Seagrass beds

Compiled by: Dieter Boedeker, Germany

1. Description of the habitat

Baltic seagrass beds are mainly characterised by the vascular plant species, Zostera marina and/or Zostera noltii, but depending on the salinity gradient they can be associated with other algae or limnic vascular plants species. Z. marina forms dense beds with trailing leaves up to 1m long, typically on sand and sandy mud (and mixed sediments). Zostera noltii forms dense beds, with leaves up to 20 cm long, typically in the hydrolittoral region and very shallow sublittoral, on mud/sand mixtures of varying consistency.

Seagrass beds are important for sediment deposition, substrate stabilization, as substrate for epiphytic species, micro-invertebrates, and as habitat for many fish species.

(EUNIS classification A 5.53; A 2.61).

2. Distribution (past and present)

Seagrass beds with Zostera marina occur in the entire Baltic Sea area, but not as far north as  the Bothnian Sea, whereas seagrass beds with Zostera noltii are restricted to the western and southern parts of the Baltic Sea area including the Bay of Mecklenburg and the Gulf of Gdansk. The most striking feature is the decrease in depth range of Zostera marina beds from 6¨m to rarely more then 2 m since the beginning of the 20^th century (Schramm 1996). 

3. Importance (sub-regional, Baltic-wide, global)

Seagrass beds are considered to be of Baltic-wide importance in the HELCOM area. They are integral part of the European Habitats Directive Annex I Habitats “1140 (Mudflats and sandflats not covered by seawater at low tide”, “1110 (Sandbanks which are slightly covered by sea water all the time)”, 1160 “Large shallow inlets and bays” and 1150 “Estuaries”.

4. Status of threat/decline

Seagrass beds belong to the most threatened marine biotopes in the Baltic Sea area. They are part of the “endangered” HELCOM biotope (HELCOM, 1998) “Level sandy bottoms dominated by macrophyte vegetation (HELCOM Code 2.2.3.2).   

5. Threat/decline factors

Eutrophication and pollution, mass occurrence of filamentous macroalgal mats, bottom trawling, sand and gravel extraction, offshore installations.

6. Options for improvement

A Baltic-wide biotope inventory and a threat assessment is needed. Seagrass beds should be considered as highly sensitive and therefore in high need of strongest protection. Eutrophication must be reduced significantly through further programmes and measures.

7. References

HELCOM (1998). Red List of Marine and Coastal Biotopes and Biotope Complexes of the Baltic Sea, Belt Sea and Kattegat - Including a comprehensive description and classification system for all Baltic Marine and Coastal Biotopes. Baltic Sea Environ­ment Proceedings 75, Helsinki Commission. 115 pp.

Schwamm W (1996). Veränderungen von Makroalgen und Seegrasbeständen. In: Lozán J L, Lampe R, Matthäus W, Rachor E, Rumohr H, Westernhagen H von (1996) Warnsignale aus der Ostsee. 150-156.