What is an underwater forest?

  Oct 3, 2022  |    

When we imagine a forest, a large area of ​​trees usually comes to mind, but… What is a forest? Do forests only exist on land?

A forest is much more than a set of trees; it is an ecosystem in which tree-shaped species predominate, but they do not necessarily have to be trees, and where many other living beings live that are related to each other, giving rise to a more complex structure or ecosystem.

These species that form structures are called "engineering species" because they "build" and shape the ecosystem, allowing other organisms such as plants, animals, bacteria, fungi, etc. to settle.
The larger and more complex the structure they form, the more different species coexist in the forest, because there are more hiding places with different light and protection conditions. For example, a natural forest of large trees offers more shelter than an area of ​​isolated bushes, so large forests tend to live in more different species of plants and animals than in a land with only bushes.




In addition, forests do not only exist on land, because in the sea there are also species that form structure - the "engineers" - such as marine plants, algae or corals.
The structure formed by these underwater forests serve as a refuge for many marine organisms, especially juveniles or other small individuals, and this attracts larger animals (among other octopuses, cuttlefish and fish such as bream and dentex) that come to feed on the small ones. For this reason, underwater forests serve as a breeding ground and/or feeding area for many fish, crustaceans (crabs, shrimps...), molluscs (octopuses, cuttlefish, shellfish, nudibranchs...) and other invertebrates such as sea urchins and starfish. Among them are many species of fishing interest.



So what are the “engineering species” that make up underwater forests? Well, it basically depends on the depth and the latitude at which they are found. In the depths, where sunlight does not reach, underwater forests are always made up of corals. 

However, in the shallower depth it depends on the latitude on Earth:

  • In tropical seas, corals also dominate at shallow depths (in symbiosis with microalgae).
  • In the temperate-cold seas they are formed by marine plants and algae, standing out for their large size the kelp forests (the “Kelp” reach up to 30 m tall!). These algae can be so large because in temperate and cold areas the waters are usually richer in nutrients than in tropical areas.
  • In the Mediterranean they are also formed by marine plants and algae. In this case, the underwater forests are smaller (up to 1 m) because the Mediterranean is a sea with very few dissolved nutrients.


Specifically, shallow Mediterranean underwater forests are made up of marine plants and algae:

  • Among marine plants, the most abundant and studied is Posidonia oceanica, which is an endemic species (that is, exclusive) to the Mediterranean. In sheltered areas it is easy to find their smaller "cousins"; Cymodocea nodosa, Zostera noltii and Zostera marina. They form extensive meadows that, although they do not have the shape of a tree, generate enough structure to be considered "engineers" (especially Posidonia).


  • Among the most important algae in the Mediterranean, a group of brown algae called Cystoseira stands out for their size and abundance, and when they are in good condition they form forests up to 1 m high (these do have a tree shape). To learn more about Cystoseira algae, you can watch this video where we interviewed Dr. Kike Ballesteros. In the Mediterranean there are also other "large" algae, such as those of the genus Sargassum and Laminaria, but they are less frequent and usually live at greater depths.



If you found interesting this information, we will be sharing soon the second part of this post: “The unknown services of an underwater forest

What services do you think these ecosystems provide to us? We will tell you soon :)


 If you want to take part in the protection of this underwater forests, please click this button and contribute to the MedGardens initiative: 



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