44
3 Study Area
corporated into sandy sediments (Hi.ittel et al. 1996). This can lead to sandtlats
with a relatively high mud content in the vicinity of mussel beds, resulting from
the export of fine grained mussel deposits. Without their activity, the fine grained
material deposited by the mussels could only accumulate in sheltered areas of the
Wadden Sea (e.g. mudflats near the coast), or would be exported from the backbarrier system towards the North Sea. Biodeposits are muddy sediments deposited
independently from the hydrodynamic energy gradients (Chap. 3.4; Flemming
1990).
The resistance of the biodeposits against erosion is governed by the ratio of faecal pellets deposited by filtration and mud particle aggregates settling out of the
water column (ten Brinke et al. 1995). These authors showed that the biogenically
deposited particles in the form of faecal pellets represent only 30 % of the entire
mud content in the upper 5-10 cm of the sediments. Consequently, the existence
and resistance against erosion of the biodeposits depends mainly on the physical
state of the muds and the local hydrodynamic conditions.
The amount of faeces and pseudofaeces produced varies strongly over the seasons depending on the filtration activity of the mussels. The lowest amounts are
produced in winter, while from spring to early summer more and more material is
accumulated (Oost 1996). This is in good agreement with the results of Kautsky &
Evans (1987) and J0rgensen (1990), which showed that the activity of the mussels
in the northern hemisphere is highest from April to September.
The accumulation and erosion of fine grained material in the vicinity of mussel
beds in the Dutch Wadden Sea is described by Oost (1996) as an annual cycle
which is controlled by variations in physical and biological conditions. Because of
storms and ice drift, mud is resuspended and/or eroded. Physical factors like wind,
temperature and tides result in a higher suspended sediment load in the water column during winter months (Dronkers 1984; De Wilde & Beukema 1984; Hickel
1984; Kragel 1997) (Chap. 3.4). De Haas & Eisma (1993) give suspended sediment concentrations during storms of 0.8 g/l, Oost (1996) mentions maximum
values of up to 50 g 1". In the context of ELA W AT, maximum suspended sediment
concentrations of 0.7 g 1" were determined. Resuspended sediments are exported
towards the open North Sea or settle again at the end of the stormy intervals.
The effects of the strong ice winter of 1995/96 showed the sediment stabilizing
function of mussel beds. After the removal of the mussel beds, deep incisions (up
to I m) into the topography developed. The lack of protection against erosion and
of sediment accumulation by the mussels, induced an increased erosion of the
former underlying sediment layers.
Although the mussel covered areas amount only to approx. I % of the total
Wadden Sea area, the biomass of the mussels comprises between 20 % and 70 %
of the total benthic biomass, depending on the area under consideration (Reise
1991). Differences exist between the East Frisian and North Frisian tidal flats in
shape and location of the mussel beds. In the backbarrier systems behind the East
Frisian island chain, the mussel beds are frequently found close to the tidal watersheds, whereas in the North Frisian tidal flats they are situated almost exclusively
in the lower intertidal zone. Both, the mean population density of the mussels and
their biomass per m
2
were at least twice as high in the Kanigshafen of Sylt compared to the East Frisian tidal flats (Dittmann 1987; Asmus 1987; Michaelis et al.
1995). The distribution pattern of the mussel beds in individual tidal areas remains
3 Study Area
corporated into sandy sediments (Hi.ittel et al. 1996). This can lead to sandtlats
with a relatively high mud content in the vicinity of mussel beds, resulting from
the export of fine grained mussel deposits. Without their activity, the fine grained
material deposited by the mussels could only accumulate in sheltered areas of the
Wadden Sea (e.g. mudflats near the coast), or would be exported from the backbarrier system towards the North Sea. Biodeposits are muddy sediments deposited
independently from the hydrodynamic energy gradients (Chap. 3.4; Flemming
1990).
The resistance of the biodeposits against erosion is governed by the ratio of faecal pellets deposited by filtration and mud particle aggregates settling out of the
water column (ten Brinke et al. 1995). These authors showed that the biogenically
deposited particles in the form of faecal pellets represent only 30 % of the entire
mud content in the upper 5-10 cm of the sediments. Consequently, the existence
and resistance against erosion of the biodeposits depends mainly on the physical
state of the muds and the local hydrodynamic conditions.
The amount of faeces and pseudofaeces produced varies strongly over the seasons depending on the filtration activity of the mussels. The lowest amounts are
produced in winter, while from spring to early summer more and more material is
accumulated (Oost 1996). This is in good agreement with the results of Kautsky &
Evans (1987) and J0rgensen (1990), which showed that the activity of the mussels
in the northern hemisphere is highest from April to September.
The accumulation and erosion of fine grained material in the vicinity of mussel
beds in the Dutch Wadden Sea is described by Oost (1996) as an annual cycle
which is controlled by variations in physical and biological conditions. Because of
storms and ice drift, mud is resuspended and/or eroded. Physical factors like wind,
temperature and tides result in a higher suspended sediment load in the water column during winter months (Dronkers 1984; De Wilde & Beukema 1984; Hickel
1984; Kragel 1997) (Chap. 3.4). De Haas & Eisma (1993) give suspended sediment concentrations during storms of 0.8 g/l, Oost (1996) mentions maximum
values of up to 50 g 1". In the context of ELA W AT, maximum suspended sediment
concentrations of 0.7 g 1" were determined. Resuspended sediments are exported
towards the open North Sea or settle again at the end of the stormy intervals.
The effects of the strong ice winter of 1995/96 showed the sediment stabilizing
function of mussel beds. After the removal of the mussel beds, deep incisions (up
to I m) into the topography developed. The lack of protection against erosion and
of sediment accumulation by the mussels, induced an increased erosion of the
former underlying sediment layers.
Although the mussel covered areas amount only to approx. I % of the total
Wadden Sea area, the biomass of the mussels comprises between 20 % and 70 %
of the total benthic biomass, depending on the area under consideration (Reise
1991). Differences exist between the East Frisian and North Frisian tidal flats in
shape and location of the mussel beds. In the backbarrier systems behind the East
Frisian island chain, the mussel beds are frequently found close to the tidal watersheds, whereas in the North Frisian tidal flats they are situated almost exclusively
in the lower intertidal zone. Both, the mean population density of the mussels and
their biomass per m
2
were at least twice as high in the Kanigshafen of Sylt compared to the East Frisian tidal flats (Dittmann 1987; Asmus 1987; Michaelis et al.
1995). The distribution pattern of the mussel beds in individual tidal areas remains