3.6 Ecosystem Engineers
43
3.6
Ecosystem Engineers: Myti/us edulis and Lanice
conchilega
Andreas Hild & Carmen-Pia Gi.inther
The two main study areas of the ecosystem project ELA W A T, the Swinnplate and
the Groninger Plate, were characterized by different dominant species, which were
the focus of scientific interest. The Swinnplate (Fig. 3.1.3) represented an area
partly covered with mussel beds of different age (Mytilus edulis L.), while the
Groninger Plate (Fig. 3.1.4) was covered with the tube-worm Lanice conchilega in
patches of varying density. Both species intluence sediment and biota in their vicinity, but in different ways. The blue mussel affects the surrounding area predominantly by the biogenic deposition of mud in the form of faeces and pseudofaeces, which are deposited in and around the mussel beds. Additionally, the
roughness of these beds reduces current velocities which results in the physical
settling of particles within the mussel beds. Currents were also reduced in patches
with L. conchilega. In this case, the tubes affected the current velocities. As a
consequence, the surrounding benthic population was modified. Therefore, both
species intluence the local hydrographic regime by their presence, while M. edulis
additionally accumulates material.
3.6.1
Myti/us edulis
Mussel beds significantly intluence the sedimentation of fine grained material in
the Wadden Sea. The mussels filter suspended matter (inorganic particles, refractory/labile detritus and living cells) out of the water column, incorporate TOC,
bacteria and phytoplankton as food (Wright et al. 1982; Frechette & Bourget 1985)
and excrete non-digested material compressed as faeces and pseudofaeces (faecal
pellets) (Verwey 1952; Haven & Morales-Alamo 1966). Faeces have passed the
intestine of the mussels while pseudofaeces have not and are rejected before.
Mytilus cdulis L. filters material in the size range of < 100 11m (Bayne et al. 1976)
to> 4 11m (sometimes material> 111m) out of the water column (Kautsky & Evans
1987). Above particle concentrations of ± 5 mg I', some of the filtered material is
rejected and excreted as pseudofaeces without passing the intestine tract (Widdows
et al. 1979). The rate of filtration amounts to 10--20 I per day per mussel (Asmus
1994). In addition to the active deposition, flocculated fine grained material settles
passively due to the roughness of the mussel beds (e.g. ten Brinke et al. 1995). In
this way, large amounts of biodeposits (faeces and pseudofaeces) as well as shells
and fine grained material are deposited within mussel beds and in the surrounding
areas. Therefore, mussel beds may be termed biosedimentary systems. Old mussel
beds in sheltered parts of the Wadden Sea may be elevated (mussel mudmounds)
compared to the surrounding tidal flats (Reineck 1994).
Sinking and/or transportation behaviour of the faecal pellets corresponds to that
of quartz particles in the silt/sand size-class (Oost 1996). Therefore, resuspended
biodeposits are transported and deposited together with sand particles and/or in-
43
3.6
Ecosystem Engineers: Myti/us edulis and Lanice
conchilega
Andreas Hild & Carmen-Pia Gi.inther
The two main study areas of the ecosystem project ELA W A T, the Swinnplate and
the Groninger Plate, were characterized by different dominant species, which were
the focus of scientific interest. The Swinnplate (Fig. 3.1.3) represented an area
partly covered with mussel beds of different age (Mytilus edulis L.), while the
Groninger Plate (Fig. 3.1.4) was covered with the tube-worm Lanice conchilega in
patches of varying density. Both species intluence sediment and biota in their vicinity, but in different ways. The blue mussel affects the surrounding area predominantly by the biogenic deposition of mud in the form of faeces and pseudofaeces, which are deposited in and around the mussel beds. Additionally, the
roughness of these beds reduces current velocities which results in the physical
settling of particles within the mussel beds. Currents were also reduced in patches
with L. conchilega. In this case, the tubes affected the current velocities. As a
consequence, the surrounding benthic population was modified. Therefore, both
species intluence the local hydrographic regime by their presence, while M. edulis
additionally accumulates material.
3.6.1
Myti/us edulis
Mussel beds significantly intluence the sedimentation of fine grained material in
the Wadden Sea. The mussels filter suspended matter (inorganic particles, refractory/labile detritus and living cells) out of the water column, incorporate TOC,
bacteria and phytoplankton as food (Wright et al. 1982; Frechette & Bourget 1985)
and excrete non-digested material compressed as faeces and pseudofaeces (faecal
pellets) (Verwey 1952; Haven & Morales-Alamo 1966). Faeces have passed the
intestine of the mussels while pseudofaeces have not and are rejected before.
Mytilus cdulis L. filters material in the size range of < 100 11m (Bayne et al. 1976)
to> 4 11m (sometimes material> 111m) out of the water column (Kautsky & Evans
1987). Above particle concentrations of ± 5 mg I', some of the filtered material is
rejected and excreted as pseudofaeces without passing the intestine tract (Widdows
et al. 1979). The rate of filtration amounts to 10--20 I per day per mussel (Asmus
1994). In addition to the active deposition, flocculated fine grained material settles
passively due to the roughness of the mussel beds (e.g. ten Brinke et al. 1995). In
this way, large amounts of biodeposits (faeces and pseudofaeces) as well as shells
and fine grained material are deposited within mussel beds and in the surrounding
areas. Therefore, mussel beds may be termed biosedimentary systems. Old mussel
beds in sheltered parts of the Wadden Sea may be elevated (mussel mudmounds)
compared to the surrounding tidal flats (Reineck 1994).
Sinking and/or transportation behaviour of the faecal pellets corresponds to that
of quartz particles in the silt/sand size-class (Oost 1996). Therefore, resuspended
biodeposits are transported and deposited together with sand particles and/or in-