Chapter 8
Fluid Dynamics in Seagrass Ecology—from Molecules
to Ecosystems
Evamaria W. Koch*
Horn Point Laboratory, University of Maryland Center for Environmental Science,
P.O. Box 775, Cambridge, MD 21613, USA
Josef D. Ackerman
Faculty of Environmental Sciences & Departments of Botany and Zoology,
University of Guelph, Guelph, ON, Canada N1G 2W1;
email: ackerman@uoguelph.ca
Jennifer Verduin
School of Biological Sciences & Biotechnology, Murdoch University, Murdoch WA 6150,
Australia; email: j.verduin@murdoch.edu.au
Michael van Keulen
School of Biological Sciences & Biotechnology, Murdoch University, Murdoch WA 6150,
Australia; email: keulen@mail.murdoch.edu.au
I. Introduction
Fluid dynamics is the study of the movement of fluids. Among other things, it addresses velocity, acceleration, and the forces exerted by or upon fluids in
motion (Daugherty et al., 1985; White, 1999; Kundu
and Cohen, 2002). Fluid dynamics affects every aspect of the existence of seagrasses from the smallest
to the largest scale: from the nutrients they obtain
to the sediment they colonize; from the pollination
of their flowers to the import/export of organic matter to adjacent systems; from the light that reaches
their leaves to the organisms that live in the seagrass habitats. Therefore, fluid dynamics is of major
importance in seagrass biology, ecology, and ecophysiology. Unfortunately, fluid dynamics is often
overlooked in seagrass systems (Koch, 2001). This
∗
Author for correspondence, email: koch@hpl.umces.edu
chapter provides a general background in fluid dynamics and then addresses increasingly larger scales
of fluid dynamic processes relevant to seagrass ecology and physiology: molecules (µm), leaves and
shoots (mm to cm), seagrass canopies (m), seagrass landscapes (100–1,000 m), and seagrasses as
part of the biosphere (>1,000 m). Although gases
are also fluids, this chapter is restricted to water
(i.e. compressed fluids), how it flows through seagrasses, the forces it exerts on the plants, and the
implications that this has for seagrass systems. Seagrasses are not only affected by water in motion,
they also affect the currents, waves and turbulence
of the water masses surrounding them. This capacity to alter their own environment is referred to as
“ecosystem engineering” (Jones et al., 1994, 1997;
Thomas et al., 2000). Readers are also encouraged
to consult a recent review by Okubo et al. (2002)
for a discussion on flow in terrestrial and aquatic
A. W. D. Larkum et al. (eds.), Seagrasses: Biology, Ecology and Conservation, pp. 193–225.
c 2006 Springer.
194
E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
vegetation including freshwater plants, seagrasses,
and kelp.
II. Fluid Dynamics: Fundamentals
The general aspects of water flow in aquatic systems
can be understood through a number of fluid dynamic concepts that have been developed largely for
steady state conditions, i.e. when there are no temporal fluctuations in the water flow (Fischer et al.,
Abbreviations
A – cross sectional area
C – celerity or phase velocity of waves
Cd – drag coefficient
Cs – concentration on the seagrass surface
Cw – concentration in the water column
D – molecular diffusivity
D – depth
DBL – diffusive boundary layer
δ – diffusive boundary layer thickness
δ D – diffusive boundary layer (=DBL)
δ I –inertial sublayer or logarithmic (log) layer
δ v –viscous sublayer
Fd – friction or viscous drag
Fp – form or pressure drag
g – acceleration due to gravity
H – water depth
H – wave height
h – canopy height
J – flux
κ – von Karman constant
l – length scale
λ – wavelength
m – mass
µ – molecular or dynamic viscosity
p – hydrostatic or dynamic pressure
Q – volume flow rate
ρ – density
REI – relative wave exposure index
Re – Reynolds number
Recrit – critical Reynolds number
St – Stanton number
T – wave period
τ – shear stress
τ o – boundary shear stress
τ W – wall shear stress
u – current velocity
u ∗ – friction velocity
Uk – critical velocity
Uo – free stream velocity
ν–kinematic viscosity
x – horizontal distance
x – principal flow direction
y – cross-stream direction
z – vertical direction or depth
z o – roughness height
1979; White, 1999; Kundu and Cohen, 2002). In the
absence of motion, seawater is described by: (i) density (ρ, i.e. mass/volume), which is used preferentially over mass (m) in fluids; (ii) kinematic viscosity
(ν) which is a measure of how easily the fluid will
flow (i.e. ν = µ/ρ, where µ is the molecular or dynamic viscosity); and (iii) hydrostatic pressure ( p),
which is a function of the depth from the water surface (i.e. p = ρgz, where g is the acceleration due
to gravity and z is the depth—note that the depth
can be the distance from the water surface to the
seafloor or the height from the seafloor to the water
surface; see below). The introduction of energy into
a fluid causes fluid motion, and the motion in natural
systems is generated by pressure gradients (d p/dz)
as result of gradients in water surface elevation or
depth (dz/dx; where x is the horizontal distance)
and/or density (dρ/dz). The major source of this energy input is the sun, which causes winds that lead
to changes in surface elevation (i.e. dz/dx; waves,
currents, and seiches in embayments), and thermal
gradients (i.e. dρ/dz) that lead to expansion, instabilities, and mixing. Other sources include inputs
of freshwater and other chemical constituents (i.e.
dρ/dz), tides and currents due to gravitation and acceleration of the earth-moon and earth-sun systems
(i.e. dz/dx), and the Coriolis force due to the earth’s
rotation (i.e. dz/dx) (Kundu and Cohen, 2002).
The flow in seawater is described with respect
to a fixed Cartesian reference frame (Eulerian perspective) with x defining the principal flow direction, y defining the cross-stream direction, and z
defining the vertical direction. Whereas it is common in geophysics to define z as the depth (i.e. with
respect to the water surface), it is equally appropriate and perhaps more informative to use height
(i.e. defined with respect to the seafloor) as the vertical direction (e.g. Ackerman and Okubo, 1993).
The volume flow rate (Q), as defined by the velocity (u) of the fluid that passes through a given cross
sectional area A (which is usually defined with respect to the x and y; i.e. dxdy), is conserved because seawater is an incompressible fluid. This continuity principle is one of the essential elements of
fluid dynamics, which, among other things, is used
to determine mass balances of water-borne materials (e.g. Hemond and Fechner, 1994). The flow of
water leads to a second type of pressure, the dynamic pressure ( p = 1/ 2 ρu2 ), which, when added
together with the hydrostatic pressure, is constant
along a flow streamline (i.e. Bernoulli’s principle).
Chapter 8 Fluid Dynamics in Seagrass Ecology
Bernoulli’s principle, which states that the sum of the
hydrostatic pressure and dynamic pressure along a
streamline are constant (Vogel, 1994), helps to explain flow-induced pressure changes (i.e. lift) that
occur within, around, and under seagrass canopies
(e.g. Nepf and Koch, 1999). Drag is another important force that acts downstream of obstacles. It has
two additive components: (i) the friction or viscous
drag that exists due to the interaction of the obstacle’s surface with the water, which can be defined
algebraically (i.e. Fd = 1/2Cd ρAu2 , where Cd is the
drag coefficient, a shape and flow dependent constant); and (ii) the dynamic, form or pressure drag
(Fp ) that exists under high flows when flows separate
from boundaries, which cannot be expressed algebraically and must, therefore, be determined empirically. As u increases, the dynamic drag contributes
a disproportionate fraction of the total drag. It is
important to note that drag is a force that operates opposite to the flow direction in that it “sucks”
a moving object upstream or a stationary object
downstream.
Water flow can exhibit a number of different
properties that depend on the temporal and spatial
scales under investigation. Water flow could either
be smooth and regular as if the fluid flows in layers
(i.e. laminar flow) or rough and irregular as if the
flow is “chaotic” (i.e. turbulent flow). This depends
on the velocity and the length scale (i.e. temporal and
spatial scale, respectively) under investigation as defined by the Reynolds number (Re = luρ/µ or more
simply Re = lu/ν; where l is the length scale appropriate for the hypothesis being tested). Re, which
is the non-dimensional ratio of inertial to viscous
forces in a fluid, defines four regimes that grade into
one another: (i) creeping flow (Re ≪ 1), which occurs at very low flows and spatial scales such as those
experienced by individual bacteria cells; (ii) laminar
flow (1 < Re < 103 ) as defined above; (iii) transitional flow (Re ∼ O(103 ); i.e. of the order of 103 ),
which involves the production of eddies and disturbances in the flow and is characterized by a critical
Re (Recrit ) defined for a particular geometry and flow;
and (iv) fully turbulent flow (Re ≫ 103 ). Associated
with these flow patterns are important differences
related to the fluid dynamic forces (e.g. friction vs.
pressure drag) and mass transfer processes (diffusion vs. advection) that operate under the different
regimes (see below; White, 1999; Kundu and Cohen,
2002). Moreover, because Re is scale dependent, it is
possible to experience multiple flow regimes simul-
195
taneously in the flow field depending on the spatial
scale under investigation. Consequently, flow is almost always turbulent at large spatial scales such
as seagrass beds, but it can also be laminar on the
scale of seagrass leaves and flowers (e.g. Ackerman
and Okubo, 1993; Koch, 1994). This not-so-subtle
distinction can influence the application and interpretation of physiological and ecological processes
in seagrass canopies (see Section III).
