206
P.R. Nienhuis
water transparency and the available vast shallow sublittoral flats
promoted the development of the sublittoral Zostera meadows in the
Dutch Western Wadden Sea and the German Northern Wadden Sea. The
intervening stretch of Wadden Sea lacked sublittoral Z. marina beds probably as a result of its more turbid water, caused by the inflowing rivers,
and by the larger tidal amplitudes resulting in stronger tidal currents.
In 1932 the ''wasting disease" of Z. marina, which started in North
America, reached the Netherlands (Den Hartog and Polderman 1975) and
the German Northern Wadden Sea (Wohlenberg 1935,1937), and it
exterminated almost completely the sublittoral Z. marina beds. The
eulittoral Zostera noltii was not affected. Only scattered eulittoral
Z. marina stands and populations in brackish water ditches and ponds on
the Wadden islands survived. From about 1940 onwards the decimated
Z. marina populations all along the North Atlantic gradually increased in
size and regained their lost grounds. Only in the Wadden Sea did this
species never succeed in re-establishing its sublittoral beds. At present,
only intertidal Z. marina stands occur. As Z. marina is incapable of
surviving frost, its eulittoral populations are mostly annual and start their
growth from seed each year (Van den Hoek et al. 1979).
Hydrographic changes caused by the closure of the Zuiderzee in 1932
are thought to have prevented the re-establishment of sublittoral
Z. marina in the Dutch Western Wadden Sea. It is not clear which
hydrographic changes were responsible, but erosion of the silty substratum favourable for the growth of Zostera marina, currents too strong for
the establishment of seeds, and a rise of the average flood level by 20 cm
are thought to be the main factors (Den Hartog 1970). It is, however,
difficult to envisage that these three factors could have completely
prevented the re-establishment of new favourable sublittoral habitats for
Z. marina. One other possible cause could be a considerable increase in
the turbidity of the seawater diminishing the light intensity below low
water level to values lower than Zostera's light compensation value
(Giesen et al. 1990a, b).
After the outbreak of the "wasting disease" in 1932 the significance of
Zostera as a primary producer went down to almost nothing (Table 7.1).
The presently remaining stands have only a local interest in the Dutch
western Wadden Sea. In the German Wadden Sea they are much more
important. The permanent disappearance of the vast sublittoral
Z. marina beds in the Dutch western Wadden Sea is an interesting example of a large-scale effect on an estuarine ecosystem by human interference,
in this case the closure of the Zuiderzee. A comparable interference,
closure of Grevelingen estuary in the southwestern Netherlands, has had
P.R. Nienhuis
water transparency and the available vast shallow sublittoral flats
promoted the development of the sublittoral Zostera meadows in the
Dutch Western Wadden Sea and the German Northern Wadden Sea. The
intervening stretch of Wadden Sea lacked sublittoral Z. marina beds probably as a result of its more turbid water, caused by the inflowing rivers,
and by the larger tidal amplitudes resulting in stronger tidal currents.
In 1932 the ''wasting disease" of Z. marina, which started in North
America, reached the Netherlands (Den Hartog and Polderman 1975) and
the German Northern Wadden Sea (Wohlenberg 1935,1937), and it
exterminated almost completely the sublittoral Z. marina beds. The
eulittoral Zostera noltii was not affected. Only scattered eulittoral
Z. marina stands and populations in brackish water ditches and ponds on
the Wadden islands survived. From about 1940 onwards the decimated
Z. marina populations all along the North Atlantic gradually increased in
size and regained their lost grounds. Only in the Wadden Sea did this
species never succeed in re-establishing its sublittoral beds. At present,
only intertidal Z. marina stands occur. As Z. marina is incapable of
surviving frost, its eulittoral populations are mostly annual and start their
growth from seed each year (Van den Hoek et al. 1979).
Hydrographic changes caused by the closure of the Zuiderzee in 1932
are thought to have prevented the re-establishment of sublittoral
Z. marina in the Dutch Western Wadden Sea. It is not clear which
hydrographic changes were responsible, but erosion of the silty substratum favourable for the growth of Zostera marina, currents too strong for
the establishment of seeds, and a rise of the average flood level by 20 cm
are thought to be the main factors (Den Hartog 1970). It is, however,
difficult to envisage that these three factors could have completely
prevented the re-establishment of new favourable sublittoral habitats for
Z. marina. One other possible cause could be a considerable increase in
the turbidity of the seawater diminishing the light intensity below low
water level to values lower than Zostera's light compensation value
(Giesen et al. 1990a, b).
After the outbreak of the "wasting disease" in 1932 the significance of
Zostera as a primary producer went down to almost nothing (Table 7.1).
The presently remaining stands have only a local interest in the Dutch
western Wadden Sea. In the German Wadden Sea they are much more
important. The permanent disappearance of the vast sublittoral
Z. marina beds in the Dutch western Wadden Sea is an interesting example of a large-scale effect on an estuarine ecosystem by human interference,
in this case the closure of the Zuiderzee. A comparable interference,
closure of Grevelingen estuary in the southwestern Netherlands, has had