196
7 Effects of the Ice Winter I YYSIYh
According to Beukema et a!. (1993), migration will not take the birds to totally
ice free areas, because cold temperatures normally affect the entire Wadden Sea,
although to a different degree. The duration of ice cover varies within the Wadden
Sea and the birds can migrate to an area of shorter ice cover duration.
7.3
The Seasonal Development 1996 (Regeneration)
Due to the effects of the ice winter, the seasonal development in 1996 was strongly
modified. In the following section results from ELA W AT will be presented supplemented by data from nearby areas.
7.3.1
The Pelagic System
After the ice melt, the salinity was unusually high (32-35 PPT) until May, which
was also due to an unusually low freshwater input (Henke 1997). Additionally an
upwelling front situated off the East Frisian Island may have caused an import of
water of higher salinity into the Wadden Sea (Henke 1997).
In spring, the turbidity of the coastal water was extremely low and at the same
time sunny weather prevailed. While in previous years the euphotic zone had a
"normal" extension of 1-2 m water depth (compare Colijn 1981; Postma 1984), it
reached down to 6-8 m water depth in spring 1996. Thus, the light conditions for
the growth of phytoplankton and microphytobenthos were extremely favourable,
even in the subtidal.
Already in February the nitrate concentration dropped below the long term average. From April onwards ammonia was also strongly reduced. In 1996 the silicate concentrations declined earlier as usual, while from March onwards the concentrations of dissolved phosphate were higher than the long term average
(compare Chap. 5.1).
Caused by the ice winter, the species composition of the phytoplankton was
clearly modified. From March onwards, a bloom of the large, cold adapted diatom
Coscinodiscu.l' ('oneillllus was observed in the Wadden Sea as well as in the German Bight. This species has the ability to produce intracellular oil. When the
bloom died down, this fat was released, producing a 640 km 2 sized lipid film on
the water surface, which remained stationary off the East Frisian Islands for about
3 weeks. The same phenomenon was observed in 1947 in the northern part of the
North Sea, when - also after an extremely cold winter - a bloom of C. coneinnus
had occurred (Gnnntved 1952). Diatom species such as Asterinellopsis glacialis or
Guinardia delicatu/a, which are representative for the coastal spring phytoplankton
bloom in normal years, were observed in low densities only (compare Chap. 5.1).
Instead of Phaeoc\'stis g/o/Josa. the cold adapted species Phaeocystis jJouchetii
was observed after the diatom bloom in 1996, most likely resulting from the very
low water temperatures (Chap. 5.1).
In 1996, the first meroplanktonic larvae were recorded later in the year compared to years with mild winters. The larval density retlected to some extent the
7 Effects of the Ice Winter I YYSIYh
According to Beukema et a!. (1993), migration will not take the birds to totally
ice free areas, because cold temperatures normally affect the entire Wadden Sea,
although to a different degree. The duration of ice cover varies within the Wadden
Sea and the birds can migrate to an area of shorter ice cover duration.
7.3
The Seasonal Development 1996 (Regeneration)
Due to the effects of the ice winter, the seasonal development in 1996 was strongly
modified. In the following section results from ELA W AT will be presented supplemented by data from nearby areas.
7.3.1
The Pelagic System
After the ice melt, the salinity was unusually high (32-35 PPT) until May, which
was also due to an unusually low freshwater input (Henke 1997). Additionally an
upwelling front situated off the East Frisian Island may have caused an import of
water of higher salinity into the Wadden Sea (Henke 1997).
In spring, the turbidity of the coastal water was extremely low and at the same
time sunny weather prevailed. While in previous years the euphotic zone had a
"normal" extension of 1-2 m water depth (compare Colijn 1981; Postma 1984), it
reached down to 6-8 m water depth in spring 1996. Thus, the light conditions for
the growth of phytoplankton and microphytobenthos were extremely favourable,
even in the subtidal.
Already in February the nitrate concentration dropped below the long term average. From April onwards ammonia was also strongly reduced. In 1996 the silicate concentrations declined earlier as usual, while from March onwards the concentrations of dissolved phosphate were higher than the long term average
(compare Chap. 5.1).
Caused by the ice winter, the species composition of the phytoplankton was
clearly modified. From March onwards, a bloom of the large, cold adapted diatom
Coscinodiscu.l' ('oneillllus was observed in the Wadden Sea as well as in the German Bight. This species has the ability to produce intracellular oil. When the
bloom died down, this fat was released, producing a 640 km 2 sized lipid film on
the water surface, which remained stationary off the East Frisian Islands for about
3 weeks. The same phenomenon was observed in 1947 in the northern part of the
North Sea, when - also after an extremely cold winter - a bloom of C. coneinnus
had occurred (Gnnntved 1952). Diatom species such as Asterinellopsis glacialis or
Guinardia delicatu/a, which are representative for the coastal spring phytoplankton
bloom in normal years, were observed in low densities only (compare Chap. 5.1).
Instead of Phaeoc\'stis g/o/Josa. the cold adapted species Phaeocystis jJouchetii
was observed after the diatom bloom in 1996, most likely resulting from the very
low water temperatures (Chap. 5.1).
In 1996, the first meroplanktonic larvae were recorded later in the year compared to years with mild winters. The larval density retlected to some extent the