Zostera noltii (Hornem.), Dwarf eelgrass, (Angiospermophyta)

Ordn. Helobiae, Fam. Zosteraceae, Syn. Zostera nana (Mert. ex Roth.)

Compiled by: Anna Karlsson, Sweden

1. Description

Zostera noltii is a small perennial aquatic plant which grows permanently submerged. It is found mainly in shallow sheltered sea bays, on muddy soft bottoms.

Zostera noltii is distinguished from Zostera marina by its smaller size, and by the colour and shape of the leaves; the leaves of Z. noltii are dull grey-green, very narrow and with a notch in the tip whereas the leaves of Z. marina are shiny green with rounded tips.

Zostera beds are – in addition to their role as a habitat for other organisms - highly productive and a major source of organic matter in coastal ecosystems. The root network of the plants stabilize the sediment and act to reduce erosion of the shoreline.

2. Distribution

Zostera noltii is a rare marine species, and within the HELCOM area its main distribution is along the Swedish west coast and the Danish coast. In Kiel Bay the species is very rare and in Mecklenburg Bay there is a small population (in the inlet Salzhaff). Outside HELCOM the distribution encompasses the European Atlantic and Mediterranean coasts, including Great Britain. A few findings from localities in southern Norway are also recorded.

Zostera beds were severely reduced in Europe due to an outbreak of epidemic disease in the 1920’s. Since then, recovery has been slow and patchy.

In Sweden the species has originally been recorded from 80 localities, but modern data exists only for 15 of these. In a recent survey in the Gothenburg-area, dwarf eelgrass was recorded in 13 of the 60 investigated bays.

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

Zostera is found in five out of the seven Annex I habitats defined in the EU Habitats Directive, and due to its distribution can be considered of Baltic-wide importance.

4. Status of threat

Z. noltii is classified as vulnerable (VU) in the Swedish Red List (2005) and as endangered (1) in the Red List  of the German Baltic Coast (1996). Outside HELCOM the species is considered heavily endangered in the Wadden Sea and has shown significant decline in Great Britain. However, Z. noltii is not listed on the OSPAR list of threatened and/or declining species and habitats.

5. Threat factors

The coastal habitats favoured by Z. noltii are under increasing threat from coastal development, pollution and other forms of human disturbance. Exploitation of shallow bays, i.e. building of roads, harbours, dredging of waterways, have thus caused local disappearances of Zostera noltii. Eelgrasses are generally not physically robust, and their root systems are easily dislodged by trampling, anchoring, digging, dredging and powerboat wash.

Eutrophication poses another threat. Nutrient enrichment encourages rapid growth of epiphytic algae which may cause severe shading of Zostera noltii (and Z. marina).

Furthermore, the recently introduced alga Graciliaria vermiculophylla is rapidly becoming established in shallow bays around Gothenburg. Thus, G. vermiculophylla could potentially be a future threat to Z. noltii.

6. Conservation measures needed

Generally, seagrass biotopes are considered a high priority for conservation measures. Localitites with Z. noltii should be protected from exploitation and nutrient loading should limited from land areas. Surveys of old localities should be performed in order to establish current status.

7. References

Gärdenfors U. (ed.) 2005: Rödlistade arter i Sverige – The 2005 Red List of Swedish Species. Swedish Species Information Centre, Uppsala.

Merck T. & von Nordheim H. 1996. Rote Listen under Artenlisten der Tiere und Pflanzen des deutschen Küstenbereichs der Ostsee. - BfN, Bonn-Bad Godesberg, Schriftenreihe für Landschaftspflege und Naturschutz 48: 1-108. R2005:11, Göteborgs Stad Miljöförvaltningen

UK Marine SACs project. http://www.ukmarinesac.org.uk/