THE IMPACT
OF MAN
Human activities can have adverse impacts on the Wadden Sea ecosystem.
The effects of human activities can be classified into three
categories, namely pollution, disturbance and habitat destruction.
A more detailed description of the status of habitats
and species can be found in Chapter 2 - 5 of the Quality
Status Report 1999.
Pollution
The relatively high level of contamination of the Wadden Sea
is caused by three main factors:
- A number of rivers, the catchment
areas of which are highly industrialized and agronomized, flow
into the Wadden Sea. The catchment area adds up to some 231,000
km2. It extends to the southeast as far as the Chechian-Austrian
border. Among the rivers are the Elbe and the IJssel, a tributary
of the Rhine. In addition a substantial part of the Rhine water
enters the Wadden Sea via the North Sea through a coastal flow
along the Dutch coast.
- The Wadden Sea is a system
which imports more sediments than it exports. The sediments originate
almost completely from the North Sea and are carriers of heavy
metals and other contaminants. Due to the net North Sea current,
a substantial part of North Sea sediments -and consequently polluting
substances- is deposited into the Wadden Sea.
- The Wadden Sea lies at the
rim of north-west Europe. An important part of its contamination
is caused by rain and dust which originate from the highly industrialized
northwest and central European countries.
Rivers are by far the largest carrier of polluting substances
from the land to the Wadden Sea. The German rivers Elbe, Weser
and Ems, together with the Dutch IJsselmeer, discharge each year
on average 60 km3 of polluted water into the Wadden Sea. The
rivers transport heavy metals, PCBs and pesticides like lindane
and large amounts of nutrients. The amount of polluting substances
is to an important degree determined by the amount of water that
is discharged by the rivers. This discharge shows large yearly
fluctuations as a result of differences in rain and snowfall
in the catchment areas (see figure). That is why it is so difficult
to determine whether or not the pollutant loads have decreased
over the past years.
In the Wadden Sea itself a general reduction in the concentration
of pollutants can be observed. Since 1983 almost everywhere in
the sediments of the Wadden Sea concentrations of heavy metals
have decreased.
The two most important nutrients are nitrate and phosphate. Of
these the concentrations of phosphate in the water of the Wadden
Sea have started to decrease in the second half of the 1980s,
mainly as a result of the use of phosphate free detergent and
water purification.
No clear reductions in the amounts of nitrate discharged into
the Wadden Sea could be determined. As a result of the imbalance
of the reduction of nitrate and phosphate changes in the ratio
of the concentration in the water of these two substances have
occurred. There are indications that this has caused an increase
in occurrence of toxic algae. It is not clear whether there are
other biological consequences.
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Disturbance
Disturbance is understood to be any activity which, by means
of mechanical, visual or acoustical action, interferes with or
influences natural behavior or processes.
Disturbance of animals results in a loss of energy and can lead
to lower breeding success and lower survival rates. When comparing
the different causes of disturbance, some types of recreation,
hunting and commercial fisheries, are regarded as having the
most impact.
The most is known about the effects of disturbance on birds and
seals. As an example, the effects on seals are illustrated here.
Disturbance on seals has the worst effects in the nursery period.
Unfortunately this time of the year coincides with the main tourist
season.
Seal pups must gain sufficient weight during the nursing period
in order to be able to survive the period in which they must
learn to become self-sustaining. During this time they have to
rely on their fat reserves to survive.
Pups are nursed on sands during low water periods. Because this
period is limited in time, interruption through disturbance results
in a reduced uptake of milk and subsequently less fat intake.
Repeated disturbance in the nursing phase lowers the chances
of survival in the subsequent phase of becoming independent.
Habitat Destruction
Through the construction of dikes and other coastal defence works,
a considerable part of the natural habitats of the Wadden Sea
was lost. In the past 50 years some 160 km2 of salt marsh was
embanked, 43 km2 of which between 1963 and 1990. To date 346
km2 of salt marsh have remained.
One of the consequences of the construction of dikes and dams
along and in rivers and river mouths has been the disappearance
of natural transition zones between salt and fresh water, the
so-called brackish water zones.
Only one natural estuary in the Wadden Sea is left, the Varde
Å in the northern Danish Wadden Sea. Another result of
the construction of dikes is the increase of the difference between
high and low water, caused by the loss of areas that flood during
high water periods.
A new threat to the Wadden Sea may be caused by the increased
sea level and the increased frequency and intensity of storms,
both of which may be the result of the greenhouse effect. These
phenomena may cause an increased erosion and submersion of salt
marshes and tidal flats.
Considerable damage to bottom structures and organisms is caused
by the cockle and mussel fishery. One of the most manifest results
has been the destruction of old natural mussel beds.
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