As indicated above, the flow conditions become
more complicated when water approaches a boundary (e.g. seagrass canopy, leaves, or seafloor, depending on the scale) or any obstacle for that matter. The
water cannot normally penetrate boundaries, except
for the most porous ones (see reviews in Boudreau
and Jørgensen, 2001; Okubo et al., 2002), and more
importantly, the water molecules directly next to a
boundary stick to the boundary rather than slip by
it. This no-slip condition leads to the development
of a velocity gradient perpendicular to the boundary (Fig. 1), as the velocity at the boundary will be
zero relative to the free stream velocity (U0 ). As the
water flows downstream, the velocity gradient will
grow in size and a slower moving layer of fluid will
develop next to the boundary, which is referred to as
the boundary layer under turbulent conditions, otherwise technically it is a deformation layer (Prandtl
and Tietjens, 1934). This boundary layer, which is
defined by velocities <0.99 U0 , has a thickness of
δ that is relatively small and can be expressed as a
function of Re and x. Initially it appears laminar in
nature, but the boundary layer will become turbulent when the local Re (Rex = ux/ν) approaches a
critical value of 3 to 5 × 105 , in the case of a flat
plate oriented parallel to the flow. In nature, this transition is accelerated by the presence of roughness
or obstacles on the boundary (Schlichting, 1979;
Nikora et al., 2002; Fig. 1) including undulations
on macroalgal blades (Hurd and Stevens, 1997). In
addition to the streamwise structure in a fully developed boundary layer, there is important vertical
structure as well. The first layer directly adjacent to
the boundary is the viscous sublayer (δ v ≈ 10ν/u ∗
where u ∗ is the friction velocity, which is a velocity
scale that provides an indication of the mass transfer
within the boundary layer) in which the forces (or
stresses if surface forces are considered) are largely
viscous, and consequently the mass transfer in this
layer is slow and dominated by diffusion, especially
within the thin diffusional sublayer (also called the
diffusive boundary layer; DBL) at the bottom of this
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E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
Fig. 1. Velocity (U ) gradient/profile adjacent to smooth (left) and rough (right) boundaries. Weak currents (solid line) generate a relatively
thick boundary layer (1) when compared with boundary layers (2) generated by faster currents (dashed curve). Names of boundary layer
zones are provided for the fast flowing water velocity profile. When the boundary (such as a seagrass leaf) is rough (e.g. due to the
presence of epiphytic organisms), a roughness height (arrow) extends the boundary layer farther into the water column. Consequently,
the flux of nutrients and carbon from the water column to the boundary is reduced.
layer (δ D ≈ ν/u ∗ (ν/D)−a , where D is the molecular
diffusivity and a is a constant equal to 1/2 or 1/3;
(Lorke et al., 2003). It is important to note that δ D
≪ δ v , which relates to the fact that the molecular
diffusion of momentum (i.e. ν ∼ 10–6 m2 s–1 ) is
much larger than the molecular diffusion of a scalar
quantity like CO2 (i.e. D ∼ 10–9 m2 s–1 ). The next
layer is the inertial sublayer or logarithmic (log) layer
(δ I ≈ 0.15δ), which is a region of exponentially increasing velocity; hence, it is dominated by inertial
forces (or stresses) and mass transfer occurs through
turbulent advection. The outer layer of the boundary
layer is the largest layer, and it represents a transition to the free stream flow (it is referred to as
the Ekman layer in situations where the Coriolis
force causes rotation of the flow; Fig. 1). Boundary layers exist embedded in one another as they
are defined by spatial scale (e.g. Ackerman, 1986;
Boudreau and Jørgensen, 2001); consequently, it is
possible to define boundary layers around plant epiphytes, flowers, leaves, canopies, and the benthos. In
this sense, there is a benthic boundary layer (BBL)
above the seagrass canopy, and separate boundary
layers around individual shoots, leaves, flowers, and
the smaller constituents described above. In addition, it is important to note that there may also be
boundary layers generated by other types of water
motion (e.g. wave current boundary layers), but this
topic is beyond the scope of this review. For biolog-
ically relevant information on this topic see Denny
(1988).
Another important consequence of the no-slip
condition at a boundary is the tractile or shearing
force that the boundary imparts on the fluid, which is
a tangential force causing rotation of the fluid next to
the boundary. A boundary or wall shear stress (τ 0 or
τ W ) is defined as the quotient of the shearing force
and the area of the boundary, and τ W = µdu/dz
within the viscous (or laminar) sublayer and τ W =
ρu2∗ in general. In practice, it is difficult to measure
the shear force or the aerial extent of the boundary
or to apply the algebraic relationships, and thus a
number of methods have been developed to measure τ directly using force balances in flow chambers or indirectly using velocity gradients based on
the law of the wall (u = u ∗ /κ ln(z/z 0 ), where κ =
0.4 is the von Karman constant, and z 0 is the roughness height; see Fig. 1; for a review of techniques
and references for the measurement of bed shear
stress see Ackerman and Hoover (2001). The velocity gradient method involves applying the law of
the wall to the velocities measured in the log layer
of the boundary layer. In this case, u ∗ is equal to κ
multiplied by the slope of the linear regression of
velocity on the natural logarithm transformed distances from the boundary, and z 0 is equal to e raised
to the value of the x intercept of the same regression.
This method has been applied successfully above and
Chapter 8 Fluid Dynamics in Seagrass Ecology
within seagrass canopies (e.g. Fonseca and Fisher,
1986; Gambi et al., 1990; Ackerman and Okubo,
1993). It is important to note that other engineering
models have been applied to rough canopies such as
corals and seagrasses with the direct measurement
of canopy friction and the use of the Stanton number, St (uptake rate by the surface/advection over
the surface), to determine the efficiency of canopy
uptake (e.g. Thomas et al., 2000; Thomas and Cornelisen, 2003). Reconfiguration of seagrass canopies
under higher flow conditions (e.g. Fonseca et al.,
1982; Ackerman, 1986), and/or unsteadiness due to
monamis (waving of the canopy; see Section V.C)
caused by an instability of the mean velocity profile
(Ackerman and Okubo, 1993; Ghisalberti and Nepf,
2002) and waves (Koch, 1996; Koch and Gust, 1999)
represents a challenge to researchers. Even so, τ is
the preferred form (over u) of expressing hydrodynamic conditions near boundaries (leaves, flowers,
sediment etc; see Nowell and Jumars, 1984).
Hydrodynamic conditions in the environment are
rarely stationary, especially in wave-dominated seagrass habitats where a more appropriate characterization of the fluid environment is that it varies in a
periodic fashion with each passing wave. Waves represent the movement of energy through a fluid and
exhibit a periodic motion, especially when viewed
at an interface (e.g. the water surface). In this case,
the passing wave (crest followed by trough) causes
a submersed object on the surface to move in a circular or orbital fashion, the diameter of which is
equal to the wave height (H ). The orbital motions
also extend downward through the fluid in a series
of orbitals that diminish in diameter with depth until
a depth (z) of 1/2λ (where λ is the wavelength) is
reached.
The classification of waves can be based on the
disturbing force that creates them, the restoring force
that destroys them, and their wavelength (Garrison,
2000). The disturbing force is the source of energy
that causes the wave, which can be (i) wind stress
acting on water surface causing capillary and gravity waves, (ii) the arrival of surge or sea wave causing swell, (iii) wind setup in an embayment creating seiches, (iv) a change in atmospheric pressure causing short-lived storm surge, and (v) large
disturbances (landslides, volcanic eruptions, earthquake) that cause seismic waves (or tsunami; the
so-called tidal waves that are actually due to gravitational inertial forces). The restoring forces that
reduce the disturbance to the water surface include
197
(a) surface tension due to the molecular cohesion of
water molecules, which works for small waves (i.e.
λ <1.73 cm; capillary waves) and, (b) gravity that
operates on larger waves (i.e. λ ≫ 1.73 cm). Whereas
the wavelength can be used to distinguish differences
among the smallest of waves, it really provides a
measure of wave size and relationship to energy; the
smaller the wavelength, the higher the energy. Some
typical relationships include (1) wind waves (λ <60–
150 m), (2) seiches (λ is large and a function of the
basin size), (3) seismic waves (λ <200 km), and (4)
tides (λ = 1/2 circumference of earth; note that tides
are caused by gravity and inertia).
