200
E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
The calculated values have not always matched the
measured values possibly due to the dependence
of St on the friction coefficient (Thomas et al.,
2000), a parameter that decreases as the seagrass
canopy bends when exposed to increasing velocities
(Fonseca and Fisher, 1986). Additionally, the St
only parameterizes the transport into the canopy,
i.e. it parameterizes the flux across the interface
defined by the top of the canopy, not the diffusive
sub-layers on individual leaves.
The discussion of fluxes of inorganic nutrients
through the DBL so far assumed steady state flows.
In nature, the thickness of the δ D tends to fluctuate
over time and space (Koch, 1994). Wave-induced
oscillatory flows and/or large-scale turbulent eddies
tend to disrupt the δ D for short periods of time (fractions of a second) during which the δ D is stripped
away and the supply of molecules near the blade
surface is replenished (Nikora et al., 2002). If these
pulses of enriched water near the seagrass leaf occur on a regular basis such as under wave-dominated
conditions, the flux of nutrients to the plant surface is
expected to be enhanced (Stevens and Hurd, 1997).
As indicated in Section II, little is currently known
about the physiological implications of δ D fluctuations on seagrass leaves.
IV. Processes at the Shoot Level (mm–cm)
When considering the hydrodynamic forces exerted
on an individual seagrass shoot, the entire canopy
(group of shoots) needs to be taken into consideration. The canopy tends to attenuate currents and
waves thereby reducing the forces exerted on individual shoots. Even at the edge of the canopy, seagrass shoots may be sheltered to a certain extent
by the presence of adjacent shoots (Granata et al.,
2001). It follows that the biomechanical properties
of seagrass shoots (a response to the forces exerted
on them) are also altered by the canopy characteristics and the capacity of the canopy to attenuate
currents and waves. Therefore, a feedback mechanism is expected between seagrass shoots, canopies,
and the fluid forces that act on them.
A. The Role of Fluid Dynamics in Epiphytic
Growth on Seagrass Shoots
Epiphytes growing on seagrass leaves are commonly
related to the nutrient concentrations in the water
column (Frankovich and Fourqurean, 1997). Epiphyte levels are even used as indicators of eutrophication (Stankelis et al., 2003). Unfortunately, little
is known about ecological factors (other than light
and nutrients) that regulate epiphytic growth on seagrass leaves (Pinckney and Micheli, 1998; see also
Borowitzka et al., Chapter 19). Due to the lack of
data on the effect of currents on seagrass epiphytes,
one can only speculate that epiphyte biomass should
increase proportionally with water flow as a result of
decreased mass transfer limitation (e.g. Cornelisen
and Thomas, 2002). But the interaction between the
grazing community and water flow also needs to be
taken into consideration as strong currents (and/or
high waves) may eliminate grazers allowing more
epiphytes to grow under strong flow conditions (an
indirect effect of water flow on epiphytes; Schanz
et al., 2002).
Only a few studies have evaluated the effect of
waves on epiphytic loading on seagrass leaves. Although no difference was found in total epiphyte
biomass in a wave-exposed and a sheltered seagrass
habitat (Pinckney and Micheli, 1998), it seems that
the composition of the epiphytic layer is responsive
to water flow. Diatoms, coralline, and some filamentous algae dominate under wave-exposed conditions,
while blue-green and other filamentous algae dominate under calm conditions (Kendrick and Burt,
1997; Pinckney and Micheli, 1998). This difference
has been attributed to the size of the epiphytes on
seagrass leaves (i.e. the influence of drag).
Natural fluctuations in water flow also affect the
epiphytic community. If an epiphytic community
develops during relatively calm conditions, species
with high drag (i.e. large area exposed to the flow)
may become dominant, but if the flow increases over
a short period of time (e.g. storms), these epiphytes
are then removed (Cambridge, 1979; Biggs, 1996).
B. Hydrodynamic Forces Exerted on Shoots
and Shoot Biomechanics
Our knowledge of the forces exerted by flowing water on seagrass shoots or the biomechanical properties of seagrass shoots is very limited. We know that
most seagrasses tolerate a wide range of water motion, from stagnant water to relatively high velocities
(100 cm s –1 , Phillips, 1980; Dierssen et al., 2003).
