The North Sea Coasts
209
Dutch Wadden Sea and in the estuaries of the rivers Elbe, Weser and
Ems, and rather common in the German Wadden Sea.
The Dutch Wadden Sea did not show a recent shift from the seagrassdominance to green algae-dominance (compare Konigshafen). However,
local accumulations of macro algae such as Enteromorpha and Ulva can
be frequently observed (Beukema 1992). No attempt to quantify the
amount of green algae and to compare historic data with recent data
have been undertaken. Obviously, local accumulations of green algae are
not considered as a management problem in the Dutch Wadden Sea.
The stretch of 200 km of sandy beaches along the west coast of the
Netherlands does not suffer from undesired accumulations of green
algae. The beaches are exposed to the open sea, and sand is transported
continuously by wind and waves. Amenity beaches are mechanically
cleaned during summer of debris and other floating objects. Seaweeds
washed ashore do not cause a serious problem.
Because of the northerly-directed residual current along the coast of
The Netherlands (Fig. 7.3) less than 5% of the original Rhine - Meuse
water reaches the estuaries and brackish lagoons in the southwest
Netherlands. Oosterschelde, Grevelingen, and Veerse Meer are therefore
mainly loaded with nutrients from diffuse sources, such as agricultural
runoff, treated waste water, and drainage canals. The saline water bodies
in the southwest Netherlands are spatially separated (compartmentalization) by dikes and sluices constructed during the "Deltaworks", a civil
engineering scheme that was in operation between 1957 and 1987, and
aimed to restrict the sea-born flow of the main estuaries for safety
reasons. Consequently, each of these (former) estuaries has its own eutrophication history and its own specific water regime. These characteristics mean that each unit has to be managed according to its particular
conditions (Nienhuis 1992; Fig. 7.10).
Some properties of present -day Dutch estuaries vary considerably
(Table 7.2). The residence time of the water masses in non-tidal
Grevelingen and Veerse Meer is long compared to the same characteristic in tidal estuaries. The net freshwater load directly derived from the
Rhine and Meuse rivers on Oosterschelde, Veerse Meer, and Grevelingen
is very small: 1 - 2% of the discharge of the rivers. The Wadden Sea
receives approximately 400 m 3 S-1 of water from the Rhine;
Westerschelde and Ems-Dollard each receive 100 to 150 m 3 S-1 of fresh
water from their tributaries. Veerse Meer experiences almost permanent
stratification, whereas in Grevelingen and Ems-Dollard only a few deep
channels are stratified during summer. The Westerschelde (marine section), Oosterschelde, and Wadden Sea estuaries are completely mixed
209
Dutch Wadden Sea and in the estuaries of the rivers Elbe, Weser and
Ems, and rather common in the German Wadden Sea.
The Dutch Wadden Sea did not show a recent shift from the seagrassdominance to green algae-dominance (compare Konigshafen). However,
local accumulations of macro algae such as Enteromorpha and Ulva can
be frequently observed (Beukema 1992). No attempt to quantify the
amount of green algae and to compare historic data with recent data
have been undertaken. Obviously, local accumulations of green algae are
not considered as a management problem in the Dutch Wadden Sea.
The stretch of 200 km of sandy beaches along the west coast of the
Netherlands does not suffer from undesired accumulations of green
algae. The beaches are exposed to the open sea, and sand is transported
continuously by wind and waves. Amenity beaches are mechanically
cleaned during summer of debris and other floating objects. Seaweeds
washed ashore do not cause a serious problem.
Because of the northerly-directed residual current along the coast of
The Netherlands (Fig. 7.3) less than 5% of the original Rhine - Meuse
water reaches the estuaries and brackish lagoons in the southwest
Netherlands. Oosterschelde, Grevelingen, and Veerse Meer are therefore
mainly loaded with nutrients from diffuse sources, such as agricultural
runoff, treated waste water, and drainage canals. The saline water bodies
in the southwest Netherlands are spatially separated (compartmentalization) by dikes and sluices constructed during the "Deltaworks", a civil
engineering scheme that was in operation between 1957 and 1987, and
aimed to restrict the sea-born flow of the main estuaries for safety
reasons. Consequently, each of these (former) estuaries has its own eutrophication history and its own specific water regime. These characteristics mean that each unit has to be managed according to its particular
conditions (Nienhuis 1992; Fig. 7.10).
Some properties of present -day Dutch estuaries vary considerably
(Table 7.2). The residence time of the water masses in non-tidal
Grevelingen and Veerse Meer is long compared to the same characteristic in tidal estuaries. The net freshwater load directly derived from the
Rhine and Meuse rivers on Oosterschelde, Veerse Meer, and Grevelingen
is very small: 1 - 2% of the discharge of the rivers. The Wadden Sea
receives approximately 400 m 3 S-1 of water from the Rhine;
Westerschelde and Ems-Dollard each receive 100 to 150 m 3 S-1 of fresh
water from their tributaries. Veerse Meer experiences almost permanent
stratification, whereas in Grevelingen and Ems-Dollard only a few deep
channels are stratified during summer. The Westerschelde (marine section), Oosterschelde, and Wadden Sea estuaries are completely mixed