Reefs shine in many colours

Reefs are submerged rock formations rising from the seafloor or 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.

Zonation of hard bottoms

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.

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

Below the sunlit zones, rocky surfaces are covered by mussels and polyps. Although the effects of ice and wave action on open shores can scour algal vegetation away from hard surfaces, these plants are replaced by new vegetation over the spring and summer months. The abundant algae in the light-filled zone then continue to provide shelter and nourishment for many invertebrates, fish, and birds.

The filamentous algal zone

Although the species composition of filamentous algae in the zone closest to the surface varies regionally, often the most common species 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 ideal growing conditions among the dense jungles 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 invertebrates.

The wrack kelp zone

The wrack kelp zone, i.e. Fucus spp., usually begins at the bottom of the filamentous algal zone, and the zones tend to blend seamlessly into each other.

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 (i.e. Gammarus spp., Idoteabaltica, Cerastoderma glaugum, Theodoxus fluviatilis). This is why the wrack is one of the most essential species in the Baltic Sea, a so-called key species.

Red algal 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 pseudoceranoides.

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 stationary filter feeders like 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 new growing sites.