Seagrasses experience each of these types of
waves, but the most common are wind waves, swell,
and tides (tides can be viewed as long waves). Wind
waves develop from capillary waves to gravity waves
as a function of the wind strength and direction and
the fetch (length of the unrestricted zone over which
the wind stress operates). Wind waves are affected by
local wind conditions, and are generally of a short
period, T (T is time it takes for a wave to pass a
fixed point). Wave action has a direct impact on the
ecosystem, with obvious effects on sediment transport, boundary layer processes and physical stresses
(Denny, 1988; Koch and Gust, 1999). There has also
been some suggestions that fetch (relative wave exposure index) is an important factor affecting seagrass on a landscape level (e.g. Fonseca and Bell,
1998; Hovel et al., 2002; Krause-Jensen et al., 2003).
Just as the size of the wave is determined by the
wavelength, the shape of the orbit is determined by
the water depth. In deep water (i.e. z > 1/2λ) the
orbits are circular, whereas in shallow water (i.e. z
<1/20λ) the orbits become elliptical or flatter due to
the influence of the bottom. Intermediate waves (i.e.
1/20λ < z < 1/2λ) are more complicated as they
combine characteristics of deep and shallow water
waves. Deep water
√ waves travel at a celerity or phase
velocity C = gλ/2π or λ/T (∼1.56 T ), but shallow water waves are slower due to the influence
of the
√
√
bottom and travel at C = gz (or 3.1 z), which is
why waves build up in shallow areas (Denny, 1988).
Waves travel in a wave train, which is a progression
of groups of waves of similar λ from the same origin.
Energy is lost by the leading wave, which eventually
dissipates, but a new trailing edge wave is created
from this energy. In deep water, the waves progress
with C ∝ λ but the wave train has a group velocity of
C ∝ 1/2λ, whereas in shallow water the celerity of
the individual waves slow until the wave and group
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E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
celerities are equal. It is important to note that waves
and wave trains are not isolated from one another;
circumstances can lead to destructive interference
with calm periods between wave trains and constructive interference with the generation of large waves
including rogue wavees due to the convergence of
many waves.
As indicated above, a number of changes occur as
waves enter the nearshore and ultimately reach the
shore. The waves become shallow water waves as
the wave train encounters the friction (shear stress)
of the bottom or seagrass canopy, the wave orbits become more elliptical in shape near the bottom, and
the wave crests become more pronounced. This can
lead to wave-induced transport in a process referred
to as Stokes drift, which may be of considerable importance in many coastal environments (Monismith
and Fong, 2004). The steepness of the wave becomes
unstable if it is greater than 1:7 (H :λ) and the water at the crest begins to travel faster than the water
near the bottom and it will break into a plunging
wave, spilling wave, or surging wave depending on
the steepness and topography of the bottom. Since
waves approach the shore at different angles, they
are unlikely to break simultaneously and may refract
from the original direction leading to the complexity
of waves experienced in coastal seagrass beds (Koch
and Gust, 1999). Realistically, the fluid dynamic conditions within these nearshore regions are affected by
a number of factors including tides and wind waves,
all of which lead to significant changes in the surface
elevation and water flow within seagrass beds. The
general predictions are that seagrasses, like other
benthic vegetation, increase the bottom shear stress
and hence have a wave dampening affect (see Section
V.B., below). This process has been relatively well
characterized for coastal kelp forests (see review in
Okubo et al., 2002), but has yet to be examined in a
thorough manner for seagrasses. Clearly, additional
efforts are needed in this area.
Whereas, the ultimate goal of studying fluid dynamic concepts is to better understand ecological
processes in seagrasses, it is important to note that
vegetative flows remain the most complex and difficult flows to describe and understand (Raupach
et al., 1991; Finnigan, 2000). Therefore, applications
in vegetated flows have typically involved steady
state condition (i.e. non turbulent), although some
progress in unsteady flows has been made with respect to seaweeds (Gaylord and Denny, 1997). Fortunately, this realization provides a challenge to those
interested in the biological, chemical, geological,
and physical processes that occur in seagrass systems.
III. Micro-Scale Processes at the Molecular
Level (µm)
As water flows through seagrass beds, a boundary
layer develops on the sediment surface as well as on
each seagrass component exposed to the moving water (leaf, short-shoot, flower) (Ackerman, 1986; Fonseca and Kenworthy, 1987; Koch, 1994; Cornelisen
and Thomas, 2002). The faster the water moves,
the thinner the diffusive boundary layer (DBL, or
δ D ) becomes (Massel, 1999; Fig. 1) and, consequently, the faster the transfer of molecules from
the water column to the sediment and/or seagrass.
It follows that when currents are weak, the flux of
molecules to the seagrass surface may be limited by
diffusion through the δ D (i.e. physical limitation).
Under those conditions, many biological sites or
enzymes in the seagrass tissue are available to assimilate molecules when/if they reach the plant’s surface
(Koch, 1994; Cornelisen and Thomas, 2002). After a
critical velocity (Uk ) is reached (Fig. 2), the transfer
Fig. 2. An example of uptake kinetics by seagrass leaves exposed
to increasing current velocities (U ). Uk is the critical current velocity at which uptake rate saturates (equivalent to Ik in photosynthesis x irradiance curves). At currents below Uk (1), uptake
is mass transfer-limited and at currents above Uk (3), uptake is
kinetically limited. A combination of both limitations may occur
at flows around Uk (2). If nutrient concentration in the water column increases, the curve is likely to shift upwards. Additionally,
other types of responses to water flow are also possible (see text).
Chapter 8 Fluid Dynamics in Seagrass Ecology
of molecules through the δ D is no longer the limiting factor. Instead, the capacity of biological uptake sites or enzymes to assimilate molecules that
reached the plant surface becomes limiting (Koch,
1994). In this case, the conditions are said to be kinetically (and not physically) limiting. When velocities
are at intermediate levels, around Uk , a combination
of physical (δ D ) and kinetic (enzymes) limitations
may influence the uptake of nutrients (Sanford and
Crawford, 2000). At velocities below 3–5 cm s–1
(Uk ), photosynthesis (i.e. carbon uptake) in Thalassia testudinum and Cymodocea nodosa is δ D limited, whereas at velocities above Uk , photosynthesis seems to be limited by the kinetics of Rubisco
(Koch, 1994). Interestingly, a similar Uk value was
found for the kelp Macrocystis integrifolia (Stevens
and Hurd, 1997). In contrast, some seagrass studies
were unable to detect a kinetic limitation in the assimilation of nutrients in flowing water (i.e. no Uk ),
instead, assimilation was δ D limited up to the maximum velocity tested: 20 cm s–1 for Thalassia testudinum and its epiphytes (Cornelisen and Thomas,
2002) and 34 cm s–1 for Zostera marina (Fonseca
and Kenworthy, 1987). This difference may be due in
part to experimental conditions. Specifically, studies
in which assimilation was only a function of velocity
were performed with entire plants rooted in sediment
and covered by epiphytes, while the experiments in
which assimilation was a function of velocity and enzyme kinetics were done with epiphyte-free leaves,
in the laboratory. For further discussion of the role
of diffusive boundary layers on photosynthesis, see
Larkum et al., Chapter 14.
Mass transfer to seagrass leaves does not only
depend on the velocity and δ D thickness but also
on: (1) the thickness of the periphyton layer (complex of debris, mucus, bacteria, algae, small animals,
and sediment particles) on the seagrass leaves (Jones
et al., 2000), (2) the reactions occurring within the
periphyton layer (Sand-Jensen et al., 1985; Jones
et al., 2000; Cornelisen and Thomas, 2002) and (3)
the concentration of the molecules in the water column adjacent to the seagrasses-periphyton complex
(Sanford and Crawford, 2000). The water interstitial to the periphyton is expected to be static (with
the exception of occasional sweep events; Nikora
et al., 2002); therefore, δ D increases linearly with
periphyton thickness (Jones et al., 2000). Consequently, the spatial scale for diffusion of molecules
from the water column to the leaf surface is longer
and δ D -limited conditions are more likely to occur.
199
The critical δ D thickness at Uk has been estimated to
be 98 µm and 280 µm for periphyton-free leaves
of Cymodocea nodosa and Thalassia testudinum,
respectively (Koch, 1994), whereas the δ D on artificial leaves with periphyton was quantified to be
950 µm in thickness (Jones et al., 2000). The δ D
limitation of molecules such as nitrogen, phosphorous and carbon may be further exacerbated by
the reactions occurring within the periphyton layer.
