82
5 Spallal and Temporal Distribution Patterns
5.1.4
Seasonal Variability
Dissolved nutrients, phytoplankton and zooplankton were analysed in ELA W AT
under different questions. Only zooplankton was studied explicitly for its seasonal
development, but nutrients and phytoplankton were sampled according to criteria,
which were not relevant for recording seasonal development. However, an attempt
is made here to describe all three parameters over the 3 study years in spite of their
varying sampling schemes.
5.1.4.1
Dissolved Nutrients
Dissolved nutrients were measured irregularly in 1994, and a description will not
be given here. In 1995, the seasonal variation of the nutrient concentration was
continuously recorded (Fig. 5.1.2). During the first months of the year concentrations of all of the measured dissolved nutrients were high. With the growth of the
algae in spring, nutrient concentrations decreased, and in some cases they reached
the detection limit. Except for phosphate all nutrient concentrations began to rise at
the end of August. The concentrations of phosphate already began to rise much
earlier in June. This increase, which was also observed in the Dutch Wadden Sea,
is due to the reduction of iron(oxy) hydroxides and the subsequent release of adsorbed phosphates (Jensen et al. 1995).
In 1996 the nutrient concentrations developed in a similar seasonal pattern as in
the previous year. With the beginning of the diatom bloom the nutrient concentrations decreased. Nitrogen compounds remained at low values until July, but phosphate and silicate increased slowly with the approach of summer (without figure).
5.1.4.2
Phytoplankton
Phytoplankton in the backbarrier system of Spiekeroog is dominated by diatoms,
which were found in variable numbers all over the year. Beside the diatoms, the
haptophycea Phaeocystis spp. occurred regularly. Dinoflagellates, which often
bloomed in the German Bight, were rarely found and occurred only in small numbers in the Wadden Sea.
In 1994, phytoplankton showed the following development (Fig. 5.1.3). As
early as March, diatoms typical for a spring situation, occurred in the water, but
only in low numbers. These were for example: Asterionellopsis glacialis, Odontella sinensis, Guinardia delicatula, Thalassiosira rotula and Rhizosolenia shrubsolei. The spring bloom was dominated by Guinardia delicatula and reached its
maximum in April with the haptophyceae Phaeocystis globosa (P. globosa:
1.4 x 10
7
cells ]', Guinardia delicatula: 1.1 x 10" cells r'). In May, June and July
cell numbers were low. In August a bloom of the diatoms Leptocylindrus danicus
and Leptocylindrus minimus was observed.
5 Spallal and Temporal Distribution Patterns
5.1.4
Seasonal Variability
Dissolved nutrients, phytoplankton and zooplankton were analysed in ELA W AT
under different questions. Only zooplankton was studied explicitly for its seasonal
development, but nutrients and phytoplankton were sampled according to criteria,
which were not relevant for recording seasonal development. However, an attempt
is made here to describe all three parameters over the 3 study years in spite of their
varying sampling schemes.
5.1.4.1
Dissolved Nutrients
Dissolved nutrients were measured irregularly in 1994, and a description will not
be given here. In 1995, the seasonal variation of the nutrient concentration was
continuously recorded (Fig. 5.1.2). During the first months of the year concentrations of all of the measured dissolved nutrients were high. With the growth of the
algae in spring, nutrient concentrations decreased, and in some cases they reached
the detection limit. Except for phosphate all nutrient concentrations began to rise at
the end of August. The concentrations of phosphate already began to rise much
earlier in June. This increase, which was also observed in the Dutch Wadden Sea,
is due to the reduction of iron(oxy) hydroxides and the subsequent release of adsorbed phosphates (Jensen et al. 1995).
In 1996 the nutrient concentrations developed in a similar seasonal pattern as in
the previous year. With the beginning of the diatom bloom the nutrient concentrations decreased. Nitrogen compounds remained at low values until July, but phosphate and silicate increased slowly with the approach of summer (without figure).
5.1.4.2
Phytoplankton
Phytoplankton in the backbarrier system of Spiekeroog is dominated by diatoms,
which were found in variable numbers all over the year. Beside the diatoms, the
haptophycea Phaeocystis spp. occurred regularly. Dinoflagellates, which often
bloomed in the German Bight, were rarely found and occurred only in small numbers in the Wadden Sea.
In 1994, phytoplankton showed the following development (Fig. 5.1.3). As
early as March, diatoms typical for a spring situation, occurred in the water, but
only in low numbers. These were for example: Asterionellopsis glacialis, Odontella sinensis, Guinardia delicatula, Thalassiosira rotula and Rhizosolenia shrubsolei. The spring bloom was dominated by Guinardia delicatula and reached its
maximum in April with the haptophyceae Phaeocystis globosa (P. globosa:
1.4 x 10
7
cells ]', Guinardia delicatula: 1.1 x 10" cells r'). In May, June and July
cell numbers were low. In August a bloom of the diatoms Leptocylindrus danicus
and Leptocylindrus minimus was observed.