The North Sea Coasts
201
1980s, Riesen and Reise (1982) and Reise and Schubert (1987) showed an
increase of mussel beds in the subtidal zone down to a depth of 20 m
and, moreover, an increase for the intertidal zone was observed by Reise
and coworkers (1989). In former times mussel beds occurred mainly
around the low-water line but repeatedly invaded oyster beds living in
the upper subtidal zone (Hagmeier and Kandler 1927). Further south in
the Wadden Sea, in the lower intertidal of Jadebusen, Michaelis (1987)
found a 2.5-fold increase in coverage of mussel beds going from the
1930s to the 1970s. An increase in phytoplankton over the last decades is
known from the western Wadden Sea (Cadee 1986) and Helgoland Bight
(Radach and Berg 1986) and mussels may have benefitted from this
improved food supply.
Few beds of Mytilus edulis were present at Konigshafen in the period
1924-1934 compared to the 1980s (Fig.7.9). After the severe winter
1962 -1963 new mussel beds spread along the low water line in eastern
Konigshafen (Ziegelmeier 1977).These became partly destroyed by ice
scouring in March 1969 and February 1979 but they always recovered
quickly. Since 1986, these intertidal mussel beds have been subjected to
heavy exploitation by the mussel fishery. Mussels are dredged from a
ship during high tide, leaving only a few small clumps of mussels on the
bare sediment. Fucus vesiculosus, f. mytili occurs in association with
mussels and concomitant with the mussel density, this alga has become
more abundant (Fig. 7.9.)
Nienburg (1927) described a bloom Enteromorpha spp. at Konigshafen
in spring and early summer, restricted to the upper intertidal zone
(Fig. 7.9.). In August, these algae disappeared. At the same time garlands
of Chaeotomorpha linum (O.F. MUller) occurred in muddy depressions
of southeastern Konigshafen. With respect to green algae, the situation
has changed dramatically since 1979 (Reise 1983a,b). Explosive growth of
Enteromorpha spp. in June/July led to the formation of thick algal mats.
They were anchored in the sand and covered a wide range of the tidal zone
until late August or September when storms removed them. According to
Reise (1983a,b, 1985) small snails (Hydrobia) and lugworms (Arenicola
marina) together playa significant role in the development of green algal
mats in the Wadden Sea. Enteromorpha spores germinate on sand grains
and on the shells of Hydrobia. The growing thalli of Enteromorpha become
attached in the feeding burrows of Arenicola and this anchoring prevents
the algae from being drifted away by tidal currents and wave action.
Nutrient concentrations in the North Frisian Wadden Sea are
considerably lower than further south along the German coast (Martens
1989). Nevertheless, several of the long-term changes observed in the
201
1980s, Riesen and Reise (1982) and Reise and Schubert (1987) showed an
increase of mussel beds in the subtidal zone down to a depth of 20 m
and, moreover, an increase for the intertidal zone was observed by Reise
and coworkers (1989). In former times mussel beds occurred mainly
around the low-water line but repeatedly invaded oyster beds living in
the upper subtidal zone (Hagmeier and Kandler 1927). Further south in
the Wadden Sea, in the lower intertidal of Jadebusen, Michaelis (1987)
found a 2.5-fold increase in coverage of mussel beds going from the
1930s to the 1970s. An increase in phytoplankton over the last decades is
known from the western Wadden Sea (Cadee 1986) and Helgoland Bight
(Radach and Berg 1986) and mussels may have benefitted from this
improved food supply.
Few beds of Mytilus edulis were present at Konigshafen in the period
1924-1934 compared to the 1980s (Fig.7.9). After the severe winter
1962 -1963 new mussel beds spread along the low water line in eastern
Konigshafen (Ziegelmeier 1977).These became partly destroyed by ice
scouring in March 1969 and February 1979 but they always recovered
quickly. Since 1986, these intertidal mussel beds have been subjected to
heavy exploitation by the mussel fishery. Mussels are dredged from a
ship during high tide, leaving only a few small clumps of mussels on the
bare sediment. Fucus vesiculosus, f. mytili occurs in association with
mussels and concomitant with the mussel density, this alga has become
more abundant (Fig. 7.9.)
Nienburg (1927) described a bloom Enteromorpha spp. at Konigshafen
in spring and early summer, restricted to the upper intertidal zone
(Fig. 7.9.). In August, these algae disappeared. At the same time garlands
of Chaeotomorpha linum (O.F. MUller) occurred in muddy depressions
of southeastern Konigshafen. With respect to green algae, the situation
has changed dramatically since 1979 (Reise 1983a,b). Explosive growth of
Enteromorpha spp. in June/July led to the formation of thick algal mats.
They were anchored in the sand and covered a wide range of the tidal zone
until late August or September when storms removed them. According to
Reise (1983a,b, 1985) small snails (Hydrobia) and lugworms (Arenicola
marina) together playa significant role in the development of green algal
mats in the Wadden Sea. Enteromorpha spores germinate on sand grains
and on the shells of Hydrobia. The growing thalli of Enteromorpha become
attached in the feeding burrows of Arenicola and this anchoring prevents
the algae from being drifted away by tidal currents and wave action.
Nutrient concentrations in the North Frisian Wadden Sea are
considerably lower than further south along the German coast (Martens
1989). Nevertheless, several of the long-term changes observed in the