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Dieren en planten

Mens en Milieu



up to 1 square meter, usually smaller


thin, flat ribbons of 'leaves' of variable shape

  • Dut: Zeesla
  • Lat: Ulva spp.
  • Eng: Ulva, sea lettuce, green laver
  • Ger: Meersalat
  • Dan: Søsalat
, Oscar Bos


During a walk on the mudflats, you are bound to find ulva growing in certain areas. The fishermen call it 'flap'. In Brittany, this green seaweed forms a plague. It is still a common species in the tidal regions around the North Sea however it has been declining in the past decennia in the Wadden Sea. This is because there are fewer nutrients in the water. Ulva readily tears off from its root, but continues to grow further just the same. Sometimes you see what looks like toilet paper on the flats or in the flood mark on the beach. In most cases, it is dried up ulva that has been bleached by the sun.

  • Tool for growing
    , Sytske Dijksen,

    Ulva, also known as sea lettuce, begins to grow in April but remains a small seaweed plant unless it is accompanied by the bacteria YM2-23. This bacteria excretes a material which triggers the ulva's growth. And once it starts to grow, it can reach an incredible size within a few months: up to 1 square meter on the Dutch tidal flats.

  • Healthy and useful
    Ulva, foto fitis, sytske dijksen

    Ulva contains lots of minerals and vitamins, such as magnesium, calcium, vitamins A, C en B12. It is eaten in many countries or processed in food. However, this is not the seaweed used for making sushi. That is purple laver.

    Its rapid growth makes ulva suitable as a living wastewater purifier: it extracts enormous amounts of nutrients from the water in a short period of time. Six hundred micrograms of nitrogen is extracted per gram of new ulva 'leaf'. Therefore, it is one of the major species used in the purification seaweed gardens encompassing salmon farms. It is already being used for purifying wastewater from biogas installations.

  • Grazers
    Jassa on ulva, FotoFitis,

    Ulva is eaten by aquatic birds and small crustaceans such as amphipods and Baltic isopods. Although, in the case of the latter group, the small creeping creatures often prefer the film of diatoms growing on the thallus. In this way, ulva profits from grazing since a clean 'leaf' receives more sunlight.

  • Deadly
    Large amount of ulva along the foot of the dike, FotoFitis,

    In the mid 1990s, there was sometime a problem with 'black spots' on the tidal flats: large surfaces had become totally depleted of oxygen and therefore lifeless due to enormous amounts of rotting ulva. It is generally assumed that this phenomenon no longer takes place since there is much less phosphate in the Wadden Sea.

    That is not the case everywhere. In Brittany, blossoming ulva led to major problems during recent summers. The coast there is made up of rocks interspersed with bays containing beaches . The packages of rotting seaweed were sometimes up to a meter thick and covered all of the beaches. Free-roaming boars died because they consumed too much sulphide emitted by the rotting seaweed. There were even two human deaths linked to the seaweed plague. Local tourism received a serious blow.

    This extreme ulva blossoming is probably related to unpurified discharges from intensive livestock farming in Brittany. Such situations will not readily take place in the Netherlands, with its long coastline and large tidal flats. However, it forms a nightmare for opponents to plans to fertilize the sea with phosphate.

  • Fishermen wisdom

    Tidal-flat fishermen have been aware of the decline of ulva in the Wadden Sea from experience. When a shrimp fisherman was asked what the biggest change has been in the Wadden Sea in the past decade, the answer was: "The fact that we can also fish shrimp in the summer nowadays. Before, starting in mid July, the net would be full with flap and it was impossible to remove any shrimp. Now we can fish throughout the year!"