Roots cause shoreline stability

28-05-2013 “Even intensively grazed seagrasses provide important coastal protection services, by reducing coastal erosion due to wave action”, concluded Marjolijn Christianen and Jim van Belzen in their article in the journal PLOS ONE of May 28. In contrast to the current assumption, seagrass roots play a substantial role in stabilizing coastal sediments, thereby decreasing coastal erosion. This conclusion has an important impact for the role of seagrass fields, protecting coastal areas by increasing waves, due to climate change.

All over the world seagrass meadows are found in shallow coastal areas where they provide space as shelter and nursery grounds for thousands of animal species. Moreover, it is generally recognized that these seagrass fields can have an important role in the protection of the shore by damping currents and waves.
Until now the protective action of seagrass was mainly linked to the amount of seagrass
shoots and leaves. Therefore, the seagrass canopy is used as proxy for the expected effectiveness in wave damping. However, the research by Christianen and Van Belzen shows that erosion can still be reduced up to 70% in areas where seagrasses are intensively grazed and, thus, have almost no above ground shoots and leaves.

Turtles
Many green sea turtles live in the tropical ocean surrounding the archipelagos islands of Eastern Kalimantan, Indonesia. They feed on the leaves of these seagrasses. In areas where the sea turtle population is extensive, the seagrass fields resemble a short mowed lawn rather than lush foliage. Therefore, it was assumed that these heavily grazed seagrass meadows were less able to protect the coast than ungrazed seagrass meadows with their well-developed canopy.

Near the island of Derawan, the researchers recorded changes in the level of the sea floor in experimental patches with and without seagrass. In addition, they sectioned off several areas of the seagrass patches with enclosures to prevent grazing by sea turtles. They found no significant difference between the grazed and ungrazed areas, indicating the important role of the belowground roots for preventing erosion instead of the canopy.

Further, the researchers used piles of sandbags to control the amount of incoming waves and found that waves are the most important driver causing erosion of the shores on these islands. Therefore, the stabilizing role of the seagrass roots is of utmost importance as it limits the water level above the shallow reef flats, thus blocking large waves from reaching and eroding the beaches.

Christianen and Van Belzen propose not to assess the value of seagrass fields based only on the presence of leaves, but also to consider their roots. Seagrass meadows that at first sight don't seem to be very substantial can be essential for coastal protection and should be valued accordingly.

Role of seagrass meadows at climate change
Coral reefs and seagrass meadows, which provide such natural coastal protection, are under pressure in many areas. We can expect that the importance of these ecosystems will increase in the future due to the apprehension of more frequent and more violent storms.

The research on natural defensive functions of coastal ecosystems is part of the PhD research by Marjolijn Christianen and Van Belzen at the Radboud University, Nijmegen, the Netherlands. NWO (Netherlands Organisation for Scientific Research) funded the practical research by Christianen in Indonesia. The research by Jim van Belzen is financed by the EU-project THESEUS, in which the Royal Netherlands Institute for Sea Research (NIOZ) works on a way to make European coastal areas climate proof in a sustainable way, by using coastal ecosystems.

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‘Low-canopy seagrass beds still provide important coastal protection services’, Marjolijn J.A. Christianen, Jim van Belzen, Peter M.J. Herman, Marieke M. van Katwijk, Leon P.M. Lamers, Peter J.M. van Leent, Tjeerd J. Bouma, PLOS ONE May 28, 2013.
Read the full article>>>

More information:
Marjolijn Christianen, University of Groningen, +31 6 43 00 47 10
Jim van Belzen, NIOZ, +31 113 577 456, +31 6 47 26 42 37
Nienke Bloksma, NIOZ Communicatie, +31 222 369 460 or +31 6 53 49 47 14

High resolution pictures are available on request from Nienke Bloksma.

Ungrazed seagrass field.
© Marjolijn Christianen

Grazed seagrass field.
© Marjolijn Christianen

Pictures above:
Experimental patch with sandbags.
© Marjolijn Christianen

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