Algae are traditionally grouped by colour pigments into green, brown and red. Grouping algae based on their colour should not be considered the only criterion for defining algal categories since all groups contain not only species of the named colour but also of other colours.

The majority of algae are defined as plants

Most of the seaweeds belong to the plant kingdom and can be characterised as simple plants. They act as plant photosynthesizers in the aquatic ecosystem and form the basis of the food chain.

Algae differ from terrestrial plants in that they have no roots at all. Instead of leaves and a stem as in higher plants, the structures of algae are called the thallus or frond. Instead of true roots, seaweeds have different attachment structures that allow them to cling onto hard substrates.

Nutrients are not transferred to the algae through these root-like structures. Instead, algae filter all they need directly from the water via the thallus.

Colourful algae bring life to the underwater landscape

Algae make the underwater habitats of the sunlit zone more colourful. They provide shelter and nourishment for many invertebrates and fish. Algae form underwater meadows and forests without which the landscape beneath the surface would be largely barren and bare.

On Finnish shores, seaweed communities of the hard sea bottom typically occur as zones where the species composition is defined by the amount of light and the site’s exposure. At the water's edge, the most common species are green algae. At depth, almost beyond the reach of sunlight, only the red algae survive.

The large brown algae are located between these two zones. In recent decades, the coastal waters have become turbid due to eutrophication, causing the traditional algal zones to narrow and even overlap.

Algae grow on both hard and soft sea bottoms

Although most algae grow attached to hard bottoms, large algae can also be found on sand and silt bottoms. Charophytes, also known as stoneworts, look more like underwater vascular plants than algae. However, since they are rootless nutrient filters they actually belong to the algae group. Likewise, wrack seaweeds torn from a hard substrate can continue to grow on soft bottoms, either loosely rooted in the mud or rolling around in loose ball-like forms.

Blue-green algae are not taxonomically algae at all

Despite their commonly used name, blue-green algae are in fact bacteria and are officially known as cyanobacteria. Although they behave in the water in a similar way to the dinoflagellates and diatoms, which are algae, cyanobacteria have a much simpler structure.