Flora of the sea dikes

Rocky coasts do not exist naturally in some of the North Sea countries, such as the Netherlands, however the flora of rocky coasts do. It has established itself on sea dikes, shipwrecks, breakwaters and artificial reefs, and consists mostly of seaweed. The position of seaweed on a dike is not arbitrary. Obvious horizontal zones can be seen on the dikes. Each level between low and high tide is inhabited by other types. Lichens also grow on sea dikes.

Growth zones on dikes

Above the high-tide mark (the so-called splash zone), and in the uppermost section of the tidal zone, one especially finds green seaweed, such as gutweed and sea lettuce and two brown seaweed types: channel wrack and small sea oak. Lower down in the tidal zone, by the low-tide mark, brown seaweed such as bladder wrack and knotted wrack are found. One will also find red seaweed in the shelter of the brown seaweed: Irish moss and calcareous red algae. Sometimes right under the low-tide mark, a whole class zone is found, namely sugar wrack and oarweed. This large brown seaweed, belonging to the family Laminaria, can form enormous seaweed fields, the so-called wrack woods. Many elegant types of red seaweed also grow in this zone. In the Netherlands, this seaweed flora is rare because the foot of the dikes below the low-tide mark is usually covered with sand or silt. It can be found in the Oosterschelde, near Vlissingen, Westkapelle, Den Helder, Texel and Terschelling. A few smaller wrack woods can be found in Germany, near Helgoland.

The zoning of the seaweed is dependent upon the colour composition of the light which penetrates the water. Light is built up of various colours. The zoning is determined by the differences in the need for light and the resistance of the various types to drying up. Green seaweed needs red light. Since this penetrates poorly, one will only find green seaweed at very shallow depths. Brown and red seaweed use blue and green light, which can penetrate deeper. That's why this seaweed grows in deeper water. The deep-lying seaweed types are not as resistant to dehydration than those growing higher up on the dike.

A resting place for animals

Seaweed play a secondary role in the food pyramids in the sea. They are literally marginal. They are sometimes consumed by animals. Seaweed fields are grazed by sea urchins and snails, such as the periwinkle. Seaweed also gives protection to an extensive community of crabs, small crustaceans, sea anemones, shellfish and other marine animals. The thick brown seaweed-packages protect them during low tide from dehydration and temperature changes.