5.4 Estuaries in the Southern North Sea Region
87
trade are located and specific habitats can be found. Thus, they are also a focus of
integrated coastal zone management.
This section provides a brief outline about selected estuaries in the southern North
Sea Region: Elbe and Weser (both in Germany) and Schelde (estuary crossing the borders
of Belgium and The Netherlands). The descriptions in this section represent the situation
and development of many estuaries around the world – see, for example, Gregory (2006);
Ducrotoy (2010) and Collins and Miller (2011).
5.4.1 Introduction
An estuary is a transition zone between a river and the open sea; fresh water is mixed with
salt water. Rivers in the North Sea are influenced by semi-diurnal tides. The consequence
is that flora and fauna have to adapt to these changing circumstances. Specialized plants
and animals can be found which are adapted to saline or to fresh water environment and
some of them are able to cope with both conditions.
Due to the fact that the transition from fresh to salt water is one of the main characteristics of estuaries in 1958 the Venice system (Anonymous, 1958) was developed to
differentiate between water bodies based on salt concentration. Attendees of an international Symposium for the Classification of Brackish Waters in 1958 agreed on four
categories: hyperhaline (≥ ± – 40 % salinity), euhaline (±40 – ± – 30 % salinity), mixohaline (± – 30 – ± – 0.5 % salinity) and limnic (< ± 0.5 % salinity). The ± indicates that
these figures should not be taken as precise limits, more as approximation.
The mixture of fresh and salt water is called brackish water body. The brackish water
body provides no strict border between the saline and fresh water environment. Mainly
driven by tides the brackish water body is oscillating in a certain reach of an estuary (see
Chap. 4). It should be understood as a brackish water zone as transition between fresh
and salt water. The extension of the brackish water zone depends on many circumstances,
for example, fresh water discharge from upstream the river, the tides, the discharge of
tributaries and of the shape of the estuary. If there is low fresh water discharge the sea
water can penetrate further upstream (Fig. 4.9).
Caused by tidal range there is also a transition between dry and wet areas. Shallow
water areas and temporarily inundated sand and mud flats exist in an estuary with endemic flora and fauna. Also anabranches and side arms are important ecological areas in
estuaries.
Figures 5.15–5.17 provide an impression that the three estuaries show differences according to the classification of the Venice system. Depending on the orientation to the sea,
human interferences and the length of the distinct zones show different extensions.
The Weser estuary has a total length of approximately 125 km and according to
morphological purposes it is divided into two parts: the Lower Weser from Bremen to
Bremerhaven (70 km) and the Outer Weser which extends from the harbor of Bremerhaven to the North Sea (55 km). According to the Venice system the Weser is divided into
87
trade are located and specific habitats can be found. Thus, they are also a focus of
integrated coastal zone management.
This section provides a brief outline about selected estuaries in the southern North
Sea Region: Elbe and Weser (both in Germany) and Schelde (estuary crossing the borders
of Belgium and The Netherlands). The descriptions in this section represent the situation
and development of many estuaries around the world – see, for example, Gregory (2006);
Ducrotoy (2010) and Collins and Miller (2011).
5.4.1 Introduction
An estuary is a transition zone between a river and the open sea; fresh water is mixed with
salt water. Rivers in the North Sea are influenced by semi-diurnal tides. The consequence
is that flora and fauna have to adapt to these changing circumstances. Specialized plants
and animals can be found which are adapted to saline or to fresh water environment and
some of them are able to cope with both conditions.
Due to the fact that the transition from fresh to salt water is one of the main characteristics of estuaries in 1958 the Venice system (Anonymous, 1958) was developed to
differentiate between water bodies based on salt concentration. Attendees of an international Symposium for the Classification of Brackish Waters in 1958 agreed on four
categories: hyperhaline (≥ ± – 40 % salinity), euhaline (±40 – ± – 30 % salinity), mixohaline (± – 30 – ± – 0.5 % salinity) and limnic (< ± 0.5 % salinity). The ± indicates that
these figures should not be taken as precise limits, more as approximation.
The mixture of fresh and salt water is called brackish water body. The brackish water
body provides no strict border between the saline and fresh water environment. Mainly
driven by tides the brackish water body is oscillating in a certain reach of an estuary (see
Chap. 4). It should be understood as a brackish water zone as transition between fresh
and salt water. The extension of the brackish water zone depends on many circumstances,
for example, fresh water discharge from upstream the river, the tides, the discharge of
tributaries and of the shape of the estuary. If there is low fresh water discharge the sea
water can penetrate further upstream (Fig. 4.9).
Caused by tidal range there is also a transition between dry and wet areas. Shallow
water areas and temporarily inundated sand and mud flats exist in an estuary with endemic flora and fauna. Also anabranches and side arms are important ecological areas in
estuaries.
Figures 5.15–5.17 provide an impression that the three estuaries show differences according to the classification of the Venice system. Depending on the orientation to the sea,
human interferences and the length of the distinct zones show different extensions.
The Weser estuary has a total length of approximately 125 km and according to
morphological purposes it is divided into two parts: the Lower Weser from Bremen to
Bremerhaven (70 km) and the Outer Weser which extends from the harbor of Bremerhaven to the North Sea (55 km). According to the Venice system the Weser is divided into