Reefs shine in many colours

Reefs as submerged rock formations rising from the seafloor or as formations of rock outcrops of organic origin. Underwater boulders, as well as the submerged portions of rocky shores and islets, are included in such reef-like, hard-bottomed habitats. Although there are no organic coral reefs on the Finnish coast, the underwater rocks and stone piles are teeming with life, particularly in the clear waters of the outer archipelago zones.

Reef zones

Reefs are typically characterised by multi-layered algal and invertebrate animal communities. The biota is determined by the salinity, the amount of light, and the openness of the habitat, hence there are large regional differences between sea areas. A special feature of the reefs in the Bay of Bothnia is the abundance of water mosses, e.g. Fontinalis spp.

Close to the water surface is a zone of annual filamentous algae dominated by green, brown, and red algae. Below this, a zone of wrack kelp commonly occurs, and finally, the deepest vegetation zone is dominated by the red algae.

Below the zones lit by sunlight, the rock surfaces are covered with mussels and polyps. Although the actions of ice and waves action on open shores can even scour away the algal vegetation completely from hard surfaces, these plants are replaced by new species over the spring and summer months. The abundant algae in the light-filled zone provide shelter and nourishment for many invertebrates, fish, and birds.

The filamentous algae zone

Although the species composition of filamentous algae in the zone nearest the water surface varies regionally, the most common species by far is the so-called mermaid’s hair alga, i.e. Cladophora glomerata. Rapid-growing filamentous algae quickly occupy rocky surfaces as the water rises and falls, and this algal zone is often very uniform.

The well-lit and warm surface waters offer the juvenile stages of invertebrate animals offer ideal growing conditions among the dense growth of filamentous algae.

 Filamentous algae swaying along with the current.
Filamentous algae can be a sign of eutrophication, but they also provide a habitat for many species.

The wrack kelp zone

The wrack kelp zone, i.e. Fucus spp., usually begins at the bottom of the filamentous algal zone, and the biota blends seamlessly from one zone into another.

The wrack kelp zone is characterised by large algal species, on and under which grow many other algae, as well as many species of invertebrates that attach themselves to the hard substrate. The wrack zone maintains an extensive community of animals, consisting especially of a variety of floating and creeping invertebrates. The most common species include crustaceans, such as amphipods and isopods, as well as the lagoon cockle and the river nerite snail.

Red algae zone

Both red and brown algae grow in the red algae zone. The most common species in this zone are clawed fork weed, the fragile red-beaded Ceramium tenuicorne, as well as low-growing ruffled species like Coccotylus truncatus, and the stalked leaf bearer, i.e. Phyllophora pseudoceranoïdes.

The algal species of this zone are characterised by their ability to survive at depths where the available light is insufficient for other plant species. Like the larger wracks, the red algae communities enliven the reef by offering extra structures, which provide shelter and food for the diverse animal community living within it.

 Red algae covered stones and boulders in shallow water, jellyfish in the background.
Red algae thrive also in shallow water close to the shore.

Blue mussel and polyp communities

The algae zones can only extend as deep as there is enough available light for them to photosynthesise. However, when photosynthesis becomes impossible, the hard surfaces become covered instead by blue mussels and polyps. In particular, mussel communities provide food and shelter for a wide variety of invertebrates and birds.

The increase in water turbidity resulting from eutrophication has caused all algal zones to narrow, and in many places, the zones even overlap. Increased sedimentation on the bottom will cause fine sediment to accumulate on the bedrock, which will make it difficult for species spread by currents to settle and successfully attach to their new growing sites.

Reef species

  • Water mosses (e.g. Fontinalis spp.) 
  • Mermaid’s hair (Cladophora glomerata
  • Wrack kelp species (Fucus spp.) 
  • Amphipod crustaceans (Gammarus spp.) 
  • Isopod crustaceans (Idoteabaltica
  • Lagoon cockle (Cerastoderma glaugum
  • River nerite snail (Theodoxus fluviatilis
  • Clawed fork weed (Furcellaria lumbricalis
  • Ceramium tenuicorne
  • Coccotylus truncatus
  • Stalked leaf bearer (Phyllophora pseudoceranoïdes)