5 I Nutrients, Algae ancl Zooplankton
81
The diatom Coscinodiscus concinnus was assigned to group two, because it
grew in the Wadden Sea under the special conditions in spring 1996 (Chaps. 3.2
and 7.4). This centric diatom is a cold adapted, large species (radius of the valve
I 10-450 11m), which occurred regularly with low numbers in the German bight
(Baars 1979; Drebes & Elbrachter 1976). Until 1996 the growth of this diatom
species was not observed in the Wadden Sea. However, C. concinnus blooms have
occurred before in the North Sea (Grontved 1952; Roskam 1970; Bauerfeind et al.
1990). Grontved reported a lipid film on the water surface of the central North Sea,
which formed after a bloom of C. concinnus. Along the Dutch coast the lipids
produced by a C. concinnus bloom caused a severe oxygen deficiency in 1964
(Roskam 1970). In May 1996 a biogenic lipid film of 640 km
2
was observed in the
southern German Bight, which could be related to the bloom of C. concinnus. Part
of the lipid film, which was transported into the Wadden Sea, as well as the end
products of the bloom were regarded as one cause for the development of the anoxic areas behind the East Frisian Islands in June 1996 (Hapner 1996; Niesel 1997;
Chap. 7.4).
The haptophyceae Phaeocystis spp. was found regularly in the Wadden Sea. But
its chance of growing under the conditions in the Wadden Sea was low, so it was
placed in group I (Niesel 1997). Losses of that species in the Wadden Sea were
caused by filtration by mussels (Smaal & Twisk 1997; Petri & Vareschi 1997).
Species like Brockmaniella brockmannii or Biddulphia alternans are considered
to be typical algae of tidal areas (Drebes 1974). These species were found regularly in the backbarrier system of Spiekeroog, but the massive growth described for
the Sylt-RomO-bight was not observed (Schneider et al. 1998). This can be caused
by different exchange rates of water in the two areas. Unpublished data from a
hydrographic model showed that the average rate of water exchange in the SyltRomO-bight was lower than in the backbarrier system of Spiekeroog. Therefore the
growth conditions for the algae are more variable in the backbarrier system. Furthermore, the higher exchange rates lead to quick transport of algae in and out of
the Wadden Sea. Pelagic algae, which need constant growth conditions for a certain amount of time, have only a low chance to establish here. Thus, the phytoplankton composition in the backbarrier system of Spiekeroog was characterized
by species imported from the North Sea and species resuspended by the currents,
wind and wave action (see de Jonge & von Beusekom 1995; Sect. 5.1.8).
5.1.3.3
Zooplankton
Zooplankton was analysed monthly in the tidal inlet of the Otzumer Balje (WP, see
Fig. 3.1.2) during low and high tide. Additionally, the variations in the zooplankton
were measured once during every hour of a tidal cycle. The variability within one
tidal cycle was so large that for most of the zooplankton species no tidal difference
could be assessed. Only the larvae of the polychaetes Lanice conchilega and Magelona papillicornis occurred in higher numbers in flood than in ebb water. Total
amounts of zooplankton individuals were mostly higher during flood than during
ebb tide. Only in the summer months an opposite situation occurred due to a higher
production of larvae of species living in the Wadden Sea (Hydrohia ulvae, spionids, cirripedes).
81
The diatom Coscinodiscus concinnus was assigned to group two, because it
grew in the Wadden Sea under the special conditions in spring 1996 (Chaps. 3.2
and 7.4). This centric diatom is a cold adapted, large species (radius of the valve
I 10-450 11m), which occurred regularly with low numbers in the German bight
(Baars 1979; Drebes & Elbrachter 1976). Until 1996 the growth of this diatom
species was not observed in the Wadden Sea. However, C. concinnus blooms have
occurred before in the North Sea (Grontved 1952; Roskam 1970; Bauerfeind et al.
1990). Grontved reported a lipid film on the water surface of the central North Sea,
which formed after a bloom of C. concinnus. Along the Dutch coast the lipids
produced by a C. concinnus bloom caused a severe oxygen deficiency in 1964
(Roskam 1970). In May 1996 a biogenic lipid film of 640 km
2
was observed in the
southern German Bight, which could be related to the bloom of C. concinnus. Part
of the lipid film, which was transported into the Wadden Sea, as well as the end
products of the bloom were regarded as one cause for the development of the anoxic areas behind the East Frisian Islands in June 1996 (Hapner 1996; Niesel 1997;
Chap. 7.4).
The haptophyceae Phaeocystis spp. was found regularly in the Wadden Sea. But
its chance of growing under the conditions in the Wadden Sea was low, so it was
placed in group I (Niesel 1997). Losses of that species in the Wadden Sea were
caused by filtration by mussels (Smaal & Twisk 1997; Petri & Vareschi 1997).
Species like Brockmaniella brockmannii or Biddulphia alternans are considered
to be typical algae of tidal areas (Drebes 1974). These species were found regularly in the backbarrier system of Spiekeroog, but the massive growth described for
the Sylt-RomO-bight was not observed (Schneider et al. 1998). This can be caused
by different exchange rates of water in the two areas. Unpublished data from a
hydrographic model showed that the average rate of water exchange in the SyltRomO-bight was lower than in the backbarrier system of Spiekeroog. Therefore the
growth conditions for the algae are more variable in the backbarrier system. Furthermore, the higher exchange rates lead to quick transport of algae in and out of
the Wadden Sea. Pelagic algae, which need constant growth conditions for a certain amount of time, have only a low chance to establish here. Thus, the phytoplankton composition in the backbarrier system of Spiekeroog was characterized
by species imported from the North Sea and species resuspended by the currents,
wind and wave action (see de Jonge & von Beusekom 1995; Sect. 5.1.8).
5.1.3.3
Zooplankton
Zooplankton was analysed monthly in the tidal inlet of the Otzumer Balje (WP, see
Fig. 3.1.2) during low and high tide. Additionally, the variations in the zooplankton
were measured once during every hour of a tidal cycle. The variability within one
tidal cycle was so large that for most of the zooplankton species no tidal difference
could be assessed. Only the larvae of the polychaetes Lanice conchilega and Magelona papillicornis occurred in higher numbers in flood than in ebb water. Total
amounts of zooplankton individuals were mostly higher during flood than during
ebb tide. Only in the summer months an opposite situation occurred due to a higher
production of larvae of species living in the Wadden Sea (Hydrohia ulvae, spionids, cirripedes).