In the short-term (minutes), this is likely due to their
capacity to bend as the velocity increases thereby
minimizing drag (by minimizing the leaf area
E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
The calculated values have not always matched the
measured values possibly due to the dependence
of St on the friction coefficient (Thomas et al.,
2000), a parameter that decreases as the seagrass
canopy bends when exposed to increasing velocities
(Fonseca and Fisher, 1986). Additionally, the St
only parameterizes the transport into the canopy,
i.e. it parameterizes the flux across the interface
defined by the top of the canopy, not the diffusive
sub-layers on individual leaves.
The discussion of fluxes of inorganic nutrients
through the DBL so far assumed steady state flows.
In nature, the thickness of the δ D tends to fluctuate
over time and space (Koch, 1994). Wave-induced
oscillatory flows and/or large-scale turbulent eddies
tend to disrupt the δ D for short periods of time (fractions of a second) during which the δ D is stripped
away and the supply of molecules near the blade
surface is replenished (Nikora et al., 2002). If these
pulses of enriched water near the seagrass leaf occur on a regular basis such as under wave-dominated
conditions, the flux of nutrients to the plant surface is
expected to be enhanced (Stevens and Hurd, 1997).
As indicated in Section II, little is currently known
about the physiological implications of δ D fluctuations on seagrass leaves.
IV. Processes at the Shoot Level (mm–cm)
When considering the hydrodynamic forces exerted
on an individual seagrass shoot, the entire canopy
(group of shoots) needs to be taken into consideration. The canopy tends to attenuate currents and
waves thereby reducing the forces exerted on individual shoots. Even at the edge of the canopy, seagrass shoots may be sheltered to a certain extent
by the presence of adjacent shoots (Granata et al.,
2001). It follows that the biomechanical properties
of seagrass shoots (a response to the forces exerted
on them) are also altered by the canopy characteristics and the capacity of the canopy to attenuate
currents and waves. Therefore, a feedback mechanism is expected between seagrass shoots, canopies,
and the fluid forces that act on them.
A. The Role of Fluid Dynamics in Epiphytic
Growth on Seagrass Shoots
Epiphytes growing on seagrass leaves are commonly
related to the nutrient concentrations in the water
column (Frankovich and Fourqurean, 1997). Epiphyte levels are even used as indicators of eutrophication (Stankelis et al., 2003). Unfortunately, little
is known about ecological factors (other than light
and nutrients) that regulate epiphytic growth on seagrass leaves (Pinckney and Micheli, 1998; see also
Borowitzka et al., Chapter 19). Due to the lack of
data on the effect of currents on seagrass epiphytes,
one can only speculate that epiphyte biomass should
increase proportionally with water flow as a result of
decreased mass transfer limitation (e.g. Cornelisen
and Thomas, 2002). But the interaction between the
grazing community and water flow also needs to be
taken into consideration as strong currents (and/or
high waves) may eliminate grazers allowing more
epiphytes to grow under strong flow conditions (an
indirect effect of water flow on epiphytes; Schanz
et al., 2002).
Only a few studies have evaluated the effect of
waves on epiphytic loading on seagrass leaves. Although no difference was found in total epiphyte
biomass in a wave-exposed and a sheltered seagrass
habitat (Pinckney and Micheli, 1998), it seems that
the composition of the epiphytic layer is responsive
to water flow. Diatoms, coralline, and some filamentous algae dominate under wave-exposed conditions,
while blue-green and other filamentous algae dominate under calm conditions (Kendrick and Burt,
1997; Pinckney and Micheli, 1998). This difference
has been attributed to the size of the epiphytes on
seagrass leaves (i.e. the influence of drag).
Natural fluctuations in water flow also affect the
epiphytic community. If an epiphytic community
develops during relatively calm conditions, species
with high drag (i.e. large area exposed to the flow)
may become dominant, but if the flow increases over
a short period of time (e.g. storms), these epiphytes
are then removed (Cambridge, 1979; Biggs, 1996).
B. Hydrodynamic Forces Exerted on Shoots
and Shoot Biomechanics
Our knowledge of the forces exerted by flowing water on seagrass shoots or the biomechanical properties of seagrass shoots is very limited. We know that
most seagrasses tolerate a wide range of water motion, from stagnant water to relatively high velocities
(100 cm s –1 , Phillips, 1980; Dierssen et al., 2003).
In the short-term (minutes), this is likely due to their
capacity to bend as the velocity increases thereby
minimizing drag (by minimizing the leaf area