Epiphytic algae tend to assimilate biologically important molecules before they reach the seagrass
surface (Jones et al., 2000; Sanford and Crawford,
2000; Cornelisen and Thomas, 2002), thereby competing for vital nutrients (Sand-Jensen et al., 1985).
If the uptake kinetics of epiphytes is more efficient
than that of seagrasses, the microalgae could potentially outcompete the seagrasses in the uptake
of nutrients (including carbon) from the water column (Sand-Jensen et al., 1985; Beer and Koch, 1996;
Cornelisen and Thomas, 2002). According to Fick’s
first law:
J=D
Cw − Cs
δD
where J is the flux of molecules, Cw the concentration in the water column, and Cs the concentration on the seagrass surface. δ D limiting conditions
become less important as the concentration of nutrients (Cw ) in the water column increases (i.e. eutrophication). Under such eutrophic conditions, uptake is controlled by the kinetics of periphyton and
seagrasses (Sanford and Crawford, 2000). As a result, one can hypothesize that as coastal waters become more eutrophic, mass transfer-limitations may
become less important to seagrasses, but this is a
complex process as the growth of the epiphytes as
a function of the nutrient concentration also needs
to be taken into account. Additionally, when uptake
rates are δ D limited, kinetic processes become less
important and the uptake rates become a function of
the planar area of seagrasses and epiphytes exposed
to water flow.
As indicated above, the Stanton number (St), a
dimensionless number, can also be used to quantify
the efficiency of a seagrass canopy to remove
nutrients from the water, as it is the flux of a
chemical to a surface divided by its advection past
the surface (e.g. Thomas et al., 2000). St can be
obtained via direct measurements of nutrient uptake
and velocity measurements, or can be calculated.
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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
Chapter 8 Fluid Dynamics in Seagrass Ecology
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 (Fd 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
201
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.
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E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
Fig. 3. Waving of seagrass leaves in wave-dominated habitats: (A) Phyllospadix torreyi at approximately 2.5 m depth at Punta Morro,
Pacific coast of Mexico and (B) Halodule wrightii at approximately 1 m depth, north of Placencia, Belize. The lines were traced over
individual leaves in order to emphasize the bending pattern. Note that the leaves are continuously moving back and forth every few
seconds with the passage of waves. Photos: E.W. Koch.
The combination of the pressure-induced upwelling of porewater and the vertical ascending flows
immediately downstream of the seagrass shoots
colonizing permeable sediments appear to generate
a slow “stream” connecting the sediment porewater
and the water column at mid-height in the seagrass
canopy (Nepf and Koch, 1999; Fig. 4). Under these
circumstances, exchanges between the sediment
Chapter 8 Fluid Dynamics in Seagrass Ecology
203
Fig. 4. Vertical ascending flows and porewater flows generated by pressure gradients around a seagrass shoot. As seagrass shoots live
in the benthic boundary layer (see velocity (U ) profile on the left), the top of the shoot experiences faster velocities and lower pressure
than the bottom of the shoot. As a result, a vertical ascending flow is generated downstream of the shoot. This water then disperses
horizontally at the point where the leaves bend over with the flow. Due to the pressure gradients generated on the sediment surface when
the flowing water impacts the seagrass shoot, water also penetrates into permeable sediments leading to a zone in which the porewater
is washed out by the overlying water. Z , distance above the sediment interface. Modified from Nepf and Koch (1999) and Koch and
Huettel (2000). It is possible that the upwelling porewater may be transported high into the water column via the above processes.
porewater and the water column are not driven by
diffusion but by advection (Huettel and Webster,
2001). This process could benefit the seagrasses
by bringing recently remineralized nutrients and
carbon to seagrass leaves (Nepf and Koch, 1999).
D. The Role of Fluid Dynamics
in Seagrass Reproduction
Pollination in water (hydrophily) is uncommon in
angiosperms, and restricted mostly to the monocotyledons, including the seagrasses (Les et al.,
1997; Ackerman, 2000, Chapter 4, this volume). It
is relevant to contrast seagrass with their freshwater
relatives. Pollination in freshwater plants involves
pollen or detached-floating anthers contacting the
stigmas of floating or partly submerged carpellate
flowers/inflorescences or submerged pollen “showering” (sedimenting) to stigmas from elevated an-
thers (Arber, 1920; Sculthorpe, 1967; Cook, 1982).
Pollination in seagrasses, however, involves pollination underwater (i.e. hydrophily or submarine pollination), in which pollination occurs through the
action of currents (Ackerman, 1995). In some cases
(i.e. shallow seagrass populations that may be exposed to the air and the single species Enhalus acaroides), pollination may occur on the water surface
(ephydrophily) when the pollen or stamens touch exposed stigmas (Cox, 1988). The concept that surface
pollination (or pollination in two dimensions) was
responsible for the evolution of seagrass pollination
modes (Cox, 1988) is unsound because it assumes
that pollen transport is random and, thus, recurrent,
which is not the case, as wind generated movements
are directional, not random (Ackerman, 1995).
Seagrasses possess a number of morphological
features that appear to be associated with submerged
or submarine pollination, notably their filamentous
pollen shapes (Ackerman, 1995), which evolved
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E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
convergently–multiple times in the Najadales and
functionally in the Hydrocharitales (Pettitt, 1984;
Ackerman, 1995, 2000). Pettitt (1984) reviewed the
research pertaining to seagrass pollination biology,
but that review was limited taxonomically with respect to pollen transport and capture mechanisms.
The situation has improved, but remains largely
limited to the mechanistic studies of submarine pollination in the north temperate species, Zostera marina (Ackerman, 1986, 1993, 1995, 1997a,b, and
2002). Fortunately, recent progress has been made
with respect to other species including Amphibolis
antarctica (Verduin et al., 1996), Posidonia australis
and Posidonia sinuosa (Smith and Walker, 2002).
The mechanics of pollination in Z. marina were
studied in a laboratory flow chamber using stroboscopic photography and in the field using physical
models of pollen and adhesive surfaces deployed in
the canopy (Ackerman, 1997a,b, 2002). Pollination
was found to occur under laminar and relatively viscous conditions at the scale of flowers (i.e. low Re;
see Niklas, 1992; Vogel, 1994), and was affected
by the emergence of female flowers from within the
inflorescence. The emergence of flowers (and other
reproductive organs in subsequent phenological processes) led to an increase in the fluid shear stress (τ )
in the local flow (Ackerman, 1997a), which caused
the filamentous pollen (2.7 mm × 7.5 µm diameter)
to rotate and cross streamlines toward female flowers (Ackerman, 1997b). The axial force responsible
for the pollen motion varied directly with the length
and aspect ratio of the pollen (Forgacs and Mason,
1958). Consequently, filamentous pollen need only
be close to female flowers to pollinate by: (i) direct interception on stigmas; (ii) rotation within one
half a pollen length of stigmas; and (iii) by being redirected through streamlines toward flowers
(Ackerman, 1997b).
Ancestral spherical pollen, on the other hand,
could only pollinate via direct interception due to the
limited axial force exerted on the spherical-shaped
body. Canopy flow conditions at the scale of leaves
and flowers were laminar, which indicates that observations would be similar to those in the laboratory
flow chamber (Ackerman and Okubo, 1993; Ackerman, 2002). Higher recovery of filamentous pollen
models compared to spherical ones also supports the
biomechanical model (Ackerman, 2002). Moreover,
field observations of pollination in Amphibolis were
consistent with the predictions from Z. marina (Verduin et al., 1996), as were results from Posidonia
(Smith and Walker, 2002). It is important to note that
seagrasses can maintain relatively high outcrossing
rates through hydrophily (Ruckleshaus, 1995; Waycott and Sampson, 1997; Reusch, 2000; Waycott et
al., Chapter 2). This is especially true for populations found under conditions that promote pollen
dispersal, such as exposed bays (e.g. Waycott and
Sampson, 1997). It is reasonable to conclude that
the strong convergence of filiform pollen morphologies in seagrasses indicates a similar convergence
in pollination mechanisms linked to fluid dynamics. Further research from a diversity of seagrass
taxa will be needed to determine the validity of this
statement.
Seed dispersal phenomena in seagrasses are somewhat analogous to pollination mechanisms in that
they are poorly recognized but have become better
understood in recent years (Van der Pijl, 1972; Orth
et al., Chapter 5). This is in part due to the clonal nature of seagrasses in which rhizomatous growth, in
addition to anchoring and binding of sediments, was
viewed as the principal mechanism of population
growth. However, the occurrence of annual populations (e.g. Keddy and Patriquin, 1978), colonization
of new areas (e.g. Turner, 1983), and recovery from
small and large-scale disturbances (e.g. Rasmussen,
1977; Inglis, 2000) via seeds has led to the realization that seed dispersal is important in this group.
