Dynamics in European and North American Soft-Bottom Mussel Beds
53
edge, a comprehensive investigation of this type has not been conducted for
soft-bottom mussel assemblages.
However, recent studies on the rocky shore,Maine, USA, where Mytilus
edulis dominates the lower intertidal and shallow subtidal zones, are
instructive (Leonard et al. 1998, 1999). They demonstrate clearly that water
flow is a dominant control agent in both these suspension-feeder communities. High-flow Mytilus edulis sites had greater recruitment of all species
with planktonic larvae than did low flow sites, leading to more mussel cover
and higher densities of snail grazers (Littorina littorea), snail predators
(Nucella lapillus), and crab predators (Carcinus maenas). At high-flow sites,
barnacle (Semibalanus balanoides) and Nucella lapillus growth rates were
faster, but mussel growth was slower, possibly because of reduced ability to
remove phytoplankton from the water column at high flow speeds. Despite the
greater abundance of predators, there were lower predation intensities and
per capita predation rates, possibly because predators had difficulty following
chemical cues under conditions of high flow. Even the Mytilus edulis size-class
structure was related to flow, with a unimodal size distribution at high-flow
sites, but a bimodal size distribution at low-flow sites due to selective predation on mid-sized mussels. These results suggest that high-flow sites were
regulated by resource-based factors, while consumer-based factors played a
more important role at low-flow sites.
Similar patterns were observed on natural, experimentally constructed,
and harvested Crassostrea virginiana (oyster) reefs at shallow subtidal sites in
North Carolina, USA (Lenihan and Peterson 1998; Lenihan 1999). The flow
environment explained 81 % of the variability in oyster growth and mortality
on experimentally constructed reefs. Location and physical structure - both
vertical profile and horizontal patchiness - influenced flow speed and
determined the balance between bottom-up and top-down control of oyster
performance.
To the extent that soft-bottom mussel beds share physical and biological
characteristics with oyster reefs and rocky shore mussel beds, they are
controlled by many of the same ecological processes. The structure of softbottom mussel beds is variable in time and space. It results from the interplay
among natural processes that kill and remove mussels (including storm
damage, ice scour, sedimentation, predation), fill in bare space (larval
settlement, juvenile and adult crawling, growth), and form the nuclei of new
patches elsewhere (larval settlement, clump dispersal). This mix of physical
disturbances and bottom-up and top-down processes is under the control of
the water flow regime, which, in turn, depends on the location and structure
of the mussel patches that make up the bed.
53
edge, a comprehensive investigation of this type has not been conducted for
soft-bottom mussel assemblages.
However, recent studies on the rocky shore,Maine, USA, where Mytilus
edulis dominates the lower intertidal and shallow subtidal zones, are
instructive (Leonard et al. 1998, 1999). They demonstrate clearly that water
flow is a dominant control agent in both these suspension-feeder communities. High-flow Mytilus edulis sites had greater recruitment of all species
with planktonic larvae than did low flow sites, leading to more mussel cover
and higher densities of snail grazers (Littorina littorea), snail predators
(Nucella lapillus), and crab predators (Carcinus maenas). At high-flow sites,
barnacle (Semibalanus balanoides) and Nucella lapillus growth rates were
faster, but mussel growth was slower, possibly because of reduced ability to
remove phytoplankton from the water column at high flow speeds. Despite the
greater abundance of predators, there were lower predation intensities and
per capita predation rates, possibly because predators had difficulty following
chemical cues under conditions of high flow. Even the Mytilus edulis size-class
structure was related to flow, with a unimodal size distribution at high-flow
sites, but a bimodal size distribution at low-flow sites due to selective predation on mid-sized mussels. These results suggest that high-flow sites were
regulated by resource-based factors, while consumer-based factors played a
more important role at low-flow sites.
Similar patterns were observed on natural, experimentally constructed,
and harvested Crassostrea virginiana (oyster) reefs at shallow subtidal sites in
North Carolina, USA (Lenihan and Peterson 1998; Lenihan 1999). The flow
environment explained 81 % of the variability in oyster growth and mortality
on experimentally constructed reefs. Location and physical structure - both
vertical profile and horizontal patchiness - influenced flow speed and
determined the balance between bottom-up and top-down control of oyster
performance.
To the extent that soft-bottom mussel beds share physical and biological
characteristics with oyster reefs and rocky shore mussel beds, they are
controlled by many of the same ecological processes. The structure of softbottom mussel beds is variable in time and space. It results from the interplay
among natural processes that kill and remove mussels (including storm
damage, ice scour, sedimentation, predation), fill in bare space (larval
settlement, juvenile and adult crawling, growth), and form the nuclei of new
patches elsewhere (larval settlement, clump dispersal). This mix of physical
disturbances and bottom-up and top-down processes is under the control of
the water flow regime, which, in turn, depends on the location and structure
of the mussel patches that make up the bed.