others.

As we have seen with dunes, these could be a source of natural regeneration of beaches. We have to had also in consideration that the destruction of seagrasses (like Posidonia) due to the urban development, bottom trawling, construction of harbours, piping and sewage pipes and the increase of anchors helps that the transported sand during a storm isn’t retained in the zone close to the beach, so this sand don’t naturally return by swell to the original position.

La destrucció de la Posidonia afavoreix l'erosió de la costa (Foto de Periodico de Ibiza).
Destruction of Posidonia contributes to the erosion of coast (Picture from Periodico de Ibiza).


Again, the occupation of beaches by buildings and other infrastructures explains why sediments can’t be spread through coast with freedom. Anyway, it is important to mention the construction of maritime structures. Effectively, we are referring to the construction of dikes, breakwaters and harbours, that constitute a barrier that produce the accumulation of sediments in the opposite side of the current direction, while it produce erosion downhill.

Les obres marítimes alteren la dinàmica del transport de sediments (Foto de Cyes).
Maritime constructions modify the dynamics of sediment transportation (Picture from Cyes).

To all this causes, we have to add the global change that, due to the rise in the sea level, is producing the disappearance of the littoral because it is sinking.


There are several possible solutions to face beach regression, but all of them have their problems:

  • Artificial regeneration of beaches with marine or terrestrial sediments. The dredging of sea floor has a negative impact on plants and animals of the zone, specially if these areas have seagrasses like Posidonia. When the origin is terrestrial, it usually come from quarries (with the posterior crushing), so its touch is not pleasant and it produces the destruction of mountains.
  • Construction of maritime structures like dikes and walls. These are not free from problems because they produce the erosion downhill.
  • Elimination of obstacles that stop the sediments or the free circulation.

Sometimes, the most prudent solution, in the case of not being any interest for population, is to leave the beach to its luck. It means to leave the natural evolution of the beach happen.