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Dieren en planten

Plants   Algae and seaweed   Green seaweed   Brown seaweed   Lichens   Lichens on rocky coasts   Phytoplankton   Plankton calendar   

Water en land

Rocky coasts   Sea dikes (biotope)   

Mens en Milieu

Lichens on the foot of a sea dike, Ecomare

Lichens on sea dikes

Lichens grow well on rocks. Just like algae, seaweed and barnacles, they form zones on the sea dikes. Seaweed can grow outside of its marine environment as long as the dikes they grow on are regularly flooded or splashed with salt water. Lichens are land organisms however there are lichen species that can also grow on the borders, such as sea dikes, as long as their not flooded for too long under salt water. These species are resistant to salt water and grow in the splash zone. Stony dikes are often streaked with horizontal strokes: yellow, orange, black (different species of lichens) and green (algae).

Rare lichen species saved during dike works


Rare Verrucaria fusconigrescens on dike stone, Sytske Dijksen, www.fotofitis.nl

There are many places along the Dutch coast where sea dikes are being raised and reinforced. The stones used for building these dikes are often replaced with cement bricks. However, some of these stones are covered in lichens. Lichens can be very sensitive to changes. Even their position in relationship to the sun is very precise. Alhough lichens can grow on cement bases, it can take hundreds of years.

On Texel, lichen experts discovered the rare species Verrucaria fusconigrescens growing on several stones along the dike in the Mok Bay. This dike was to be raised and reinforced between August 2009 and February 2010. The organization responsible for the works was informed about this rare species. Not only were the stones temporarily stored so that they still faced the sun in the exact same position while the dike was under construction, GPS coordinates were noted before removing the stones so that they could be returned to their exact same position on the newly restored dike.
In the photo, the dark spots are Verrucaria fusconigrescens.