1\6
5 Spallal allli Temporal DlstrIbutlOIl Patterns
was built in July by cirrepedia, polychaetes and copepods. In June large amounts
of bivalve larvae were observed (3237 larvae 1001".)
The decrease of the zooplankton individuals in early June 1996 could have been
caused by the lipid film, formed by the diatom C. concinnu.l' (see above Sect.
5.1.3). Residues of the lipid film were observed as a white substance in the water
and on the sediment surface in the Wadden Sea. Such fats can clog the respiration
organs of organisms and cause their death.
5.1.5
Interannual Variability
In the Wadden Sea the variability between the years can be high. But some recurrent patterns can be observed nearly every year. The nutrient concentrations are
high in winter, decrease in spring because of the phytoplankton bloom and usually
a phosphate maximum is observed in summer (de Jonge & Postma 1974). Phytoplankton is characterized by a spring bloom, which is dominated by diatoms. The
diatom bloom is followed by a bloom of flagellates, usually Phaeocystis spp. Often
a second diatom bloom is considered in summer (Cadee 1986).
A comparison of the data taken over three years of observation showed a very
high variability of plankton concentrations (e.g. no spring bloom in 1995). This
high variability made it impossible to take one year as a reference year for the
evaluation of stability properties. Rather, each year had to be treated as a special
case.
Because of the different intensity and composition of the phytoplankton bloom
in each year, the development of the zooplankton varied respectively. The year
1994 was characterized by a spring and summer bloom of diatoms. The spring
bloom was followed by a bloom of Phaeocystis globosa. This temporal sequence is
considered to be typical for the Wadden Sea (e.g. Hickel et al. 1992; Elbrachter et
al. 1994). In the zooplankton, larvae of molluscs were scarcely represented,
whereas copepods and polychaete larvae occurred with mean abundances.
The seasonal development of nutrient concentrations observed in 1995 followed
a typical trend for this area (e.g. Gillbricht 1988). But instead of a spring bloom of
phytoplankton a long summer bloom was observed. The zooplankton species reacted differently to the lack of the spring bloom and the high cell numbers in
summer. Larvae of molluscs appeared later in the year and polychaete larvae were
only found in small numbers.
The year 1996 was characterized by low temperatures in the beginning of the
year, a bloom of C. cOllcinnus and a mass occurrence of Phaeocystis pouchetii.
Many copepod species reached the highest individual numbers of the study period
in that year. Larvae of polychaetes and bivalves reacted differently depending on
the species.
This large variability on the base of the food web implies that species on higher
trophic levels have to face not only small scale variations of food abundances, but
large scale variations between the years as well.
5 Spallal allli Temporal DlstrIbutlOIl Patterns
was built in July by cirrepedia, polychaetes and copepods. In June large amounts
of bivalve larvae were observed (3237 larvae 1001".)
The decrease of the zooplankton individuals in early June 1996 could have been
caused by the lipid film, formed by the diatom C. concinnu.l' (see above Sect.
5.1.3). Residues of the lipid film were observed as a white substance in the water
and on the sediment surface in the Wadden Sea. Such fats can clog the respiration
organs of organisms and cause their death.
5.1.5
Interannual Variability
In the Wadden Sea the variability between the years can be high. But some recurrent patterns can be observed nearly every year. The nutrient concentrations are
high in winter, decrease in spring because of the phytoplankton bloom and usually
a phosphate maximum is observed in summer (de Jonge & Postma 1974). Phytoplankton is characterized by a spring bloom, which is dominated by diatoms. The
diatom bloom is followed by a bloom of flagellates, usually Phaeocystis spp. Often
a second diatom bloom is considered in summer (Cadee 1986).
A comparison of the data taken over three years of observation showed a very
high variability of plankton concentrations (e.g. no spring bloom in 1995). This
high variability made it impossible to take one year as a reference year for the
evaluation of stability properties. Rather, each year had to be treated as a special
case.
Because of the different intensity and composition of the phytoplankton bloom
in each year, the development of the zooplankton varied respectively. The year
1994 was characterized by a spring and summer bloom of diatoms. The spring
bloom was followed by a bloom of Phaeocystis globosa. This temporal sequence is
considered to be typical for the Wadden Sea (e.g. Hickel et al. 1992; Elbrachter et
al. 1994). In the zooplankton, larvae of molluscs were scarcely represented,
whereas copepods and polychaete larvae occurred with mean abundances.
The seasonal development of nutrient concentrations observed in 1995 followed
a typical trend for this area (e.g. Gillbricht 1988). But instead of a spring bloom of
phytoplankton a long summer bloom was observed. The zooplankton species reacted differently to the lack of the spring bloom and the high cell numbers in
summer. Larvae of molluscs appeared later in the year and polychaete larvae were
only found in small numbers.
The year 1996 was characterized by low temperatures in the beginning of the
year, a bloom of C. cOllcinnus and a mass occurrence of Phaeocystis pouchetii.
Many copepod species reached the highest individual numbers of the study period
in that year. Larvae of polychaetes and bivalves reacted differently depending on
the species.
This large variability on the base of the food web implies that species on higher
trophic levels have to face not only small scale variations of food abundances, but
large scale variations between the years as well.