The light in the water
Have you considered that algae growing in the sea can have different colours? If you look in the aquariums around you, you might see some algae that are green, some that are brown, but also several that are red! Why is that? Well, that is because the different algae use different parts of the light spectrum for their photosynthesis – the process that all plants and algae use to create oxygen and energy from light and carbon dioxide.
In the water, the light does not reach as far as in the air. This is because the water contains many different particles, phytoplankton and zooplankton, bacteria and dead organic matter. The further down into the water you get, the more sunlight has been absorbed by the particles, and the darker it becomes. The difference in light gives rise to many different habitats and living conditions for the organisms that live in the water.
The deeper into the water, the darker it becomes. Algae of different colors dominate at various depths, as they are adapted to live in varying amounts of light. Green algae require the most light, followed by brown, and finally red algae.
Photo: Bengt-Littorin-CC-BY
Green algae, like this gutweed, use chlorophyll for photosynthesis. Chlorophyll requires a lot of light - that's why green algae need to live as close to the water surface as possible.
Photo: W.carter-CC-BY-SA
Cladophora glomerata is another green algae that lives near the water surface. It grows in thin threads and can make the water appear green and murky.
Green algae can also be single-celled - in that case, individual algae cells are not visible to the naked eye. However, many single-celled green algae together can form a greenish soup that obscures light for organisms living deeper down.
Photo: Patrik-Nylin-CC-BY-SA
Both green, brown, and red algae can grow near the water surface, but only green algae must do so for their survival. Here, various colors coexist in the same location, hosting different species such as sea lettuce, sugar kelp, oarweed, serrated wrack, and filamentous red algae.
Photo: W.carter-CC-BY-SA
Green algae have the greatest need for light
It is brightest close to the surface. Green algae and green plants grow and thrive there. Green algae use green pigment to capture light for their photosynthesis. Many green algae are small and unicellular, and cannot be seen with the naked eye. But other green algae are multicellular, and can form large layers of thin threads, and some species resemble lettuce!
In the zone where green algae and green plants live, lots of habitats and hiding places for various animals are formed. If the sea contains too much nutrient, there is a risk that some species of green algae will grow so much that they block the light for the other species. This is called algal blooms, and eventually it leads to major problems for many species in the sea.
Brown algae can live deeper below the surface than green algae but can also thrive just at the water's surface.
Photo: Rickard-Tornblad-CC-BY-SA
Bladderwrack is a common brown algae in Swedish waters that forms important habitats for various animals both near the surface and further below.
Kelp is a group of brown algae that can form vast, dense kelp forests beneath the surface, providing habitats for numerous other organisms.
Photo: Pbsouthwood-CC-BY-SA
An extensive brown algae forest beneath the surface in the Mediterranean Sea.
Photo: Arnaud-Abadie-CC-BY
Where enough light reaches, green algae can coexist with brown algae a bit below the surface. Here, the brown algae "dead man's rope" is surrounded by the filamentous green algae Cladophora glomerata.
The brown kelp forests
Brown algae are a large group of multicellular algae. The seaweed species bladderwrack, which is very common in the Baltic Sea, Öresund and along the Swedish West coast, is a brown alga. The brown algae use a different pigment than green algae for their photosynthesis, and this allows them to live deeper down than green algae are capable of. Despite this, brown algae such as bladderwrack also thrive near the surface, but they can survive much further down as well. A brown alga called kelp can form huge kelp forests that are perfect homes for many other species, and our bladderwrack builds large underwater thickets where fry and other animals can seek shelter.
Red algae dominate where the light is no longer sufficient for either green or brown algae.
Photo: Peter-Southwood-CC-BY-SA
Among the red algae, there is a group called coralline algae - they build up calcium plates in a similar way to how some corals construct their skeletons. Here, a red coralline algae is seen growing in a coral cave.
Photo: Diego-Delso-CC-BY-SA
It's not only the red algae that display red colors in the depths where red algae dominate. Many other species have also developed red hues to better blend in with the red algae, such as this coralline sculpin.
Here, several different species of red algae are visible in the waters off the coast of New Zealand.
Bild: Peter-Southwood-CC-BY-SA
Red algae can also be bushy, creating important hiding places for various small animals.
Photo: Rickard-Zerpe-CC-BY
As it gets darker, red algae also find it increasingly difficult to survive with the limited light. Eventually, other organisms take over the space. Here, red algae and coral animals, such as the species dead man's fingers, are visible.
Photo: Learning-by-action-CC-BY-SA
At a depth of 50 meters in the Koster Sea on the Swedish West coast, brittle stars and Anthozoas thrive. At such depths, light rarely reaches enough for any algae to survive.
Photo: Geological-Survey-of-Sweden-SGU-CC-BY
Here, at a depth of 80 meters in the Koster Sea, stands a sea anemone. At these depths, there's never enough light for algae to survive in Swedish waters.
Photo: Geological-Survey-of-Sweden-SGU-CC-BY
Red algae, mussels and polyps
Down where there is no longer enough light for brown algae, red algae dominate. Red algae use a pigment that can take advantage of the light that penetrates deepest down in the water, before it becomes too dark for any photosynthesis at all. Just like the brown algae, red algae can grow close to the surface as well, but they are more abundant further down.
In Swedish waters, red algae can live down to a depth of 40 metres, while in warmer waters they can be found as far down as 80 meters – and in some cases even deeper! Most red algae are multicellular, but some species are unicellular organisms. Red algae also form habitats that are very valuable for the organisms that live at these depths.