Filamentous algae bottoms are nurseries for invertebrate animals

As their name suggests, the filamentous algae grow as single, thin strands that, under favourable conditions, form broad zones on almost all hard surfaces. Although filamentous algae zones can be found from the waterline down to almost beyond the reach of light, such algae are most abundant in fairly shallow water, in depths of less than four metres.

There is an abundance of different kinds of green algae at the waterline, among which both filamentous brown and red algae can also be found. The most common shallow species of shallow filamentous algae include the green mermaid’s hair, the brown algae Pilayella littoralis and Ectocarpus siliculosus, and the fragile red-beaded Ceramium tenuicorne.

As depth increases and the light fades, the annual filamentous algae are increasingly replaced by larger, perennial algae, such as wrack kelps and clawed fork weed. Even though the filamentous algae thrive among them, the larger algae dominate the landscape and the community.

Filamentous algae are excellent at adaptation

The filamentous algae are characterised by rapid growth and can occupy the rocky surfaces cleared by winter ice or waves more effectively than the larger perennial algae. Filamentous algae benefit from eutrophication and flourish alongside humans in the changing conditions of the Baltic Sea.

Filamentous algae can also attach and grow on the surfaces of other seaweeds such as wrack kelps and fork weeds, as well as on any other hard surface, such as dock structures, boat hulls, and buoy chains.

The multiform and species diversity of filamentous algal bottoms varies in space and time

The green alga, known as mermaid’s hair, i.e. Cladophora glomerata, is generally a dominant species of filamentous algal bottoms. However, particularly after long and cold winters, it may have to give up its growth area to the so-called golden sea hair alga (Dictyosiphon foeniculaceus). In late autumn, it will, in turn, be replaced by the red-beaded algae, i.e. Ceramium tenuicorne.

The filamentous algae bottoms are often covered by a thin layer of diatoms (plant plankton), which attracts large numbers of tiny grazers to feed within the shelter of the filaments. For example, crustaceans such as amphipods, ostracods, and snails, as well as insect larvae feed on both the diatoms and the filamentous algae underneath.

However, moving between the jungle of thin filaments is difficult, and as the invertebrates grow, they move to the shelter of larger algae.

Rihmalevävyöhykkeen joukossa kasvaa muutama rakkohauru. — Pyaiella littoralis is commonly found in the Baltic Sea, except in the Bay of Bothnia.

Species of filamentous algae bottoms

Mermaid’s hair (Cladophora glomerata)
Pilayella littoralis
Ectocarpus siliculosus
Ceramium tenuicorne
Golden sea hair (Dictyosiphon foeniculaceus)

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Fish species of the Finnish marine areas

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Benthic animals live on both hard and soft seafloors

Blue mussels are one of the most important and common benthic animals in the Baltic Sea