Can we talk about deserts under water?
If you imagine a desert, you may picture an arid place where water is scarce at extreme levels. You could also imagine a quite inert place, with little life: this connects with the concept of "underwater desert". However contradictory it may seem, there are deserts under the sea, places with little life that unfortunately are more and more frequent.
‘Underwater deserts’ are large inert areas of bare rock in which, at most, we find some sea urchins. The bad news is that they have not always been like this. They used to be areas with algae or marine plants forests, highly productive and very rich in biodiversity since they were the habitat of many species (places to feed, reproduce or rest). Although we can find deserts all over the planet, they are much more common in warmer zones.
The existence of these deserts indicates that the ocean is in danger. Unfortunately, it is an expanding problem. The more underwater deserts we have, the less resilient the sea is against climate change. The structure generated by algal forests - similar to that generated by terrestrial forests - gives life to a large number of species, including the fish we eat. In addition, losing underwater forests implies reducing the quality of the waters, which has infinite consequences that can impact various sectors such as fishing, tourism, leisure ...
Size matters to determine the existence of an ‘underwater desert’. Small naked patches with groups of urchins are not consider 'deserts', not even if there are several patches close to each other. However, in these areas it is necessary to monitor whether, over time, these patches merge and the inert area widens substantially. An ‘underwater desert’ can have different magnitudes, but it requires at least a few meters without any type of vegetation to be considered as such.
Deserts have a direct relationship with sea urchins since they eat algae. Specifically, the species Arbacia lixula and Paracentrotus lividus. The first is the most common in the Mediterranean - you probably had a spike on your body sometime-; the second, on the other hand, is more common in the Canary Islands and the Atlantic Ocean.
Why are sea urchins more abudant now than before? One of the most important reasons is that they do not have as many predators, there are not may fishes large enough to be able to eat a medium-sized sea urchin. These species of fish - such as the Dentex dentex or the Diplodus sargus - are no longer so abundant due to overfishing, which is one of the main causes of the existence of ‘underwater deserts’.
Other causes are global warming, poor water quality, invasive species and the increasingly recurring heat waves, as happened in Australia, in 2012: a heat wave raised the average water temperature by 2 degrees for 10 weeks, which caused the collapse of the ecological system of the area and the disappearance of the underwater forest, leaving a desert of size equal to 150,000 soccer fields.
Early detection is extremely important, as is monitoring and gathering information. We need to identify all the affected areas to know what happens and what causes it. For this, different techniques are used such as the use of aerial drones -very useful when the waters are clear-, satellite images, or marine citizen science. Through marine citizen science anyone can help, including you. In the project "Hidden deserts" of this platform you can collaborate by informing the scientific teams about a possible "underwater desert" with or without presence of sea urchins. It is as simple as uploading a photograph, the position and some extra information, such as the depth and the presence of sea urchins and other fish, to the web. This is how you can contribute to identify incipient deserts.
We must tackle the enormous problem of ‘underwater deserts’ and each of us can do our bit, at least, by spreading the word.
Want to know more? Listen our podcast episode nº13 "Desiertos submarinos" at Ivoox or Spotify.
Photo: Underwater desert at Cap de Creus Natural Park by Club d'Immersió Biologia (CIB-UB) . Source: Observadores del Mar.