208
P.H. Nienhuis
these birds in winter might also be responsible for the recent gradual
decline of the eulittoral stands. These waterfowl also feed on the subterranean parts of Zostera and they are thus capable of eradicating Zostera
stands (Van den Hoek et aL 1979).
In the intertidal as well as the subtidal zone of the Wadden sea
multicellular algae play a subordinate role. The sand and mudflats are
mostly devoid of any obvious, macroscopic algal growth. Concentrations
of macroscopic algae tend to occur where the tidal currents have low
velocities and wave action is limited. Intertidal seagrass vegetations as
well as macroscopic algae congregate in near-shore zones and on the
tidal watersheds. The algae grow attached either to Zostera leaves and
rhizomes or to scattered shells and shellfragments (Michealis 1969,1970).
The macro algal vegetation consists mainly of Enteromorpha and Ulva
species starting their development in April/May, and having explosive
growth in June/July, and declining in August/September. Small, scattered plants and full-grown sediment-covered plants ensure survival
during winter, showing vegetative proliferation during springtime after
being uncovered. Full-grown plants, whether or not growing attached to
shells, are repeatedly removed by the incoming and receding tidal
streams which causes continuously changing vegetation patterns (Van
den Hoek et al. 1979).
Recent data show that the biomass of Ulva species may vary with a
factor 15 within the course of a year. Development of Ulva in the
Wadden Sea seems to depend largely on the course of the summer.
Bright and calm summers are favourable for its development. In 1988
and 1989, both years having a bright summer, Ulva was notably present.
Mussel beds are an important substrate for Ulva species. In June, many
of the plants become afloat, mainly due to the activities of the mussel
farmers starting to harvest mussels with dredge nets. Currents determine whether Ulva can be found on the flats or in the tidal channels.
In the Dutch Wadden Sea there are indications for an increase in the mean
annual biomass of Ulva over the last 30 years (De Jonge et al. 1993).
Recent information on seagrasses is summarized by De Jonge and De
Jong (1992) Philipp art et aL (1992) and Philipp art (1995). Seagrasses
have become rare in the Dutch Wadden Sea. The sublittoral populations
never recovered after the 1930s but also the intertidal populations are
very thin nowadays (only several hundreds of hectares over an area of
hundreds of km 2 ). The recent destruction of small and patchy habitats by
shellfish-fisheries activities, is mainly held responsible for the almost
total extinction of both Zostera marina and Z. noltii. The habitat map
published by Dijkema (1991) allows a comparison between the Danish,
German and Dutch Wadden Sea: nowadays seagrasses are rare in the
P.H. Nienhuis
these birds in winter might also be responsible for the recent gradual
decline of the eulittoral stands. These waterfowl also feed on the subterranean parts of Zostera and they are thus capable of eradicating Zostera
stands (Van den Hoek et aL 1979).
In the intertidal as well as the subtidal zone of the Wadden sea
multicellular algae play a subordinate role. The sand and mudflats are
mostly devoid of any obvious, macroscopic algal growth. Concentrations
of macroscopic algae tend to occur where the tidal currents have low
velocities and wave action is limited. Intertidal seagrass vegetations as
well as macroscopic algae congregate in near-shore zones and on the
tidal watersheds. The algae grow attached either to Zostera leaves and
rhizomes or to scattered shells and shellfragments (Michealis 1969,1970).
The macro algal vegetation consists mainly of Enteromorpha and Ulva
species starting their development in April/May, and having explosive
growth in June/July, and declining in August/September. Small, scattered plants and full-grown sediment-covered plants ensure survival
during winter, showing vegetative proliferation during springtime after
being uncovered. Full-grown plants, whether or not growing attached to
shells, are repeatedly removed by the incoming and receding tidal
streams which causes continuously changing vegetation patterns (Van
den Hoek et al. 1979).
Recent data show that the biomass of Ulva species may vary with a
factor 15 within the course of a year. Development of Ulva in the
Wadden Sea seems to depend largely on the course of the summer.
Bright and calm summers are favourable for its development. In 1988
and 1989, both years having a bright summer, Ulva was notably present.
Mussel beds are an important substrate for Ulva species. In June, many
of the plants become afloat, mainly due to the activities of the mussel
farmers starting to harvest mussels with dredge nets. Currents determine whether Ulva can be found on the flats or in the tidal channels.
In the Dutch Wadden Sea there are indications for an increase in the mean
annual biomass of Ulva over the last 30 years (De Jonge et al. 1993).
Recent information on seagrasses is summarized by De Jonge and De
Jong (1992) Philipp art et aL (1992) and Philipp art (1995). Seagrasses
have become rare in the Dutch Wadden Sea. The sublittoral populations
never recovered after the 1930s but also the intertidal populations are
very thin nowadays (only several hundreds of hectares over an area of
hundreds of km 2 ). The recent destruction of small and patchy habitats by
shellfish-fisheries activities, is mainly held responsible for the almost
total extinction of both Zostera marina and Z. noltii. The habitat map
published by Dijkema (1991) allows a comparison between the Danish,
German and Dutch Wadden Sea: nowadays seagrasses are rare in the