On the hard seabed, dominant red algae use the last glimmers of light

The red algae zone is usually located from depths of two to ten metres on reefs and underwater rocky outcrops below the wrack kelp community, where other algae are rarely found.

Bottom communities dominated by red algae are often diverse and composed of many species

The most common large red algae species along the coast, include the clawed fork weed (Furcellaria lumbricalis), black and purple siphon weeds, as well as the fragile and red-beaded Ceramium tenuicorne. The exposed rocky shores of the outer archipelago may also include low-growing, ruffled red algae such as Coccotylus truncatus and the stalked leaf bearer, i.e. Phyllophora pseudoceranoides.

In addition to red algae, such hard-bottom communities also include brown algae, such as wrack kelps, and the delicate brush-like clumps of Battersia arctica. Moving deeper, blue mussels also become more common. Here too, gleaming white barnacles, now a well-established alien species dot the hard seafloor dominated by the red algae.

The largest perennial red algae, i.e. the clawed fork weed, provides a growing surface for many filamentous algae to grow upon, such as the red-bead alga, as well as brown filamentous algae species like Pylaiella littoralis and Ectocarpus siliculosus. In well-lit areas, the familiar green alga, known as mermaid’s hair (Cladophora glomerata) also grows on top of fork weed. A red algae community composed of more than one species of red algae may often be revealed growing under the yellow or brownish filamentous algal growth by gently moving the surface vegetation aside.

As with filamentous algae, pieces of clawed fork weed that are ripped off from their growing surface by waves or ice can be carried to shallow sandy bottoms and form loose algal carpets with ruffled red algae (C. truncatus) and wrack kelps (Fucus spp.). Depending on the water currents, these loose algal mats can either remain on the seabed or may drift to the shoreline to form banks of seaweed, which are a lush growing medium for beach plants.

Red algae communities, as well as mixed communities of red algae and wrack kelps, serve as spawning grounds for Baltic herring (Clupea harengus membras) on reefs and exposed rocky shores. Many benthic animals also thrive among such algal communities.

Bright crimson red algae growing on rocks. — The hard bottoms dominated by colourful red algae are an eye catching sight.

Red algae thrive best in brackish water with the highest salinity

Almost all of the red algae on the Finnish coast are of marine origin. The richest and most diverse communities of red algae can be found in the outer archipelagos of the Bothnian Sea, Archipelago Sea, and the Western Gulf of Finland, where the waters are clear and are sufficiently high in salinity.

Many other algae species thrive in good growing conditions, and the red algae communities are delicately mixed with the surrounding wrack kelp and filamentous algal communities. Bottoms dominated by red seaweeds are often the deepest occurring algal communities, underneath which the seafloor is dominated by blue mussels and polyps.