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Z. marina (leaf): 20 to 50 centimeters long, 2-9 millimeters wide
Z. noltii (leaf): 4 to 25 centimeters long, 0.5-1.5 millimeters wide




June through the autumn




root stock, seed spread by water, birds and fish




coastal waters in the entire northern hemisphere

  • Dut: Groot zeegras
  • Lat: Zostera marina
  • Eng: Eelgrass, seawrack
  • Ger: Gemeines Seegras (Großes Seegras, Gewöhnliches Seegras)
  • Dan: Havgræs (almindelig bændeltang)
  • Dut: Klein zeegras
  • Lat: Zostera noltii
  • Eng: Dwarf eelgrass
  • Ger: Zwerg-Seegras (Kleines Seegras)
  • Dan: Havgræs (dværg-bændeltang)
Eelgrass, Foto Fitis,


Eelgrass is a seed plant and not a seaweed. It has roots and seeds just like land plants. There are two species of eelgrass growing in Dutch waters: Zostera marina and Zostera noltii. Up till 1932, there were extensive eelgrass fields in the northern Zuiderzee. The local people called these fields 'weed forelands' since they referred to eelgrass as 'seaweed'. When Wieringen was still an island, many of the residents earned their income by harvesting and marketing eelgrass. It was used in many ways, for example as filling for mattresses and cushions. Many dikes along the Zuiderzee coasts were made from pressed eelgrass.

On Texel

A small section of an eelgrass dike has been preserved in the nature reserve Ottersaat, just north of Oudeschild. There is an old eelgrass shed displayed at Kaap Skil, the former Maritime and Beachcombing museum, where they used to store eelgrass. In earlier times, Texel and Wieringer eelgrass fishermen often fought over the use of the eelgrass fields, which were located on the flats in the western Wadden Sea.

  • Differences between Zostera noltii and Zostera marina
    Illustration of placement of eelgrass species in the intertidal zone, Marieke van Katwijk

    Besides the length and width of the leaves, the two species and forms of eelgrass grow at different depths. The leaves of Z. noltii are more slender and no longer than 25 cm. Z. noltii grows in the zone between high and low tide - the intertidal zone. Because Z. noltii is exposed during low tide, it must withstand the strongly varying conditions, such as fluctuations in temperature and salinity. Z. noltii grows in the lower areas of salt marshes together with salicornia. Lower down in the tidal zone, it grows together with the flexible form of Z. marina.

    The leaves of Zostera marina can reach up to 1 meter in length and are 0.5 cm wide. There are two forms. The narrow-leaved, flexible form grows in the deeper part of the intertidal zone and is annual. The robust form grows deeper down, past the low tide line. This form is perennial and if it is exposed at low tide, it is only for a very short period of time. The robust Z. marina is the species which was suffered from the disease and has disappeared from the Wadden Sea. It is also the species that was so important for the 'weed industry'. 

    There is another visible difference between the robust and flexible form of Z. marina. The base of robust perennial eelgrass stands erect in the water, even when it is exposed. The flexible form is 'flexible' and lies flat on the bottom, just like Z. noltii.

  • Biotope in itself
    , Mike Baird, via

    Eelgrass beds are a biotope in itself. That makes them very important for the marine ecology. By taking up lots of CO2, they play an important role in storing this material in the seawater environment. Because eelgrass holds down sediment, it helps to protect the coast. The fields are very important for numerous species of animals. Wigeon, brent geese, mute swans and coots graze the plants. Since the large beds of eelgrass disappeared in the Netherlands in the 1930s, many of these birds now forage on farmed grasslands on the other side of the dike, to the annoyance of many farmers.
    Other animals also feed on eelgrass or use it for other purposes. Baltic isopods forage on the Z. marina. Pipefish and seahorses thrive best in eelgrass fields, hiding from predators among the long leaves.  Furthermore, those beds submerged during low tide function as a nursery for young fish.

    Just like land plants, eelgrass gets its energy from the sun via photosynthesis. Therefore, clear water is essential. However, clear water is often poor in nutrients and eelgrass needs nutrients to grow. To compensate, the plant has an extensive root system for extracting the necessary supplements out of the sea bottom. Cloudy water caused by eutrophication or whirling particles from a changing water current is reason for this species to disappear.
    When water is rich in nutrients, algae and seaweed can prevail and compete better with eelgrass. These species do not need to exert energy into a root system. More algae in the water means less light available for eelgrass. Instead of strengthening its roots, the eelgrass then uses the greater nutrient supply to produce longer leaves that reach the surface for light, This makes it even more vulnerable during storms.

  • Underground collaboration

    Eelgrass and sulphide is a deadly combination. Nevertheless, eelgrass grows in muddy bottoms where this toxic material is also found. Till recently, it was a mystery as to how this was possible. But during a joint research project between the Dutch marine research institute NIOZ and a couple of universities, this mystery has been unveiled.

    Earlier studies of fossils dating back some 100 million years ago already showed that there was a relationship between a clam species from the Lucinidae family (Loripes lacteus), bacteria and eelgrass. These small shellfish, which are found among the roots of eelgrass, 'cultivate' special bacteria in their gills. The bacteria use sulphide to grow and therefore remove the material before it invades the eelgrass.

