52
J.A. Commito and N.M.J.A. Dankers
vestigators were not able to establish the reasons why the three musselbed sites had seabed drag coefficients that differed from each other by as
much as an order of magnitude. The authors concluded that site differences in
spatial distribution and density must have caused the wide variation in flow
regime.
Butman et al. (1994) used live Mytilus edulis to simulate a mussel bed in a
laboratory flume and determined the contours of turbulent stress in front of
and over the bed at different flow velocities. They used young mussels all of
about the same length (2.7±0.2 cm), a simplifying first step, and it would be
useful to expand the range of mussel sizes and bed surface configurations to
determine how flow changes over different types of beds. Several field and
laboratory studies by Frechette and co-workers on intertidal Mytilus edulis
beds in the St. Lawrence River estuary, Quebec, Canada, have demonstrated
that boundary layer flow regimes are strongly affected by bed surface topography, such that the delivery, depletion, and resuspension of particulate
organic matter (chlorophyll a and phaeopigments) are enhanced because of
roughness provided by the mussels themselves (Frechette and Bourget
1985a,b; Frechette et al.1989; Frechette and Lefaivre 1990; Frechette and Grant
1991; Frechette et al. 1992; Frechette and Despland 1999). Similarly, Widdows
et al. (1998) showed that Mytilus edulis bed structure regulated biodeposition
and erosion rates at an intertidal site in the outer Humber estuary in England.
Studies of Mytilus edulis beds in two Dutch estuaries (Dame et al. 1991) and
the German Wadden Sea (Asmus and Asmus 1991; Asmus et al. 1992) have
shown that differences in benthic-pelagic flux rates are strongly flow-dependent and occur over even small spatial scales, suggesting that differences in bed
structure might be important.
2.6 Top-Down vs Bottom-Up Control of Soft-Bottom MusselBed Community Structure
We have presented some information indicating that the physical structure of
mussel beds plays a role in determining the delivery and depletion of larvae,
food particles, and oxygen, as well as the rates of biodeposition and erosion.
These are resource-based, "bottom-up", control processes. On the other hand,
mussel-bed structure also affects the abundances of herbivores and carnivores and their rates of consumption. These are consumer-based, "top-down",
control processes. We need to understand the interactions between musselbed structure and water flow if we want to learn how top-down and bottomup factors combine to regulate the population performance of mussels and
soft-bottom community structure where mussels are present. To our knowl-
J.A. Commito and N.M.J.A. Dankers
vestigators were not able to establish the reasons why the three musselbed sites had seabed drag coefficients that differed from each other by as
much as an order of magnitude. The authors concluded that site differences in
spatial distribution and density must have caused the wide variation in flow
regime.
Butman et al. (1994) used live Mytilus edulis to simulate a mussel bed in a
laboratory flume and determined the contours of turbulent stress in front of
and over the bed at different flow velocities. They used young mussels all of
about the same length (2.7±0.2 cm), a simplifying first step, and it would be
useful to expand the range of mussel sizes and bed surface configurations to
determine how flow changes over different types of beds. Several field and
laboratory studies by Frechette and co-workers on intertidal Mytilus edulis
beds in the St. Lawrence River estuary, Quebec, Canada, have demonstrated
that boundary layer flow regimes are strongly affected by bed surface topography, such that the delivery, depletion, and resuspension of particulate
organic matter (chlorophyll a and phaeopigments) are enhanced because of
roughness provided by the mussels themselves (Frechette and Bourget
1985a,b; Frechette et al.1989; Frechette and Lefaivre 1990; Frechette and Grant
1991; Frechette et al. 1992; Frechette and Despland 1999). Similarly, Widdows
et al. (1998) showed that Mytilus edulis bed structure regulated biodeposition
and erosion rates at an intertidal site in the outer Humber estuary in England.
Studies of Mytilus edulis beds in two Dutch estuaries (Dame et al. 1991) and
the German Wadden Sea (Asmus and Asmus 1991; Asmus et al. 1992) have
shown that differences in benthic-pelagic flux rates are strongly flow-dependent and occur over even small spatial scales, suggesting that differences in bed
structure might be important.
2.6 Top-Down vs Bottom-Up Control of Soft-Bottom MusselBed Community Structure
We have presented some information indicating that the physical structure of
mussel beds plays a role in determining the delivery and depletion of larvae,
food particles, and oxygen, as well as the rates of biodeposition and erosion.
These are resource-based, "bottom-up", control processes. On the other hand,
mussel-bed structure also affects the abundances of herbivores and carnivores and their rates of consumption. These are consumer-based, "top-down",
control processes. We need to understand the interactions between musselbed structure and water flow if we want to learn how top-down and bottomup factors combine to regulate the population performance of mussels and
soft-bottom community structure where mussels are present. To our knowl-