Dynamics in European and North American Soft-Bottom Mussel Beds
43
speed up the rate at which mussel beds coalesce. Some studies have shown
that predators may consume Mytilus edulis more effectively on smooth beds
than on rough, irregular beds (Reusch and Chapman 1997; Dolmer 1998), so
surface topography may be as important as clump size and shape.
Horizontal and vertical bed structure also affect removal and death of
mussels by physical processes. For example, the removal of spatially aggregated Geukensia demissa by winter ice was inversely related to mussel density
(Bertness and Grosholz 1985). Mytilus edulis individuals at the perimeter of
mussel clumps on hard substrate were more susceptible to storm damage
than were those in the center (Asamushi, Japan: Tsuchiya and Nishihira 1986).
Studies on several species of mussels in Europe and on both coasts of North
America have demonstrated that the risk of dislodgement was higher for
individuals projecting above the bed surface or for mussels to which other
organisms, especially algae, were attached (Witman and Suchanek 1984;
Witman 1987; Dolmer and Svane 1994). Juvenile Geukensia demissa suffered
lower rates of suffocation by shifting sediment when in the presence of adults
because the larger individuals provide a safe location above the sediment
surface to which juveniles can migrate (Bertness and Grosholz 1985; Lin
1991). Albrecht and Reise (1994) and Albrecht (1998) demonstrated that the
presence of algae growing on K6nigshafen mussel beds contributed to increased sedimentation, which reduced mussel density and altered the bed
surface profile. Algal abundance was under the control of Littorina littorea
grazing, which increased with larger mussel patch size. As these studies all
show, the removal or death of mussels depends upon patch size and shape in
aerial view and surface roughness in profile.
Although mussel beds are fragmented by storms and ice scour, this
change in patch size and shape sets the stage for beds to "heal" by
recruitment of new mussels and growth of existing mussels. Flume studies
with Mytilus edulis have shown that boundary-layer flow changes
dramatically at the margins of mussel patches where they project above the
sediment (Butman et al. 1994). Larval recruitment was greater to small
patches and to the edges of large patches than to the centers of large patches
of Mytilus edulis on sand bars in two Danish fjords (Svane and Ompi 1993).
Flow-mediated delivery of phytoplankton to Mytilus edulis was increased as
a result of bed roughness caused by the surface topography of the mussels
themselves in an intertidal bed in the St. Lawrence River estuary, Quebec,
Canada (Frechette et al. 1989). Mussels at the edges of patches typically grow
faster and reach larger sizes than those in the center (Geukensia demissa:
Bertness and Grosholz 1985; Mytilus edulis from California on hard
substrate: Okamura 1986; Mytilus edulis on soft substrate: Svane and Ompi
1993). These patterns demonstrate that the benefits of group living (lower
mortality due to protection from predation, ice scour, and sediment burial)
may be counterbalanced by the costs of slower growth in the centers of
43
speed up the rate at which mussel beds coalesce. Some studies have shown
that predators may consume Mytilus edulis more effectively on smooth beds
than on rough, irregular beds (Reusch and Chapman 1997; Dolmer 1998), so
surface topography may be as important as clump size and shape.
Horizontal and vertical bed structure also affect removal and death of
mussels by physical processes. For example, the removal of spatially aggregated Geukensia demissa by winter ice was inversely related to mussel density
(Bertness and Grosholz 1985). Mytilus edulis individuals at the perimeter of
mussel clumps on hard substrate were more susceptible to storm damage
than were those in the center (Asamushi, Japan: Tsuchiya and Nishihira 1986).
Studies on several species of mussels in Europe and on both coasts of North
America have demonstrated that the risk of dislodgement was higher for
individuals projecting above the bed surface or for mussels to which other
organisms, especially algae, were attached (Witman and Suchanek 1984;
Witman 1987; Dolmer and Svane 1994). Juvenile Geukensia demissa suffered
lower rates of suffocation by shifting sediment when in the presence of adults
because the larger individuals provide a safe location above the sediment
surface to which juveniles can migrate (Bertness and Grosholz 1985; Lin
1991). Albrecht and Reise (1994) and Albrecht (1998) demonstrated that the
presence of algae growing on K6nigshafen mussel beds contributed to increased sedimentation, which reduced mussel density and altered the bed
surface profile. Algal abundance was under the control of Littorina littorea
grazing, which increased with larger mussel patch size. As these studies all
show, the removal or death of mussels depends upon patch size and shape in
aerial view and surface roughness in profile.
Although mussel beds are fragmented by storms and ice scour, this
change in patch size and shape sets the stage for beds to "heal" by
recruitment of new mussels and growth of existing mussels. Flume studies
with Mytilus edulis have shown that boundary-layer flow changes
dramatically at the margins of mussel patches where they project above the
sediment (Butman et al. 1994). Larval recruitment was greater to small
patches and to the edges of large patches than to the centers of large patches
of Mytilus edulis on sand bars in two Danish fjords (Svane and Ompi 1993).
Flow-mediated delivery of phytoplankton to Mytilus edulis was increased as
a result of bed roughness caused by the surface topography of the mussels
themselves in an intertidal bed in the St. Lawrence River estuary, Quebec,
Canada (Frechette et al. 1989). Mussels at the edges of patches typically grow
faster and reach larger sizes than those in the center (Geukensia demissa:
Bertness and Grosholz 1985; Mytilus edulis from California on hard
substrate: Okamura 1986; Mytilus edulis on soft substrate: Svane and Ompi
1993). These patterns demonstrate that the benefits of group living (lower
mortality due to protection from predation, ice scour, and sediment burial)
may be counterbalanced by the costs of slower growth in the centers of