BS13 Red-listed marine habitats

>> Background information

Status of underwater habitat types

The most acute threat to Baltic Sea habitat types is eutrophication. According to the first red-list assessment of Finland's habitat types completed in 2008 more than half of the marine underwater habitat types were evaluated as threatened and one third as near-threatened. Only one habitat type was evaluated as of least concern in all parts of the coast. Unlike most other habitat types, the filamentous algal zone found in shallow water (hydrolittoral) has benefited from the large-scale nutrient enrichment of the Baltic Sea.

The most threatened of Finland's underwater habitat types are Red Algae and Bladder Wrack communities on hard subtsrates as well as Eelgrass and Blue Mussel communities. In addition to a country-wide assesment, the viability of underwater communities was assessed separately for five different sea areas: Gulf of Finland, Archipelago Sea, Bothnian Sea and The Quark and Bothnian Bay. In terms of these, the status of underwater communities seems to be worst in the southern parts (Gulf of Finland and Archipelago Sea) and best in the northern parts (The Quark and Bothnian Bay) of the Finnish marine area.

In general, underwater communities are poorly known when compared with other habitat types. The present classification of Baltic Sea habitat types is rather coarse. Several habitat types also remain inadequately known at least in some parts of the coast.

Impacts of eutrophication

Eutrophication changes underwater communities in many ways. The increased availability of nutrients increases primary production, including the growth of algae. Abundant algae turns the water more turbid causing less light to fall on plants at the seabed. As a result, underwater vegetation zones begin to shrink. The epiphytic algae growing on underwater plants also become more abundant and further deteriorate the plant communities.

More organic matter falls on the seabed as primary production increases in the upper water colums. Decomposing the increased amount of organic matter consumes more oxygen and increases the occurence of anoxic seabed areas. This leads to detrimental or even lethal effects on the whole local fauna (see also BS7).

In addition to eutrophication, the habitat types of Baltic Sea are threatened by construction, foreign species and oil and chemical transportation together with other sea traffic.