Chapter 8 Fluid Dynamics in Seagrass Ecology
201
exposed to the flow; Sand-Jensen, 2003). Reproductive shoots of Zostera marina were found to be
approximately one order of magnitude stiffer than
macroalgae but two to three orders of magnitude
less stiff than trees (Patterson et al., 2001), allowing
seagrass shoots to bend and, as a result, minimize
drag (Fonseca et al., 1982; Sand-Jensen, 2003). In a
terrestrial grass (Arundinaria tecta), the sheath contributed 33% of the overall bending stiffness (Niklas,
1998). Perhaps the sheath surrounding the base of
seagrass shoots also increases the bending stiffness
of seagrass shoots making them stiffer than macroalgae. In the long term (weeks), seagrasses likely acclimate to water flow through growth changes in
anatomy and morphology such that drag, breakage,
and dislodgement are minimized. Eutrophication has
been shown not to alter the tensile strength that
Zostera marina leaves can withstand (Kopp, 1999).
Another aspect to be considered in the estimation
of the drag exerted on seagrass shoots is the epiphytes that colonize their leaves. Epiphytes on a red
alga (Odonthalia floccosa) increased the drag exerted on the macrophyte (Ruesink, 1998). It is likely
that this would also happen with epiphytes on seagrass shoots.
The risk of being dislodged due to excessive drag
is highest during storm events when waves and currents are at their maximum (F d α u
2 ). Massive loss
of Zostera marina is expected when currents reach
values above 4 m s –1 (Kopp, 1999). One mechanism
to cope with this risk seems to be the existence of
a few strong (reproductive) seagrass shoots that can
resist extreme events and protect the other shoots in a
population (Patterson et al., 2001). Within a population, vegetative male shoots of Phyllospadix torreyi
tended to be dislodged at lower flows than the female
plants (Williams, 1995).
The forces exerted on seagrasses exposed to waves
are more complex than those in unidirectional flows.
As water in waves accelerates in different directions
during the course of each wave, organisms exposed
to such unsteady flows are subjected to acceleration reaction forces as well as drag (Koehl, 1984).
These forces are higher than in unidirectional flows
at the same instantaneous velocity (Koehl et al.,
1991) and the maximum drag occurs at a different time than the maximum acceleration reaction
forces. In these wave-swept environments, a long
flexible shape (Fig. 3) can minimize the forces exerted on the anchoring system (roots in the case of
seagrasses). This is confirmed by the finding that
leaves of Posidonia australis become longer as wave
exposure increases (Larkum, 1976). The long leaves
tend to move in one direction during the passage of
a wave. If the leaves begin to move in the other direction before they are fully extended (Fig. 3), they
will sway back and forth with the waves without imposing too much drag on the roots (Koehl, 1984).
C. Water Flow Around Seagrass
Shoots—Ecological Implications
Seagrass shoots are obstructions to flowing water.
When considering the vertical scale, seagrass shoots
are exposed to a gradient of velocities in the canopy
and benthic boundary layers. Due to the no-slip condition, the slowest flows are found near the sediment
surface and the strongest flows near the top of the
canopy. As a result of this vertical difference in velocities and the horizontal differences in upstream
versus downstream velocities around a shoot, a vertical pressure gradient develops on the downstream
side of the seagrass shoot: high pressure near the
bottom where the currents are relatively slow and
low pressure farther up in the water column where
currents are stronger. This leads to the development
of significant ascending flows (i.e. as high as 15%
of ambient) immediately downstream of seagrass
shoots (Fig. 4; Nepf and Koch, 1999). Pressure gradients around shoots (Huettel and Gust, 1992) can also
lead to the intrusion of water into permeable sediments upstream of the shoot (high pressure zone;
Koch and Huettel, 2000) and porewater upwelling
downstream of the shoot (low pressure zone) (Nepf
and Koch, 1999). For example, around a single Thalassia testudinum shoot exposed to a current speed of
10 cm s –1 , water was found to penetrate 2.5 cm into
the permeable sediments (Fig. 4); a depth an order
of magnitude larger than that affected by diffusion
(Jørgensen and Boudreau, 2001). This flow-induced
intrusion of water into seagrass-colonized permeable sediments may bring organic particles (Huettel et al., 1996) closer to the root zone and remove
toxic compounds from the sediments such as sulfide
(Koch, 1999a). Most of the impact of water intrusion
into the sediment occurs over the first 6 h which coincides with semi-diurnal tides (Koch and Huettel,
2000). Therefore, the exchange between the sediments and the water column seems to be maximized
in seagrass habitats in which the current direction
changes every 6 h.
