- Flora of sea dikes, Ecomare
Sea dike flora
A natural Dutch coast consists of sand and mud. The Netherlands doesn't have a rocky coast. However, thanks to the man-made sea dikes, dams, shipwrecks and breakwaters, the flora growing on natural rocky coasts also grows along the Dutch coast. Most of the flora consists of green, brown and red seaweed. Seaweed species don't grow arbitrarily on these rocky slopes. There are obvious horizontal zones. Each zone between low and high tide is suitable for specific species. Other flora found on sea dikes are land plants which tolerate salt. These are various species of lichens and salt marsh plants that take root between the cracks of the rocky dike.
On Texel
- , Sytske Dijksen, www.fotofitis.nl
There is too much sand and mud for a varied growth of seaweed to develop along the majority of the Wadden Sea coast. Texel is no exception. Mud makes the water too murky so that light cannot penetrate while sand scours the plants. The richest seaweed vegetation grows on the dike by 't Horntje, close by the NIOZ building. The Marsdiep channel runs along this dike, where the water washes away any sand or mud. Here you find sea belt and oarweed, the large brown seaweed species in the lowermost zone. Just like many plants on Texel, due to the severe conditions, these seaweeds don't grow as large as they do in other environments.
Above the average high tidal zone, unusual species of lichens that are resistant to salt water are found on the dike.
- Growth zones on dikes
- Gutweed growing on the dike, Foto Fitis, www.fotofitis.nl
Seaweed zoning is dependent upon the color composition of the sunlight which penetrates the water. Sunlight is made up of various colors. Zoning is determined by the seaweed's necessity for the kind of light and the resistance to dehydration. Green seaweed needs red light. Red light penetrates water poorly, so green seaweeds grow at very shallow depths. Since this zone is often exposed to air, green seaweeds can stand dehydration to a certain degree. Brown and red seaweeds use blue and green light to grow, colors which penetrate deeper. That's why these seaweeds grow in deeper water. The deep-lying seaweed species are not as resistant to dehydration than those growing in shallower waters.
Walking down a dike from land, above the high-tide mark (the so-called splash zone) and in the uppermost section of the high-tide zone, the first seaweed you find is mostly gutweed and sea lettuce (green seaweeds) and possibly channel wrack and small sea oak (brown seaweeds). The brown seaweeds bladder wrack and knotted wrack grow lower down in the tidal zone, by the low-tide mark. Several red seaweeds grow in the shelter of brown seaweed: Irish moss and calcareous red algae. Sometimes right under the low-tide mark, you find sugar wrack and oarweed. These large brown seaweeds, members of the Laminaria family, can form enormous beds, so-called wrack woods. Many elegant species of red seaweed also grow in this zone. However, this seaweed flora is rare in the Netherlands since the foot of the dikes below the low-tide mark is usually covered with sand or silt. Nevertheless, they grow in the Oosterschelde, near Vlissingen, Westkapelle, Den Helder, Texel and Terschelling. Several smaller wrack woods can be found in Germany, near Helgoland.
- A resting place for animals