There is considerable diversity in seed biology
and ecology related to the polyphyletic nature of
seagrasses (e.g. Les et al., 1997) and their evolutionary innovations. For example, 7 of the 12/13
genera of seagrasses have dormant seeds, with geocarpy (releasing seeds under the sediments) occurring in Halodule, Cymodocea, and Halophila (Inglis,
2000). Geocarpy appears to facilitate recovery from
disturbances in Halodule, where dispersal involves
near-bed saltational movements analogous to sediment transport, and high densities of seeds accumulate in dugong feeding depressions (Inglis, 2000).
Other species, which have reproductive organs elevated above the seafloor, such as Z. marina, have
small, negatively buoyant seeds that sink in still water, and that likely move on the order of 1–10 m
horizontally in the water column, depending on the
canopy flow (see Okubo et al., 2002). Sometimes
seeds (5–13% of seeds in Long Island Sound) are
released with a bubble of gas (presumably from the
lacunar spaces in the infructescence), which can extend dispersal distances on the order of 10–100 m
(Churchill et al., 1985). Longer distance dispersal
Chapter 8 Fluid Dynamics in Seagrass Ecology
205
Fig. 5. Vertical velocity (U) profiles (thick solid lines) in seagrass canopies exposed to 5 cm s−1 (A), 10 cm s−1 (B) and 20 cm s−1 (C).
Z , distance above the sediment interface. Note that, as velocity increases, the angle of bending of the canopy increases and the canopy
height (dashed horizontal line) decreases. Based on a flume experiment using a short (16 cm) and dense (1,000 shoots m−2 ) Zostera
marina canopy (Gambi et al. 1990).
(e.g. 100 m–10 km) occurs through the floatation of
buoyant diaspores (i.e. fruits and seeds) and detached
infructescences and/or whole reproductive shoots,
which “sweepstake” because of the positive buoyancy provided by the lacunar gas system (e.g. Harwell and Orth, 2002; Lacap et al., 2002; see review
in Orth et al., Chapter 5). In the latter case, seeds
may be released from the plant while it is floating,
or when the plant reaches the shore. Lastly, seeds
may be transported in the guts of birds, sea turtles,
and fish (e.g. Baldwin and Lovvorn, 1994), leading
to potentially large dispersal distances on the order
of 100 m–1,000 km, especially in migrating birds.
It is interesting to note the morphological/
ecological specializations that evolved in Phyllospadix, which is found in wave-exposed environments where it is a dominant successional species
(Wyllie-Echeverria and Ackerman, 2003). In this
case, dispersal is facilitated by wave action by an undescribed mechanism, but recruitment is limited to
areas that have branching macrophytes with cylindrical thalli onto which the bifid, barbed fruits attach (Turner, 1983). This is a form of obligate facultative succession in that there is no recruitment
in the absence of the correct type of turf-forming
algae.
V. Processes at the Canopy Level (m)
Fluid dynamic processes at the canopy level are the
most studied and best understood. Based on more
than two decades of research, it is well-accepted that
seagrass beds attenuate currents and waves and, as
a result, tend to accumulate organic and inorganic
particles as well as spores and larvae in the canopy.
These concepts are now being refined and new questions linking water flow and seagrasses are being addressed. This section provides a general background
on classical concepts and then focuses on new trends
in fluid dynamics at the canopy level.
A. Attenuation of Currents by Seagrass
Canopies: A Classical Concept
The presence of seagrass canopies in the benthic
boundary layer (BBL) alters the roughness of the
bottom (Fonseca and Fisher, 1986; Nepf and Vivoni,
2000; Granata et al., 2001). As a result, the vertical
flow profile shown in Fig. 1 is altered (Fig. 5), especially when the plants occupy a large portion of
the water column, i.e. when H/h <10 (H is the water depth and h the canopy height; Nepf and Vivoni,
2000). Reduced flows are common within the canopy
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E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
Fig. 6. Vertical velocity (U) profile (solid line) showing relative flow intensification near the bottom, which is a result of the vertical
seagrass biomass (B) distribution (shaded area). Z , distance above the sediment interface. Adapted from Ackerman and Okubo (1993;
Zostera marina). Also observed for Thalassia testudinum (koch, 1996), Amphibolis griffithii (van Keulen, 1997), and Amphibolis
antarctica (Verduin and Backhaus, 2000).
due to the deflection of the current over the canopy
and a loss of momentum within the canopy (Fonseca
et al., 1982; Fonseca and Fisher, 1986; Gambi et al.,
1990; Koch, 1996; Wallace and Cox, 1997; Koch and
Gust, 1999; Verduin and Backhaus, 2000; Peterson
et al., 2004). As a result, depending on the seagrass
species and shoot density, water speed in the canopy
can be 2 to >10 times slower than outside the bed
(Ackerman, 1986; Gambi et al., 1990). This process
can also trap water within dense seagrass canopies
during low tide, leading to a water height difference
between vegetated and adjacent unvegetated areas
(Powell and Schaffner, 1991). Velocities within seagrass canopies are commonly <10 cm s–1 but can be
as high as 100 cm s–1 (see review by Koch, 2001).
Even relatively short seagrasses (Zostera novazelandica, 15 cm) or beds with relatively low densities
(Zostera marina, 100–200 shoots m–2 ) still seem
to reduce velocity (Worcester, 1995; Heiss et al.,
2000).
When measuring velocities at a relatively fine
scale (cm), flow intensification near the bottom (i.e.
relatively faster flows in the region of the sheaths
or vertical stems, Fig. 6) may be observed depending on the vertical biomass distribution (Ackerman
and Okubo, 1993; Koch, 1996; Koch and Gust,
1999; Nepf and Vivoni, 2000; Verduin and Backhaus, 2000; van Keulen and Borowitzka, 2002). This
is due to the fact that the sheaths (e.g. Thalassia testudinum and Zostera marina) or stems (e.g. Amphibolis griffithii and A. antarctica) are less effective in
reducing the flow and extracting momentum (Fig. 6)
than the vegetated regions above the sheaths and
stems that are filled with leaves. Similarly, velocities
increase near the top of the canopy as the leaf area
is reduced and eventually disappears.
A number of canopy flow models have been applied to terrestrial plant canopies using empiricallyfit parameters to modify the law of the wall (review in
Okubo et al., 2002). This approach has been recently
Chapter 8 Fluid Dynamics in Seagrass Ecology
207
Fig. 7. Wave attenuation (open circles) as a function of water depth/tidal fluctuation (black boxes). Wave attenuation was based on the
significant wave height in a Ruppia maritima bed in comparison to an adjacent unvegetated area at Bishop’s Head Point, Chesapeake
Bay, USA. Note that these data were collected in June when the plants were reproductive. Wave attenuation was highest at low tide
when the canopy occupied the entire water column. Negative wave attenuation represents periods in which wave height was larger in
the vegetated site than the unvegetated site. Source of data: E.W. Koch.
applied to Z. marina with some success, although
there were a number of inconsistencies with field observations as would be expected (Abdelrham, 2003;
Peterson et al., 2004). This type of approach provides some indication of the general pattern of flow
within an eelgrass canopy, but its utility will likely
be limited by species-specific differences in canopy
vegetative profiles and the lack of detailed studies of
canopy flow in these systems. Future development
will need to apply the mixing layer analogy. Realistically, the ability to model canopy flow phenomena
is a goal that speaks to the need for detailed canopy
flow profiles in the laboratory and the field.
B. Wave Attenuation by Seagrass Canopies:
A Concept in Development
tween 20 and 76% over 1 m length when the plants
were occupying the entire water depth (Fonseca and
Cahalan, 1992), whereas field studies measured values between 1.6 and 80% (Koch, 1996; Prager and
Halley, 1999). Our general understanding is that
wave attenuation is highest when seagrasses occupy
a large portion (>50%) of the water column (Ward
et al., 1984; Fonseca and Cahalan, 1992; Fig. 7),
but reduction in wave energy (15 s waves) has also
been observed in a 5 m deep Amphibolis antarctica
bed (Verduin and Backhaus, 2000) and reduction of
orbital velocities (3–4 s waves) at a 15 m deep Posidonia oceanica bed (Granata et al., 2001) where the
plants occupied only a small portion of the water
column.
