Keywords:
barrier dune
dune cliff
primary dune
redevelopment
drift line
beach cleaning
Contact: A. Gerlach, University of Oldenburg, FB
7 AG ter. ecology, Germany
Location: Germany, North Sea, Lower Saxony
The island of Spiekeroog
Spiekeroog belongs to the inhabited East Frisian islands in the
German North Sea. These islands form together with the Dutch West
Frisian islands a classical system of protecting sandy barrier-islands
(dune islands) and back-barrier tidal flats. The island was probably
formed from sandplates only some 2000 years ago and covers about
20 km² and is 10 km long from the west to the east. The distance
to the mainland is about 5 km with a tidal flat area in between.
Spiekeroog can be divided into three areas: salt marshes, the
relatively young and thus highly dynamic Ostplate, and the old
dune area. Until the 18th century the changed currents accumulated
sand mainly in the part of the Ostplate. The old dunes show nowadays
only very little change, the dune succession consists of a typical
dune system (xerosere only) under atlantic climatic conditions.
The primary lime content of the sand is very low (0.4-0.8 %),
so the carbonate-poor sands from the beachplain and foredunes
become acid very quickly.
Observations
On 27/28 February 1990, the high tide of a severe onshore storm
reached the foot of the dunes and damaged or even eroded completely
most of the primary dunes. In March 1990, the University of Ol-denburg
in Germany started to survey the drift material. The study focused
mainly on the mineral nutrients. In the litter to a depth of 0.30
m, normally a content of around 11 g/m² N was found, whereas
in the drift line the amount was about 50 g/m². In total,
N stocks therefore increased almost five fold by the drift line.
Following carbon, hydrogen and oxygen, nitrogen is the quantitatively
most important nutrient for plants. While C, H and O are usually
available in amounts sufficient for the metabolism of plants,
nitrogen can often be a limiting factor for plant growth. The
nutrient input of phosphorous with 3 g/m² is not as
high, still a strong fertilizing effect is expected.
Above this point, the drift line contains viable seeds and roots,
both of annual drift line vegetation (e.g. Cakile maritima)
and amounts of seeds, roots or rhizomes of typical dune-forming
grasses, such as Elymus farctus, E. arenaria and
Ammophila arenaria. This plant material may be regarded
as an es-sential ressource for the redevelopment of primary dunes
and for the readvance of the dune cliff, ferti-lized by the released
mineral contents by decomposition. Furthermore, the drift line
itself forms an effective trap for windblown sand.
Because of the relative stability of the old dunes on Spiekeroog,
beach cleaning should be avoided to force the natural biogenic
redevelopment of the dunes.
Spiekeroog
References
- Gerlach, A. (1992): Dune cliffs: A buffered system. In: Carter,
Curtis & Sheehy-Skeffington (eds.). Coastal Dunes: 51-55.
Balkema. Rotterdam. (BNd92).
- Gerlach, A. (1993): Biogeochemistry of nitrogen in a coastal
dune succession on Spiekeroog (Germany) and the impact of climate.
Phytocoenologia 23: 115-127. Berlin-Stuttgart. (31R5Sp93)
- Gerlach, A., E.A. Albers & W. Broedlin (1994): Development
of the nitrogen cycle in the soils of a coastal succession.
Acta Botanica Neerlandica 43 (3): 189-203. (31R5Sp94).
- Isermann, M. & H. Cordes (1992): Changes in dune vegetation
on Spiekeroog (East Frisian islands) over a 30 year period.
Carter, Curtis & Sheehy-Skeffington (eds.). Coastal Dunes:
273-281. Balkema. Rotterdam. (BNd92).
