The North Sea Coasts
203
range of the subtidal zone; (4) increased abundance of infaunal species
in the sandy bottoms of the intertidal and subtidal zone.
Positive responses by the benthos to enhanced eutrophication have
also been observed in other regions of the Wadden Sea (Michaelis 1978,
1986; Beukema and Cadee 1986; Madsen and Jensen 1987). On the other
hand, deleterious effects attributed to hypoxia are known from sites close
to effluents of organic wastes (Essink 1984; Essink and Beukema 1986).
In the North Frisian Wadden Sea, the prevailing physical conditions
with strong tidal currents and wave action normally prevent hypoxia. In
the vicinity of mussel beds, however, resuspended biodeposits may cause
oxygen deficiencies in the interstitial water. Unfortunately, in the deep
tidal channels where the sessile epifauna has declined, oxygen concentrations have not been measured (Reise et al. 1989).
Not all of the observed long-term changes in the benthos around Sylt
can be attributed to eutrophication. Erosion contributed to the loss of
narrow-zoned communities in the upper intertidal, and possibly also to
losses in the epifauna of the deep channels. Fisheries activities almost
certainly affected oyster- and musselbeds and Sabellaria reefs. The
historical comparisons presented cannot be conclusive with regard to the
causes of the observed long-term changes (Reise et al. 1989).
The Wadden Sea area between the rivers Ems, Weser and Elbe has
also been quite intensively studied from the 1930s onwards (Linke 1939;
Michaelis et al. 1992). Much information is not published in international journals, and hence is rather inaccessible.
Until 1980, macro algae represented a constant but inconspicuous
element in the intertidal Wadden Sea ecosystem (Michaelis 1969, 1970,
1978, 1987; Hauser and Michaelis 1975; Koeman 1975). In some of the
older reports a tidal-flat blooming is mentioned, caused by an explosive
spring growth of green algae. Early symptoms of increasing macro algal
abundance were already seen by Michaelis (1978), but it is only from
1989 onwards that green algae increasingly reach high densities in many
localities.
Aerial surveys from 1990 onwards revealed that each year during
summer roughly 15% of the intertidal area between the rivers Ems and
Weser are covered by green algae. The maximum biomass varies between
20 to 110 g dry weight m -2. The dominating green algae are Ulva
scandinavica, U. lactuca, Enteromorpha prolifera subsp. prolifera and
subsp. radiata and E. intestinalis. The seasonal dynamics of the green
algal vegetation changes from year to year, with regard to the start of the
growing season and the duration of the vegetative period (May-October;
Kolbe et al. 1995). Data from the German Wadden Sea suggests that in
"macroalgae years" the concentrations of nitrate in the watercolumn are
203
range of the subtidal zone; (4) increased abundance of infaunal species
in the sandy bottoms of the intertidal and subtidal zone.
Positive responses by the benthos to enhanced eutrophication have
also been observed in other regions of the Wadden Sea (Michaelis 1978,
1986; Beukema and Cadee 1986; Madsen and Jensen 1987). On the other
hand, deleterious effects attributed to hypoxia are known from sites close
to effluents of organic wastes (Essink 1984; Essink and Beukema 1986).
In the North Frisian Wadden Sea, the prevailing physical conditions
with strong tidal currents and wave action normally prevent hypoxia. In
the vicinity of mussel beds, however, resuspended biodeposits may cause
oxygen deficiencies in the interstitial water. Unfortunately, in the deep
tidal channels where the sessile epifauna has declined, oxygen concentrations have not been measured (Reise et al. 1989).
Not all of the observed long-term changes in the benthos around Sylt
can be attributed to eutrophication. Erosion contributed to the loss of
narrow-zoned communities in the upper intertidal, and possibly also to
losses in the epifauna of the deep channels. Fisheries activities almost
certainly affected oyster- and musselbeds and Sabellaria reefs. The
historical comparisons presented cannot be conclusive with regard to the
causes of the observed long-term changes (Reise et al. 1989).
The Wadden Sea area between the rivers Ems, Weser and Elbe has
also been quite intensively studied from the 1930s onwards (Linke 1939;
Michaelis et al. 1992). Much information is not published in international journals, and hence is rather inaccessible.
Until 1980, macro algae represented a constant but inconspicuous
element in the intertidal Wadden Sea ecosystem (Michaelis 1969, 1970,
1978, 1987; Hauser and Michaelis 1975; Koeman 1975). In some of the
older reports a tidal-flat blooming is mentioned, caused by an explosive
spring growth of green algae. Early symptoms of increasing macro algal
abundance were already seen by Michaelis (1978), but it is only from
1989 onwards that green algae increasingly reach high densities in many
localities.
Aerial surveys from 1990 onwards revealed that each year during
summer roughly 15% of the intertidal area between the rivers Ems and
Weser are covered by green algae. The maximum biomass varies between
20 to 110 g dry weight m -2. The dominating green algae are Ulva
scandinavica, U. lactuca, Enteromorpha prolifera subsp. prolifera and
subsp. radiata and E. intestinalis. The seasonal dynamics of the green
algal vegetation changes from year to year, with regard to the start of the
growing season and the duration of the vegetative period (May-October;
Kolbe et al. 1995). Data from the German Wadden Sea suggests that in
"macroalgae years" the concentrations of nitrate in the watercolumn are