C. Monamis
“Seagrasses are able to modify current flow and sediment composition, yet little information exists describing their effect on waves.” This statement with
which Fonseca and Cahalan (1992) started their paper more than 10 years ago is still true today. Many
papers begin by describing the importance of seagrasses, including their capacity to attenuate waves,
but the studies which led to this generalization are
few. A flume study measured wave attenuations be-
Canopy flow is complex because it is a function of
the drag or resistance exerted by the vegetation on
the fluid, which is likely to vary spatially due to
stem spacing and vertically due to the vegetation
profile (Okubo et al., 2002). One important consequence of this realization is the propagation of wavelike oscillations or monamis (mo = aquatic plant;
nami = wave; Ackerman and Okubo, 1993) caused
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E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
by Kelvin-Helmholtz instabilities that generate large
coherent vortices at the interface between the canopy
and the overlaying water column, where the velocity
profiles exhibit an inflection point. These vortices
can penetrate into the canopy (Ghisalberti and Nepf,
2002) and interact with the flexible and buoyant
plants in a hydroelastic response due to the buoyancy
created by gas-filled lacunae and propagate downstream over the canopy causing the shoots to wave
in a coherent manner (Ghisalberti and Nepf, 2002).
This synchronous motion of seagrasses results in enhanced vertical transport between the water column
and the canopy (Ghisalberti and Nepf, 2002). The
ecological consequence of monamis is the potential
to increase larval recruitment (Grizzle et al., 1996)
and nutrient uptake.
contrast, when seagrasses are exposed to monamis in
unidirectional flows, or wave-dominated conditions,
the back and forth motion of the leaves (Fig. 8C, D,
and E) enhances vertical exchange (Koch and Gust,
1999; Ghisalberti and Nepf, 2002). This suggests
that nutrient uptake should be highest at the upper
portion of the canopy (Nepf and Vivoni, 2000), but
this is also where the oldest and least biologically
active portions of the seagrass leaves are located.
Perhaps the vertical mixing zone at the top of the
canopy is more important in the recruitment process
bringing larvae and spores into the canopy (Grizzle
et al., 1996) than in the flux of nutrient and carbon
molecules.
D. Water Flow and Nutrient Uptake
in Seagrass Canopies
The ecological role of seagrass canopies in the ecology of benthic and pelagic organisms is becoming increasingly clear, especially in the case of crustaceans
and fish (e.g. Kenyon et al., 1999; Thayer et al., 1999;
Etherington and Eggleston, 2000; Nagelkerken et al.,
2001). Many of these organisms are ecologically and
economically important species that settle in or on
the leaves and shoots of seagrasses as epiphytes for a
portion of their life history (Eckman, 1987; Borowitzka and Lethbridge, 1989; Newell et al., 1991;
Grizzle et al., 1996). Larval settlement appears to
be a function of larval supply (i.e. flux), the fluid
dynamic interaction with boundaries on which settlement occurs, and larval behavior (e.g. Abelson
and Denny, 1997; Okubo et al., 2002). As indicated
above, flows through vegetated areas are quite complex, and consequently, a mechanistic understanding
of faunal recruitment in seagrass canopies is lacking
at present, although recent efforts have been directed
to these ends (e.g. Palmer et al., 2004). Fortunately,
there are a number of processes and taxonomic systems including sediment dynamics in which the role
of canopy flow has been examined.
A recent estimate from the field indicates that the
potential for particle contact with a leaf surface approaches certainty under particular flow conditions
in a Zostera marina canopy (Ackerman, 2002; see
Section IV.D). The situation is more complex in
terms of larval settlement. The general pattern that
emerges is that settlement is higher in vegetated areas
in the case of bivalve larvae (e.g. scallops) settling
on Z. marina leaves (Eckman, 1987) and filamentous benthic algae (Harvey et al., 1995). For example, blue mussel recruitment on Z. marina leaves can
Nutrient uptake at the canopy level is a function of
water velocity. Increasing velocities lead to higher
uptake of ammonium in Thalassia testudinum and
Halodule wrightii (Thomas et al., 2000) and their
epiphytes (Cornelisen and Thomas, 2002). At the
same time, as velocity increases, the leaves bend, decreasing the obstruction/friction of the canopy. As a
result, the efficiency of the canopy to remove ammonium from the water column decreases at high
velocities (Thomas et al., 2000), unless leaf flapping
due to monamis or the orbital motion of waves cause
mixing within the canopy (Wallace and Cox, 1997;
Koch and Gust, 1999; Ghisalberti and Nepf, 2002).
The depth that the upper, highly turbulent flow at
the top of the canopy (with relatively high nutrient
levels) penetrates into the canopy is a function of
the percent of the water column that is occupied by
the canopy and the density of the shoots (Nepf and
Vivoni, 2000). Dense canopies occupying most of
the water column have narrow zones of high turbulence “skimming flow” at the top of their canopies.
Therefore, the flux of nutrients to areas deep within
the canopy may be limited.
Mixing and vertical exchange between the water
column and the seagrass canopy are a function of the
prevailing hydrodynamic conditions and also of how
seagrasses respond to them by bending, flexing, waving, etc. When leaves bend under strong currents, the
canopy height decreases collapsing the leaves onto
each other, “closing” the canopy (Fig. 8A and B) and
limiting vertical exchange (Koch and Gust, 1999). In
E. Faunal Recruitment in Seagrass Canopies
Chapter 8 Fluid Dynamics in Seagrass Ecology
209
Fig. 8. Bending cycles seagrass canopies undergo when exposed to unidirectional (A and B) and oscillatory (C, D and E) flows. During
a flood tide, leaves bend in the direction of the flow (A) and hours later, during the ebb tide, leaves bend in the opposite direction (B).
These are considered “closed” canopies where conditions within the canopy are relatively stagnant. In contrast, in wave-dominated
habitats, seagrass leaves bend in one direction (C), become somewhat upright (D) and bend in the opposite direction (E) in a matter of
seconds (wave period). This process leads to rapid “opening” (vertical leaves) and “closing” (bent leaves) of the canopy and enhances
the exchange between the canopy and the water column above it (Koch and Gust, 1999). The intermediate phase (D) is non-existent in
habitats characterized by swell—leaves just sway back and forth, never becoming upright or fully extended (see Fig. 3A).
be considerable (i.e. >90 postlarvae per cm of leaf;
Newell et al., 1991; reviewed in Ackerman et al.,
1994), and it has been suggested that the monamis
facilitate high settlement rates through increasing
the likelihood of larval encounter with undulating
leaves and the mixing that occurs under monami conditions (Grizzle et al., 1996). Clearly, plant–animal
interactions mediated by the interaction of seagrass
canopies and fluid dynamics is an important subject
where future inquiry is warranted.
F. Direct and Indirect Effects of Tides
on Seagrass Canopies
Seagrasses exist in areas that are affected by tidal
flow, which can lead to desiccation of leaves, and
limit the depth distribution and light availability of
habitats. Tides can also intensify or relieve the effect of waves on seagrass canopies. As indicated in
Section II, most of a wave’s energy (orbital velocity)
is located near the water surface. Therefore, assuming an equal wave climate, seagrass canopies are
exposed to more wave energy during low tides than
at high tides (e.g. Ochieng and Erftemeijer, 1999;
Koch, 2001; Krause-Jensen et al., 2003; Middelboe
et al., 2003). The degree of tidal exposure and the
capacity of different seagrass species to tolerate desiccation and high light levels (including ultraviolet-B
radiation) affect their minimum depth of distribution
(Stapel et al., 1997; Koch, 2001). Additionally, tides
also affect the light availability in seagrass habitats. During periods of high or low tide at noon,
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E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
seagrasses are exposed to extreme light levels in the
middle of the day and intermediate light levels in
the morning and afternoon (ebb or flood), i.e. these
plants will be exposed to long hours of saturating
light. In contrast, when high tides occur in the morning and afternoon (semi-diurnal tides), the light levels in the middle of the day may be saturating (ebb
or flood), but the number of hours of saturating light
will be reduced due to the high water during the remainder of the day (Koch and Beer, 1996). These
scenarios are also complicated by turbidity (Koch
and Beer, 1996).
G. Self-Shading in Seagrass Canopies
Exposed to Tides, Currents and Waves
Although seagrass leaves contain gas-filled lacunae
and tend to become erect in the water column, the
leaves will comply with the flow generated by tidal
currents and waves. During low tide, when seagrass
leaves in the intertidal area rest on top of each other,
self-shading is at its maximum. This process is reversed as the tide returns to its full level. Under
unidirectional flows, the degree of bending in the
direction of the flow is a function of the magnitude of the current (Fonseca et al., 1982). It follows
that self-shading is expected to be highest when currents are strongest and the leaves are collapsed onto
each other. Leaf flapping also occurs under wavedominated conditions (Koch and Gust, 1999), relieving some of the self-shading as flecks of light
penetrate the leaf mass or canopy at the frequency
of flapping.
