Keywords:
onshore barrier dune
water extraction
dune slack
vegetation
Location: The Netherlands, province of North-Holland
Water extraction in the North-Holland Dune Reserve (NHDR)
The NHDR is owned by the province of North Holland and managed
as a nature reserve by PWN Water Supply Company North-Holland.
It has a total area of about 5300 ha and is situated along the
North Sea coast, northwest of Amsterdam.
The groundwater extraction in the NHDR started in 1885 (for drinking
water supply). After the foundation of the PWN in 1920, the water
distribution network expanded rapidly. The artificial recharge
of groundwater was introduced in 1940 because the dunes dryed
out on large scale and increased risk of pumping brackish water
from deeper levels. Pre-purified water from the river Rhine and
the lake Ijsselmeer has been pumped into canals of two so-called
infiltration areas with a total surface area of about 200 ha.
Unfortunately, this process leads to eutrophication of the groundwater
and the development of unnatural, ruderal vegetation alongside
the infiltration area because of the nutrient content of the water.
In 1990 the extraction was strongly diminished and nowadays, the
water extraction in the area is reduced to less than 10 % of its
peak levels in the 1950s. The present policy focuses on the restoration
of the natural hydrological conditions.
Observation
Both field observations in three dune slacks (‘Oceaan’, ‘Kil’
and ‘Reggers Sandervlak’) in the NHDR along a vegetation gradient
and experimental research were done with respect to the effects
of changing the soil water content on soil processes and vegetation.
The observed dune slacks are more or less influenced by changed
in groundwater level and not by external input of nutrients. In
incubation experiments the effects of different soil water contents
on soil processes and on the growth of three dominant plant species
(Schoenus nigricans, Calamagrostis epigejos and
Molinia caerulea) in addition to different nutrient contents
(nitrogen and / or phosphate) were analised. Furthermore, the
decomposition of the litter of these three plants was studied.
Results
Undisturbed wet dune slacks have a vegetation with low productivity
and high species diversity which is related to their low nutrient
levels. When management operations in the dune slacks are considered,
the soil water content (present and intended) and the phosphate
and nitrogen contents of both soil and plant material have to
be taken into account. A rise of the groundwater level may influence
dehydrogenase activity, ammonium, nitrate and phosphate concentrations.
If saturation of the (top)soil is reached, net nitrogen mineralization
will decrease but the concentration of available phosphorus, manganese
and DOC (dissolved organic carbon concentration) will increase.
In addition the quality of the groundwater will also influence
the availability of phosphate. This implies that additional management
tools to remove phosphorus may be necessary. Mowing and harvesting
of plant material are advised as they remove phosphorus without
disturbing the soil structure. Further the effects of rewetting
will differ between different vegetation zones. Zones dominated
by Schoenus nigricans may have good conditions for regeneration
due to low dehydrogenase activity and low amounts of some nutrients.
Seasonal variation in soil processes indicates that the moment
of rewetting in the year will also influence the success. Increasing
the groundwater level to the surface (inundated condition) will
oppress more competitive species and favour the regeneration of
the less competitive wet dune slack vegetation.
It can be concluded, that only a rise in groundwater level will
create a good starting situation for regeneration of wet dune
slacks (high soil water content, low net nitrogen mineralization
rate). In zones with a more ruderal vegetation additional management
may be necessary.
| North-Holland
Dune Reserve Vegetation Structure |
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North-Holland
Dune Reserve Geomorphology |
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References
- PWN waterleidingbedrijf & EUCC (1992): The North Holland
dune reserve. Coastline special. Coastline 1992 ½: 18-32.
(KJd),
- van Beckhoven, K. (1992): Effects of groundwater manipulation
on soil processes and vegetation in wet dune slacks. In: Carter,
Curtis & Sheehy-Skeffington (eds.). Coastal Dunes: 273-281.
Balkema. Rotterdam. (BCD92).
- van Beckhoven, K. (1995): Rewetting of coastal dune slacks:
effects on plant growth and soil processes. Thesis Vrije Universiteit
Amsterdam. The Netherlands. (33BCd95).
