3.6 Ecosystem Engineers
4S
relati vely constant, but the size of the mussel population tluctuates strongly from
year to year (Dankers & Koelemaij 1989; Nehls & Thiel 1993). Following calculations of Dankers & Koelemaij (1989), the water volume of the Dutch Wadden
Sea is filtered by the mussel population every 8-9 days, in years with large
amounts of mussels. This takes about I month in years with a small mussel population. In the last years a severe decrease in the mussel population has been observed both for the Dutch (Beukema 1993; Dankers 1993) and the East Frisian
Wadden Sea (Michaelis et al. 1995; Herlyn 1996). Possible causes are: increased
predation, algal blooms, fishing, pollution and parasite infestation. The continuous
existence of a mussel bed is also subject to the intluence of mechanical disturbances like storm events, ice drift and the seasonal cycle of physical water parameters as well (Verwey 1952). Nehls & Thiel (1993) discuss the influence of storm
events on the distribution and frequency of mussel beds. They distinguish between
highly dynamic, exposed mussel beds and mussel beds in wind sheltered areas that
can exist for a long tIme period.
In sediments containing biodeposits a fauna prevails that is adapted to oxygen
deficiency as well as high TOC- and H 2 S-concentrations. The sediment below
mussel beds represents a habitat with a fauna consisting of a small number of species with many individuals. By contrast the epifauna that only exists on mussel
beds in the Wadden Sea, exhibits a high diversity (Dittmann 1987, 1990).
Remineralization processes are intensified in the deposits of mussel beds compared to the surrounding sandtlats (Asmus et al. 1990). The cycle of production
and consumption of organic material is accelerated (Dankers & Zuidema 1995). In
situ measurements have shown the possibility of high release rates of inorganic
nutrients from mussel beds into the water column (Dame & Dankers 1988; Dame
et al. 1989; Asmus et al. 1990; Asmus 1994; Prins & Smaal 1994; Zurburg et al.
1994). In this case, the mussels are only responsible for a part of the whole rem ineralization. The major part is controlled via the activity of micro-organisms and
meiofauna living in the faeces and the pseudofaeces (Dankers et al. 1989; Kosfeld
1989). Biodeposits reveal a high bacterial activity (Dahlback & Gunnarsson 1981;
Grenz et ai. 1990). The nutrients which are released during mineralization are
available for primary production again. In this way mussel beds stimulate the
growth of phytoplankton (Asmus & Asmus 1991; Prins & Smaal 1994). A comprehensive study of the role of mussel beds in deposition and/or consumption of
organic matter and the release of inorganic nutrients was published by Smaa1 &
Prins (1993).
Due to the habitat heterogeneity of the mussel bed, the biodiversity is higher
compared to mudtlats. Since mussel beds intluence and modify their surrounding
environment due to their presence and activity, they can be regarded as ecosystem
engineers in the sense of Jones et ai. (1994). In conjunction with the increasing
impoverishment of fine grained material in the backbarrier systems in the course of
diking and land reclamation (Flemming & Nyandwi 1994), the areas influenced by
biogenic sedimentation gain a special significance in the Wadden Sea (potential
substitute for mudtlats). However, these areas represent no precise analogue to
mudflats, since they are subject to a strong temporal and spatial variability. In
addition, the benthic communities in mussel beds and tidal flats enriched with
biodeposits are different from natural mudflats (Kroncke 1996).
4S
relati vely constant, but the size of the mussel population tluctuates strongly from
year to year (Dankers & Koelemaij 1989; Nehls & Thiel 1993). Following calculations of Dankers & Koelemaij (1989), the water volume of the Dutch Wadden
Sea is filtered by the mussel population every 8-9 days, in years with large
amounts of mussels. This takes about I month in years with a small mussel population. In the last years a severe decrease in the mussel population has been observed both for the Dutch (Beukema 1993; Dankers 1993) and the East Frisian
Wadden Sea (Michaelis et al. 1995; Herlyn 1996). Possible causes are: increased
predation, algal blooms, fishing, pollution and parasite infestation. The continuous
existence of a mussel bed is also subject to the intluence of mechanical disturbances like storm events, ice drift and the seasonal cycle of physical water parameters as well (Verwey 1952). Nehls & Thiel (1993) discuss the influence of storm
events on the distribution and frequency of mussel beds. They distinguish between
highly dynamic, exposed mussel beds and mussel beds in wind sheltered areas that
can exist for a long tIme period.
In sediments containing biodeposits a fauna prevails that is adapted to oxygen
deficiency as well as high TOC- and H 2 S-concentrations. The sediment below
mussel beds represents a habitat with a fauna consisting of a small number of species with many individuals. By contrast the epifauna that only exists on mussel
beds in the Wadden Sea, exhibits a high diversity (Dittmann 1987, 1990).
Remineralization processes are intensified in the deposits of mussel beds compared to the surrounding sandtlats (Asmus et al. 1990). The cycle of production
and consumption of organic material is accelerated (Dankers & Zuidema 1995). In
situ measurements have shown the possibility of high release rates of inorganic
nutrients from mussel beds into the water column (Dame & Dankers 1988; Dame
et al. 1989; Asmus et al. 1990; Asmus 1994; Prins & Smaal 1994; Zurburg et al.
1994). In this case, the mussels are only responsible for a part of the whole rem ineralization. The major part is controlled via the activity of micro-organisms and
meiofauna living in the faeces and the pseudofaeces (Dankers et al. 1989; Kosfeld
1989). Biodeposits reveal a high bacterial activity (Dahlback & Gunnarsson 1981;
Grenz et ai. 1990). The nutrients which are released during mineralization are
available for primary production again. In this way mussel beds stimulate the
growth of phytoplankton (Asmus & Asmus 1991; Prins & Smaal 1994). A comprehensive study of the role of mussel beds in deposition and/or consumption of
organic matter and the release of inorganic nutrients was published by Smaa1 &
Prins (1993).
Due to the habitat heterogeneity of the mussel bed, the biodiversity is higher
compared to mudtlats. Since mussel beds intluence and modify their surrounding
environment due to their presence and activity, they can be regarded as ecosystem
engineers in the sense of Jones et ai. (1994). In conjunction with the increasing
impoverishment of fine grained material in the backbarrier systems in the course of
diking and land reclamation (Flemming & Nyandwi 1994), the areas influenced by
biogenic sedimentation gain a special significance in the Wadden Sea (potential
substitute for mudtlats). However, these areas represent no precise analogue to
mudflats, since they are subject to a strong temporal and spatial variability. In
addition, the benthic communities in mussel beds and tidal flats enriched with
biodeposits are different from natural mudflats (Kroncke 1996).