Short-term (seconds) flecks of light referred to
as “lightflecks” (Fig. 9), are also generated in areas
exposed to waves. The light that reaches the water
surface is focused at the crest of the waves and dispersed at the trough of the waves (Wing and Patterson, 1993; Fig. 9). This results in “dancing lights” in
shallow areas such as seagrass beds due to the propagation of lightflecks. The frequency of lightflecks
resembles that of the passing waves (Wing and Patterson, 1993). Although the effect of lightflecks on
seagrass productivity was never tested, productivity
is likely to be enhanced as has been demonstrated in
phytoplankton and macroalgae (Dromgoole, 1988;
Greene and Gerard, 1990; Wing and Patterson, 1993;
Wing et al., 1993).
H. Seagrass Canopies as Depositional
Environments: Not a Universal Concept
Historical evidence of seagrass beds as depositional
environments is borne out of the loss of seagrasses
(due to wasting disease or grazing) resulting in
the erosion of sediments (Rasmussen, 1977; Hine
et al., 1987). Many authors suggested that seagrass
canopies are areas where sediments deposit and accumulate (Grady, 1981; Almasi et al., 1987; Patterson and Black, 1999; Gacia and Duarte, 2001),
largely due to the reduction of velocity and turbulence intensity (e.g. Fonseca et al., 1983; Ackerman and Okubo, 1993; Worcester, 1995; Verduin
and Backhaus, 2000; Granata et al., 2001), i.e. a
reduction in stress on the sediment surface due to
reduced flow speed within the canopy that leads to
a reduction in resuspension and thus, an increase in
accumulation (Lopez and Garcia, 1998). This accumulation can be seasonal, especially during summer when seagrasses are at their maximum density,
but in winter, when the plants disappear or decrease
in density, resuspension may be greater than deposition (van Keulen and Borowitzka, 2003). Often,
the presence of seagrass rhizomes and roots in the
sediment through the winter is sufficient to stabilize the sediments. The size of the seagrass species
also affects sediment accumulation. Although bigger species may be better for sediment deposition
(Fonseca and Fisher, 1986), small seagrasses such
as Halophila decipiens and Zostera novazelandica
can still alter the sediments they colonize (Fonseca,
1985, 1989; Heiss et al., 2000).
More recent efforts have focused on the role of
seagrass canopies in trapping and retaining sediments, which appears to be related to sediment contact with leaves (Gacia et al., 1999; Agawin and
Duarte, 2002). Epiphytic layers on seagrass leaves
may contribute to the entrapment of particles in seagrass beds by increasing the roughness of the canopy
and increasing the boundary layer on the leaf surface; i.e. expanding the area in which water flow is
reduced thereby facilitating the entrapment of particles (Vermaat et al., 2000). Whereas, seagrasses are
generally viewed as agents that trap particles and
stabilize the sediments, resuspension, especially of
fine sediments with high organic content, can occur
under high wave exposure and current flow (Fonseca
et al., 1983; Fonseca and Bell, 1998). In highly waveexposed sites where seagrasses do not attenuate
Chapter 8 Fluid Dynamics in Seagrass Ecology
211
Fig. 9. Lightflecks form in areas where the surface of the water is not flat, instead, it has waves (A). The light that reaches the water
surface (vertical arrows above wave) is focused by wave crests and is dissipated by wave troughs. In the area where the light is focused,
lightflecks form. Under the right combination of water depth and wavelength (i.e. water depth in the focusing range), lightflecks can be
observed on the seafloor as seen in a shallow Thalassia testudinum bed in the Florida Keys, USA (B). Photo: E.W. Koch.
212
E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
Fig. 10. Patch of Zostera marina at Crown Breach, Alameda, California, USA, containing coarser sediment than the surrounding area.
This may be a result of turbulence generated by the seagrass leaves (Koch, 1993). As a result, finer particles are resuspended while
coarser particles remain. Note that this patch was located at a water depth of 0.9 m and is a single genet (i.e. a true clonal unit) as it
originated from one single seed that germinated in February 2003. Photo taken in August 2003 by Mark Fonseca.
water flow as effectively as in unidirectional (tidal)
flows (Koch and Gust, 1999), sediment characteristics within and outside seagrass beds differed little to
none (Hoskin, 1983; Edgar and Shaw, 1991; Koch,
1999b; van Keulen and Borowitzka, 2003). Actually,
in some cases, sediment in a vegetated area can be
coarser than in the adjacent unvegetated area (Koch,
1993; Fig. 10).
VI. Hydrodynamically-Mediated Processes
at the Landscape Level (100–1000 m)
A number of studies have analyzed how seagrass
canopies alter local hydrodynamic conditions and,
with that, affect their own productivity, associated
biota, sediments, and the water column surrounding them. In contrast, hydrodynamic studies in sea-
grass habitats at the landscape level are less common. While most studies at the canopy level assume
that the shoot density is homogenous (Vidono et al.,
1997), at the landscape level, it is becoming clear that
seagrasses are spatially heterogeneous (Robbins and
Bell, 1994). In this section, we address flow-related
causes of seagrass heterogeneity and hydrodynamic
consequences of seagrass patchiness.
A. Seagrass Landscapes and the Substrates
they Colonize
Although seagrass landscapes represent a simpler
system than terrestrial landscapes in terms of species
diversity and structure (Robbins and Bell, 1994), a
mosaic of different seagrass species, shoot characteristics, associated biota, and sediment elevations and
types exist. Responses of individual plants to water
Chapter 8 Fluid Dynamics in Seagrass Ecology
213
Fig. 11. Diagram of seagrass distribution in habitats characterized by high wave energy. In shallows areas where waves are felt on the
bottom (1), sediment movement is constant (arrows) not allowing seagrasses to become established. In this area (1), some seagrasses
are capable of colonizing non-shifting substrates such as rocks. In contrast, in deeper areas sheltered from the waves (2, i.e. below
the maximum wave penetration depth represented by the dashed line), sediment movement is reduced allowing seagrasses to become
established. The maximum depth of distribution of the seagrasses is limited by light availability (3).
motion accumulate and may be the basis of seagrass landscape patterns perceived at coarser scales
of resolution (Fonseca, 1996). Hydrodynamic forces
may also affect seagrass habitat requirements such
as light availability, sediment characteristics, and
substrate stability (Ben Alaya, 1972; Cooper, 1982;
van Katwijk and Hermus, 2000; Fig. 10), thereby
altering the pattern of distribution even further
(feedback).
In shallow areas with relatively high wave energy, the substrate is usually characterized by coarse
shifting sand (Dan et al., 1998) and/or rocks. The
shifting sediments remain unvegetated due to continuous erosion and burial of recruits (Shepherd
and Robertson, 1989; Hemminga and Duarte, 2000;
Frederiksen et al., 2004). Some seagrasses are able
to colonize rocks (a stable substrate) in these shallow, high wave-energy areas by modifying their root
system. The genus Phyllospadix, which colonizes
the rocky intertidal in the north Pacific (WyllieEcheverria and Ackerman, 2003), is an example
of this. However, in Corsica (Mediterranean), Posidonia oceanica is found on rocks in shallow areas where sand grains are moving back and forth
every few seconds due to the passage of waves
(Fig. 11) (Koch, personal observation), yet it colonizes soft substrates at depths below the maximum
wave penetration depth. Other seagrasses capable
of colonizing hard as well as soft substrates are
Thalassodendron ciliatum (Bandeira and Nilsson,
2001) in Mozambique (Bandeira, 2002) and Amphibolis antarctica in Australia (Ducker et al., 1977).
What makes one species more adaptable to different
substrates than others is presently unknown. Landscape level studies for seagrasses on rocky substrates are limited; therefore, the remaining discussion will address seagrass meadows colonizing soft
substrates.
B. Hydrodynamically Generated Patchiness
in Seagrass Meadows
The mosaic of patterns observed in seagrass landscapes is often a result of natural perturbations such
as erosion and burial by sand waves (Harlin and
Thorne-Miller, 1982; Fonseca et al., 1983; Marbà
et al., 1994; Marbà and Duarte, 1995; Fonseca and
Bell, 1998; Bell et al., 1999; Fig. 12) and/or disturbances caused by fauna (Orth, 1975; Ogden, 1980;
Preen, 1995), storms (Preen et al., 1995; Fonseca and
Bell, 1998) and/or disease (den Hartog, 1987). Anthropogenic causes (eutrophication, boat and mooring scars, fishing gear scars) can also contribute to
seagrass patchiness (Cambridge, 1975; Walker et al.,
1989; Creed and Filho, 1999; Orth et al., 2002). Here
we will focus on flow-related generation of patchiness.
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E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
Fig. 12. Complex seagrass landscape due to the presence of sand waves (arrow) in a Zostera marina bed at Horn Harbor, Chesapeake
Bay, USA. Photo: R.J. Orth.