    During recent studies, researchers performed some laboratory tests using these animals. They compared pots of clams living with and without eelgrass and pots of eelgrass with and without the clams. Not only did the eelgrass grow better in combination with the shellfish, the shellfish also prospered more in the pots with eelgrass. The plant provided oxygen to the shellfish via its roots while the bacteria associated with the shellfish got rid of the sulphide.

    Clams from the Lucinidae family are not found in the Wadden Sea. Therefore, eelgrass growing here must deal with the sulphide without the help of the bacteria.

  • Seaweed fisheries in the Netherlands
    Unloading the seaweed harvest, harbour Den Oever., Historische Vereniging Wieringen

    People found several ways to use eelgrass in earlier days. It made great compact piling material for building dikes. Whale hunters used bales of eelgrass as a means for plugging leaks. It was also used as filling for mattresses and cushions (a pinch of sea wormwood kept the fleas away). That made 'fishing' eelgrass an important way of life. It was mowed and the floating material was collected (fished). This fishery was (mistakenly) called seaweed fishing.

    The oldest reports of seaweed fishing date back to around 1300 A.D. For centuries, it was an important source of income for the inhabitants along the coastal tidal flats. The eelgrass was washed several times to get rid of the salt and then dried in special sheds. It was very labor-intensive. When the eelgrass disappeared in the Wadden Sea after 1932, seaweed fisheries ended as a means of existence for the island inhabitants.

  • Eelgrass disease

    Up till 1932, large areas of the entire Wadden Sea were covered with eelgrass. In the Netherlands, there was more than 150 square kilometers (15,000 hectare). However in 1932, a disease infected the leaves and root tubers of the plant, probably caused by the Labyrinthula zosterae fungus. It wasn't just local, having struck just about all of the fields of seawrack (Z. marina) along the Western European as well as the North American Atlantic coasts.

    Less than a square kilometer of eelgrass survived in the Wadden Sea. Although it has not yet recovered, there is still hope. Beds of Z. noltii had doubled in size in the southern and central Wadden Sea between 2001 and 2008. There were only local increases of seawrack noted. Beside the few growing areas in the harbor of West-Terschelling and by the Hond/Paap in the Eems estuary, seawrack has also been found along the Groningen coast. This could mean that the characteristic eelgrass fields for the Wadden Sea is making a comeback.

    Some biologist assume that the disappearance of the eelgrass was caused by a combination of factors. Turbidity, less sunlight, changing currents and pollution probably all played a role. The summers of 1931 and 1932 were extremely gloomy. The sand and clay dumped to build the Zuiderzee Causeway increased the turbidity in the Wadden Sea. The combination of relatively little sun and increased turbidity limited the amount of sunlight that reached the leaves of the Z. marina. The currents also changed their pattern, whereby erosion and sedimentation might have disrupted plant growth. Polluted water (toxic materials and eutrophication) most likely also contributed to the disappearance of the eelgrass fields.

    The infectious disease left fewer traces in the delta region. Before the Grevelingen was closed off to the sea in 1971, there were around 1000 hectare fields of Z. marina. After the closure, the fields expanded explosively, probably due to the protection from tidal movements and the greater clarity in the water. The total surface area of eelgrass in 1978 in the Grevelingen was 4000 to 5000 hectare. There have since been large fluctuations, which scientists are still unable to clarify.

  • Re-introduction in the Wadden Sea

    When one speaks of eelgrass in the Wadden Sea nowadays, they are referring to the flexible form of Zostera marina. The robust form is still extinct here. The only possible spot where it may be growing is in the vicinity of the Hindenburg dam by the German Wadden Island Sylt.

    Experiments for re-introducing eelgrass have been taking place since 1993, with varying results. Eelgrass grows in places that are either periodically or permanently under water and where the current is weak. Most of the beds in the international Wadden Sea are presently found in the northern part in the shelter of the islands or sandbanks. In comparison, beds in the southern and central areas are few and smaller. There are scattered areas where eelgrass also strives, such as the Jadebusen (mainly Z. noltii) and along the coast between the Weser and the Elbe (Z. marina).

    In September 2011, the Waddenvereniging started a project to bring back eelgrass (Z. marina) in three areas of the Dutch Wadden Sea. They gathered clumps containing seeds of the flexible form of Zostera marina growing around the German island Sylt. The clumps were scattered off the coast of Groningen, the harbor of Schiermonnikoog and the Balgzand near Den Helder.

    In the third year, when no more manual seeding took place, the researchers found that the density and expanse of the eelgrass fields had increased naturally. Furthermore, there were even several new finds, such as by Rottum. However, the fields are still very vulnerable to erosion and weather conditions, which determine the success of seed production.

    While there were years in which the two species of eelgrass were transplanted with much success, the fields also frequently just disappear. For example the field of Zostera noltii on the Balgzand, which had expanded in 13 years' time into tens of hectares, disappeared in 2007. The annual Z. marina produces lots of seeds but it only survives one winter. Furthermore, there must be at least five plants per square meter for successful pollination. That is why researchers from the Radboud University in Nijmegen advise large-scale transplantation so that the chances of success are as great as possible.