Microphytobenthos in Contrasting Coastal Ecosystems: Biology and Dynamics
109
5.3.2 Physical Dynamics
Broadly, the nature of the sediment reflects the hydrodynamic forces that
control sediment deposition and erosion. High-energy conditions prevent
fine sediments being deposited and therefore coarse sediments predominate
while, under low-energy conditions, only fine sediments are carried to the site
and deposited. However, the forces of tides and winds behave in an episodic
manner bringing in sediments of varied nature depending on the conditions.
Under these circumstances, mixed flats can develop where both fine particles
and coarse sediment are deposited resulting in poorly sorted sediments
(Yallop et al. 1994). In fact, most natural intertidal sediments consist of a mixture of various sediment types. Biological processes have a role in developing
and maintaining intermediate sediment beds in two ways. Firstly, biogenic
products may add to the sediment matrix, for example, shell fragments,
skeletal components (spicules), and cell walls (diatom frustules). In some situations, the sediment matrix is entirely organic in origin (e. g., coral sands and
diatom ooze). Secondly, organic secretions add to the cohesion of the sediments (Sutherland et al.1998a; Paterson and Black 1999). Thus,fine sediments
can be trapped and retained despite current velocities that would normally
lead to their resuspension (Faas et al.1992; Underwood and Paterson 1993).
The sediment bed responds to hydrodynamic forcing and the flow over
sediments creates a boundary layer (Paterson and Black 1999). Flow within
the boundary layer over the bed can be described as laminar, smooth
turbulent or rough turbulent (Fig. 5.2; Paterson and Black 1999). Laminar flow
is rare under natural conditions in coastal waters (Brown et al. 1999); therefore, smooth or rough turbulent flow predominates. The transition between
smooth and rough turbulent conditions is influenced by the velocity of the
free stream flow and by the roughness of the bed. For smooth turbulent
conditions, a small region near the bed experiences laminar flow (the viscous
sub-layer); this layer breaks down on transition to rough turbulent flow. For a
given flow, a smooth surface may help to maintain smooth turbulent flow
while a rough surface promotes a transition to rough turbulent conditions
(Fig. 5.2; Vogel 1994).
Organisms also influence flow (Paterson 1997) and, even at a microbial
level, the effects are significant (Grant and Gust 1987; Dade et al. 1990). The
interactions are complex; it is not simply that the bed resists flow more
strongly because of the organic secretion or network effects, but also that the
stress experienced on the bed as a result of water flow varies depending on the
nature of the bed (Fig. 5.2). A rough bed will experience greater stress for the
same overall flow than a smooth bed. This has important consequences for
biota at the sediment surface. Under rough turbulent conditions, turbulent
eddies impact the bed and the likelihood of sediment (or organism!) erosion
is enhanced. No viscous sub-layer can be retained and the flux of material is
109
5.3.2 Physical Dynamics
Broadly, the nature of the sediment reflects the hydrodynamic forces that
control sediment deposition and erosion. High-energy conditions prevent
fine sediments being deposited and therefore coarse sediments predominate
while, under low-energy conditions, only fine sediments are carried to the site
and deposited. However, the forces of tides and winds behave in an episodic
manner bringing in sediments of varied nature depending on the conditions.
Under these circumstances, mixed flats can develop where both fine particles
and coarse sediment are deposited resulting in poorly sorted sediments
(Yallop et al. 1994). In fact, most natural intertidal sediments consist of a mixture of various sediment types. Biological processes have a role in developing
and maintaining intermediate sediment beds in two ways. Firstly, biogenic
products may add to the sediment matrix, for example, shell fragments,
skeletal components (spicules), and cell walls (diatom frustules). In some situations, the sediment matrix is entirely organic in origin (e. g., coral sands and
diatom ooze). Secondly, organic secretions add to the cohesion of the sediments (Sutherland et al.1998a; Paterson and Black 1999). Thus,fine sediments
can be trapped and retained despite current velocities that would normally
lead to their resuspension (Faas et al.1992; Underwood and Paterson 1993).
The sediment bed responds to hydrodynamic forcing and the flow over
sediments creates a boundary layer (Paterson and Black 1999). Flow within
the boundary layer over the bed can be described as laminar, smooth
turbulent or rough turbulent (Fig. 5.2; Paterson and Black 1999). Laminar flow
is rare under natural conditions in coastal waters (Brown et al. 1999); therefore, smooth or rough turbulent flow predominates. The transition between
smooth and rough turbulent conditions is influenced by the velocity of the
free stream flow and by the roughness of the bed. For smooth turbulent
conditions, a small region near the bed experiences laminar flow (the viscous
sub-layer); this layer breaks down on transition to rough turbulent flow. For a
given flow, a smooth surface may help to maintain smooth turbulent flow
while a rough surface promotes a transition to rough turbulent conditions
(Fig. 5.2; Vogel 1994).
Organisms also influence flow (Paterson 1997) and, even at a microbial
level, the effects are significant (Grant and Gust 1987; Dade et al. 1990). The
interactions are complex; it is not simply that the bed resists flow more
strongly because of the organic secretion or network effects, but also that the
stress experienced on the bed as a result of water flow varies depending on the
nature of the bed (Fig. 5.2). A rough bed will experience greater stress for the
same overall flow than a smooth bed. This has important consequences for
biota at the sediment surface. Under rough turbulent conditions, turbulent
eddies impact the bed and the likelihood of sediment (or organism!) erosion
is enhanced. No viscous sub-layer can be retained and the flux of material is