201
exposed to the flow; Sand-Jensen, 2003). Reproductive shoots of Zostera marina were found to be
approximately one order of magnitude stiffer than
macroalgae but two to three orders of magnitude
less stiff than trees (Patterson et al., 2001), allowing
seagrass shoots to bend and, as a result, minimize
drag (Fonseca et al., 1982; Sand-Jensen, 2003). In a
terrestrial grass (Arundinaria tecta), the sheath contributed 33% of the overall bending stiffness (Niklas,
1998). Perhaps the sheath surrounding the base of
seagrass shoots also increases the bending stiffness
of seagrass shoots making them stiffer than macroalgae. In the long term (weeks), seagrasses likely acclimate to water flow through growth changes in
anatomy and morphology such that drag, breakage,
and dislodgement are minimized. Eutrophication has
been shown not to alter the tensile strength that
Zostera marina leaves can withstand (Kopp, 1999).
Another aspect to be considered in the estimation
of the drag exerted on seagrass shoots is the epiphytes that colonize their leaves. Epiphytes on a red
alga (Odonthalia floccosa) increased the drag exerted on the macrophyte (Ruesink, 1998). It is likely
that this would also happen with epiphytes on seagrass shoots.
The risk of being dislodged due to excessive drag
is highest during storm events when waves and currents are at their maximum (F d α u
2 ). Massive loss
of Zostera marina is expected when currents reach
values above 4 m s –1 (Kopp, 1999). One mechanism
to cope with this risk seems to be the existence of
a few strong (reproductive) seagrass shoots that can
resist extreme events and protect the other shoots in a
population (Patterson et al., 2001). Within a population, vegetative male shoots of Phyllospadix torreyi
tended to be dislodged at lower flows than the female
plants (Williams, 1995).
The forces exerted on seagrasses exposed to waves
are more complex than those in unidirectional flows.
As water in waves accelerates in different directions
during the course of each wave, organisms exposed
to such unsteady flows are subjected to acceleration reaction forces as well as drag (Koehl, 1984).
These forces are higher than in unidirectional flows
at the same instantaneous velocity (Koehl et al.,
1991) and the maximum drag occurs at a different time than the maximum acceleration reaction
forces. In these wave-swept environments, a long
flexible shape (Fig. 3) can minimize the forces exerted on the anchoring system (roots in the case of
seagrasses). This is confirmed by the finding that
leaves of Posidonia australis become longer as wave
exposure increases (Larkum, 1976). The long leaves
tend to move in one direction during the passage of
a wave. If the leaves begin to move in the other direction before they are fully extended (Fig. 3), they
will sway back and forth with the waves without imposing too much drag on the roots (Koehl, 1984).
C. Water Flow Around Seagrass
Shoots—Ecological Implications
Seagrass shoots are obstructions to flowing water.
When considering the vertical scale, seagrass shoots
are exposed to a gradient of velocities in the canopy
and benthic boundary layers. Due to the no-slip condition, the slowest flows are found near the sediment
surface and the strongest flows near the top of the
canopy. As a result of this vertical difference in velocities and the horizontal differences in upstream
versus downstream velocities around a shoot, a vertical pressure gradient develops on the downstream
side of the seagrass shoot: high pressure near the
bottom where the currents are relatively slow and
low pressure farther up in the water column where
currents are stronger. This leads to the development
of significant ascending flows (i.e. as high as 15%
of ambient) immediately downstream of seagrass
shoots (Fig. 4; Nepf and Koch, 1999). Pressure gradients around shoots (Huettel and Gust, 1992) can also
lead to the intrusion of water into permeable sediments upstream of the shoot (high pressure zone;
Koch and Huettel, 2000) and porewater upwelling
downstream of the shoot (low pressure zone) (Nepf
and Koch, 1999). For example, around a single Thalassia testudinum shoot exposed to a current speed of
10 cm s –1 , water was found to penetrate 2.5 cm into
the permeable sediments (Fig. 4); a depth an order
of magnitude larger than that affected by diffusion
(Jørgensen and Boudreau, 2001). This flow-induced
intrusion of water into seagrass-colonized permeable sediments may bring organic particles (Huettel et al., 1996) closer to the root zone and remove
toxic compounds from the sediments such as sulfide
(Koch, 1999a). Most of the impact of water intrusion
into the sediment occurs over the first 6 h which coincides with semi-diurnal tides (Koch and Huettel,
2000). Therefore, the exchange between the sediments and the water column seems to be maximized
in seagrass habitats in which the current direction
changes every 6 h.