Shallow marine bays teem with life

Large shallow marine bays have been identified as important habitats throughout the Baltic Sea region and also across Europe. They are part of the habitat types defined by the EU Habitats Directive, which member states are required to protect.

In addition to being valuable in themselves, shallow bays also provide ecosystem services beneficial to humans, such as fish and recreational opportunities. Bays also contribute to the overall health of the Baltic Sea: their abundant vegetation retains nutrients from runoff and helps protect the sea from eutrophication.

Shallow marine bays host a diverse range of species

Shallow marine bays teem with life. Their shores are often lined with reed beds, while the outer waters are dominated by submerged and floating-leaved plants. The plant diversity is overall rich and varied.

Among the vegetation lives a variety of invertebrate animals. Some burrow into the muddy seabed, others climb on plants or swim among them. Many insects lay their eggs in the warm and calm waters of the bays, where their larvae develop before eventually taking flight as adult insects.

Among fish, especially pike, perch, and cyprinids seek out shallow coastal waters to spawn. Their fry begin life in the sheltered and gentle conditions of the bays. Larger fish, as well as many water and shorebirds, gather to feed on fish fry and invertebrates burrowed in the seabed. Due to the rich birdlife, many large shallow marine bays have been included in the national bird water protection program.

Plant communities dominated by pondweeds and watermilfoils

In shallow and sheltered bays, the seabed vegetation is often dominated by either watermilfoils or pondweeds. Pondweed-dominated plant communities are common along the entire coast, although the species composition varies depending on the openness of the site and the salinity of the water.

Watermilfoil-dominated communities typically occur on sheltered muddy bottoms, but also on open mixed substrates. Watermilfoils tolerate eutrophication and turbidity slightly better than pondweeds, but both plant groups can also coexist in the same communities.

Both watermilfoil and pondweed habitats are defined in Finland as protected habitat types. Their condition has remained stable, and this is expected to continue in the future.

Endangered aquatic plants thrive in shallow marine bays

With some luck, one might find an endangered plant species in shallow bays, as most of Finland’s coastal endangered aquatic plants grow in shallow shoreline waters. Many endangered species enjoy special legal protection, and it is prohibited to destroy or degrade their habitats. By protecting natural and ecologically valuable marine bays, biodiversity can be effectively promoted and the ecosystem services provided by the bays safeguarded.

Eutrophication and heavy use degrade the condition of bays

Large marine bays are popular recreational destinations: warm and shallow waters, along with the sheltered nature of bays, encourage boating and the construction of summer cottages along the shores. However, the heavy use of bays, shoreline modifications, and maintenance of boating routes degrade their natural state.

In addition to dredging and shoreline construction, shallow bays face other pressures. These include the general eutrophication trend in the Baltic Sea, reed overgrowth caused by excessive nutrient loads, and invasive species. The condition of large shallow bays along Finland’s coast has therefore been assessed as poor.

The status of wide shallow bays in Finland is bad, and no positive development is in sight. The assessment is based on a 2025 evaluation of the conservation status of habitat types under the EU Habitats Directive for the period 2019–2024.

Osion sisältö

Watermilfoil habitats

Pondweed habitats