Bladder wrack bottoms – the lush groves of the Baltic Sea

Bladder wrack communities are a familiar sight to anyone active along the coast. With its forest-like vegetation, which borders rocky shores, wracks, i.e. Fucus spp., are the largest and most prominent algae community in the Baltic Sea and have been the subject of much research.

Wracks occur at depths of 0.5 to 5 metres and form dense habitats if the water salinity exceeds 4 ‰. The accumulation of loose sediment on growing surfaces, as well as competition for space with rapidly growing filamentous algae, can prevent the propagation of Fucus species.

The potential growth depth of wracks is limited by the depth of light penetration and the abrasion effect of waves and ice in shallow water. It is assumed that due to climate change, the general warming of seawater, as well as the alleviation of ice winters will to contribute to the permanent establishment of wrack species on shallow bottoms. By contrast, there has been a decline in wrack communities in recent decades.

Habitat affects the size of wracks and their canopy

The thallus or frond of wrack kelp in good condition generally grows from 20 to 60 cm long. However, in sheltered conditions, the tallest canopies can even exceed one metre in length.

In open and/or low salinity habitats wracks form fewer branches and often remain low-growing in appearance. In the eastern Gulf of Finland, especially at the edge of the archipelago zones, wrack kelps often grow to only 10-15 cm in length and do not reproduce sexually.

In exposed locations, the wrack canopy often lacks the characteristic bladders or gas-filled blisters, which support the foliage in sheltered areas, keeping it vertical in the water.

The sizes of the so-called Fucus forests may vary from a few square metres to several hectares. The size of the vegetation is most influenced by the quality of the seabed, the available light, and the nutrient content of the seawater. The density of this seaweed's canopy also varies, with an average of 21 adult plants per square metre.

Bladder wrack is often an indicator of a good status of a habitat.

Wrack seaweed habitats are important to many other species

The wrack seaweeds form one of the key habitats of the hard bottom of the Baltic Sea, by maintaining and protecting a large number of associated species. Several other algae can be found living on or under wrack kelps, such as the green alga, known as mermaid’s hair, as well as brown filamentous algae like Ectocarpus siliculosus and Elachista fucicola. There are also several species of fish which shelter and feed in the wrack’s canopy, such as the viviparous blenny and the three-spined stickleback.

Algae and animal species of the wrack seaweed bottoms

  • Mermaid’s hair alga (Cladophora glomerata
  • Ectocarpus siliculosus
  • Elachista fucicola 
  • Amphipod crustaceans (Amphipoda) 
  • Herbivorous grazing crustaceans (Isopoda) 
  • Water snails (Gastropoda) 
  • Viviparous blenny (Zoarces viviparus
  • Three-spined stickleback (Gasterosteus aculeatus)