5 I Nutrients, Algae and Zooplankton
79
5.1.3
Tidal Variations
5.1.3.1
Dissolved Nutrients
The backbarrier system of Spiekeroog is strongly int1uenced by the North Sea in
contrast to the Dutch Wadden Sea (de Jonge et al. 1993). The t1uvial input is only
of minor importance along the East Frisian coast. Consequently the nutrient concentrations at high tide ret1ect the situation of the North Sea coastal zone. The
nutrient concentration in the Wadden Sea showed a large variability: The magnitude of the values was comparable to those of other Wadden Sea areas, but often
about one order of magnitude higher than in the adjacent coastal zone (Martens &
Elbrachter 1998). Especially for phosphate and silicate strong gradients were observed. Maximum values were measured mostly during low tide (Table 5.1.2).
During every tide nutrient poor coastal waters mixes with nutrient rich Wadden
Sea water and results in a nutrient export to the coastal zone.
Dissolved nutrients like silicate, phosphate and nitrogen were measured in
spring and summer hourly at different sites in the backbarrier system (Otzumer
Balje, breakwater). Dissolved phosphate and silicate showed a regular pattern: At
high tide the concentrations of nutrients were low. At low tide and during the beginning t1ood, concentrations of nutrients increased (Fig. 5.1.1).
Differences in the nutrient concentrations were caused by a variety of processes.
Chemical and microbial conversion takes place in the sediment and in the water
column. Currents near the sediment boundary can lead to an exchange of pore
water (Shum 1993; Huttel et al. 1996), which is enhanced by the bioturbation of
several worms and mussels (Asmus & Asmus 1991; Dame et al. 1991). Furthermore, the nutrient concentrations are increased by heterotrophic processes (Asmus
& Asmus 1998). This can explain the higher nutrient concentration in the Wadden
Sea as compared to the coastal zone.
The above regular pattern was not found for the dissolved nitrogen. The concentrations of nitrate, nitrite and ammonia varied irregularly during a tidal cycle.
The reason for this variability is the high reactivity of nitrogen. Under anoxic conditions, which occur close to the sediment surface microbial denitrification takes
place. In this process nitrate is conversed to molecular nitrogen.
Table 5.1.2 Maximum concentrations of dissolved nutrients measured in the Wadden Sea.
during tlood tide at the hreakwater in July 1995. Minimum concentrations of all nutrients
reached their detection limits and were always measured in spring. (Data from Niesel 1997)
Phosphate
Silicate
Nitrate
Nitrite
Amlllonia
[I-Jlnolllj
6
100
60
65
79
5.1.3
Tidal Variations
5.1.3.1
Dissolved Nutrients
The backbarrier system of Spiekeroog is strongly int1uenced by the North Sea in
contrast to the Dutch Wadden Sea (de Jonge et al. 1993). The t1uvial input is only
of minor importance along the East Frisian coast. Consequently the nutrient concentrations at high tide ret1ect the situation of the North Sea coastal zone. The
nutrient concentration in the Wadden Sea showed a large variability: The magnitude of the values was comparable to those of other Wadden Sea areas, but often
about one order of magnitude higher than in the adjacent coastal zone (Martens &
Elbrachter 1998). Especially for phosphate and silicate strong gradients were observed. Maximum values were measured mostly during low tide (Table 5.1.2).
During every tide nutrient poor coastal waters mixes with nutrient rich Wadden
Sea water and results in a nutrient export to the coastal zone.
Dissolved nutrients like silicate, phosphate and nitrogen were measured in
spring and summer hourly at different sites in the backbarrier system (Otzumer
Balje, breakwater). Dissolved phosphate and silicate showed a regular pattern: At
high tide the concentrations of nutrients were low. At low tide and during the beginning t1ood, concentrations of nutrients increased (Fig. 5.1.1).
Differences in the nutrient concentrations were caused by a variety of processes.
Chemical and microbial conversion takes place in the sediment and in the water
column. Currents near the sediment boundary can lead to an exchange of pore
water (Shum 1993; Huttel et al. 1996), which is enhanced by the bioturbation of
several worms and mussels (Asmus & Asmus 1991; Dame et al. 1991). Furthermore, the nutrient concentrations are increased by heterotrophic processes (Asmus
& Asmus 1998). This can explain the higher nutrient concentration in the Wadden
Sea as compared to the coastal zone.
The above regular pattern was not found for the dissolved nitrogen. The concentrations of nitrate, nitrite and ammonia varied irregularly during a tidal cycle.
The reason for this variability is the high reactivity of nitrogen. Under anoxic conditions, which occur close to the sediment surface microbial denitrification takes
place. In this process nitrate is conversed to molecular nitrogen.
Table 5.1.2 Maximum concentrations of dissolved nutrients measured in the Wadden Sea.
during tlood tide at the hreakwater in July 1995. Minimum concentrations of all nutrients
reached their detection limits and were always measured in spring. (Data from Niesel 1997)
Phosphate
Silicate
Nitrate
Nitrite
Amlllonia
[I-Jlnolllj
6
100
60
65