The disturbances that lead to patchiness in seagrass landscapes range in scale from the complete
but local destruction of seagrass ecosystems (Dan
et al., 1998) to the creation of smaller (10’s of meters) unvegetated depressions in continuous meadows, termed “blowouts” (Patriquin, 1975; see review
in Short and Wyllie-Echeverria, 1996; Fonseca and
Bell, 1998). The magnitude and frequency of disturbances is what determines the degree of patchiness of a seagrass meadow (Fonseca et al., 1983;
Fonseca and Bell, 1998; Hemminga and Duarte,
2000). It can therefore be assumed that more extreme events would result in erosion and complete
or partial loss of seagrasses (with little recovery),
whereas periods of reduced disturbance may result
in coalescence of the patches and the formation of
more continuous meadows (Fonseca et al., 1983;
Kirkman and Kirkman, 2000). This concurs with numerous reports of widespread loss of meadows as a
result of hurricanes and cyclones (Birch and Birch,
1984; Williams, 1988; Rodriguez et al., 1994; Preen
et al., 1995; Moncreiff et al., 1999; Whitfield et al.,
2002), and the extensive colonization of seagrasses
(pioneering or climax species) during periods of reduced disturbance (Kirkman, 1985; Kendrick et al.,
2000). Note that some studies report no damage to
seagrasses after the passage of hurricanes (Thomas
et al., 1961; Tilmant et al., 1994; Dawes et al., 1995).
In areas with continuous high wave energy, seagrass ecosystems can be: (i) non-existent (Dan et al.,
Chapter 8 Fluid Dynamics in Seagrass Ecology
1998); (ii) depth restricted (when sufficient light is
available, seagrasses colonize areas below the maximum wave penetration depth; Krause-Jensen et al.,
2003; Middelboe et al., 2003); (iii) dominated by
more robust species (e.g. Amphibolis griffithii and
Posidonia coriacea); and (iv) patchier as the disturbance of high waves may hinder the lateral expansion of some seagrass beds (Kendrick et al.,
2000; Frederiksen et al., 2004). In contrast, in sheltered waters, seagrass meadows tend to be more
continuous and are colonized by relatively more
fragile species (e.g. some Posidonia spp) (Kirkman
and Kuo, 1990). Under calm conditions, creation of
openings in meadows appears not to lead to further
wide-scale loss, although regrowth into the damaged
areas can be slow (Walker et al., 1989; Meehan and
West, 2000; Orth et al., 2002). The degree of wave
exposure can be quantified by applying the relative
wave exposure index (REI) first developed by Keddy.
This index takes into account the wind direction and
intensity and fetch and has been successfully linked
to landscape features in seagrass habitats (Fonseca
and Bell, 1998; Fonseca et al., 2000, 2002; Hovel
et al., 2002; Krause-Jensen et al., 2003; Frederiksen
et al., 2004). When wave-dissipating structures (e.g.
sand bars, sills, coral or oyster reefs) occur in the
seagrass system, the bathymetry may also have to be
taken into account in order to properly estimate the
wave exposure using the REI.
A mixture of unvegetated and densely vegetated
areas in close proximity may characterize intermediate disturbance regimes. For example, Posidonia sinuosa meadows in habitats of moderate water flow are
characterized by dense rows of plants interchanged
with strips of bare sand (Bridgwood, 2002). A similar
gradient of patterns has been reported for meadows
of Zostera marina subject to gradients in velocity
and wave energy (Fig. 12), with the proposal that
the structural integrity of the habitat would deteriorate with increasing habitat fragmentation (den Hartog, 1971; Fonseca et al., 1983; Fonseca and Bell,
1998).
Another common cause for complex seagrass
mosaic patterns is the hydrodynamically-mediated
movement of sand waves and sand dunes through
seagrass meadows (Marbà et al., 1994; Walker et al.,
1996; Bridgwood, 2002; Paling et al., 2003; van
Keulen and Borowitzka, 2003; Frederiksen et al.,
2004). Changes in sediment height, a result of water
flow, can be significant and rapid (10’s of cm over periods of hours) (Paling et al., 2003), and larger sand
215
dunes can travel through meadows on a time scale
of months (Walker et al., 1996; Bridgwood, 2002;
van Keulen and Borowitzka, 2003). The degree to
which these large amounts of sediment negatively
affect the seagrasses creating unvegetated patches
depends on their tolerance for sedimentation, the
amount of sediment deposited, and the period the
plants remain buried. Sedimentation rates of 2 to
13 cm year–1 can be coped with by large (e.g. Enhalus acoroides) as well as by fast growing (e.g.
Halophila) seagrasses as well as by plants with vertical stem elongation (e.g. Cymodocea nodosa, C.
serrulata and Syringodium isoetifolium) (Vermaat
et al., 1996). Subaqueous dune migration appears
to maintain Cymodocea meadows in a continuous
state of colonization which is ultimately responsible for the characteristic patchy landscape (Marbà
and Duarte, 1995). Even when sediments completely
cover the leaves of small, slow-growing plants, some
seagrasses are able to survive as long as the sediment is removed by currents or waves in a matter of
weeks (Halodule wrightii survived after being buried
for 2 months; Phillips, 1980). Other seagrasses such
as Posidonia oceanica are able to increase vertical
growth from 5 to 7 mm year–1 to 52 mm year–1 when
(partially) covered by sand waves (Boudouresque
and de Grissac, 1983). In contrast, Zostera marina
seems to have little or no tolerance to sedimentation
regardless of the sediment type (Mills and Fonseca,
2003).
C. Effect of Patchiness on Flow
and Flow-Related Processes
When considering processes in seagrass meadows,
spatial homogeneity is usually assumed, but in nature, patchy seagrass landscapes are more common
than homogenous ones. This patchiness may have
a major effect on physical, geological, chemical,
and biological processes. For example, gaps in the
canopy allow an accelerated influx of water into
the canopy (Granata et al., 2001), replenishing nutrients, introducing spores and propagules, eroding
sediment and increasing mixing in general. The distance currents can penetrate the edge of a seagrass
bed before becoming equilibrated with respect to
momentum has been estimated to be between 1 m
(Fonseca and Fisher, 1986) and 50 boundary layers
(assumed to be equal to the height of the canopy;
Nowell and Jumars, 1984; Granata et al., 2001).
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E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
These values can be affected by the density of the
bed and the vertical distribution of biomass. In dense
beds, unidirectional water flow is smoothly directed
over the top of the seagrass canopy as “skimming
flow” (Nowell and Jumars, 1984; Fonseca and Kenworthy, 1987), effectively trapping a layer of water within the canopy (Koch and Gust, 1999), i.e.
increasing the residence time. Under such conditions, nutrient concentrations within the vegetation
may be quite low (Moore et al., 1996). In contrast, a
reduction in shoot density leads to increased flow intrusion and velocity within the canopy (van Keulen,
1997). Therefore, reducing seagrass density could
permit an increase in turbulence and mixing within
the meadow, with an associated increase in nutrient
exchange and uptake, and the potential for increased
sediment resuspension. This subject needs further
attention.
VII. Hydrodynamic Processes at the
Meso-Scale Level (>1,000 m)
A. Seagrasses in the Biosphere
Seagrass meadows are one of several plant communities found in coastal areas around the world.
Marshes and mangroves line the intertidal area of
shorelines of seagrass-colonized temperate and tropical systems, respectively. The hydrodynamic functions of each of these communities can be quite
similar: attenuation of currents and waves leading
to deposition of particles and the stabilization of
the substrate (Knutson et al., 1982; Knutson, 1988;
Massel et al., 1999; Möller et al., 1999). When considering these communities as a part of a larger
coastal ecosystem, interactions between seagrasses
and marshes/mangroves as well as with adjacent animal communities such as oyster and coral reefs start
to emerge (see Section VII.B).
Water masses may have traveled over extensive
distances, interacting with pelagic and benthic organisms before they reach seagrass beds. For example, during the flood tide, water masses may travel
over sediments colonized by a variety of microalgae and benthic organisms such as a coral polyps
and reefs before reaching the seagrass bed. In contrast, during ebb flow, the water that reaches seagrass
beds may have resided in an estuary, a mangrove, or a
marsh system for a period of time. Each of these plant
and animal communities tends to alter the water mass
in its chemical and/or physical properties directly or
indirectly (Bulthuis et al., 1984). When these waters are then transported into seagrasses meadows,
they also affect these plant communities. Therefore,
a link between seagrasses and adjacent plant and animal communities is expected.
B. Linking Seagrasses and Adjacent
Communities via Water Flow
1. Linking Seagrasses and Adjacent Systems
via Tidal Fluxes
Tidal flows link terrestrial, estuarine, and marine
systems. The residence time of a water mass in
a seagrass habitat (determined by the tidal fluxes)
may have a profound effect on seagrass distribution.
Short residence times (days) allow pollutants and
excess nutrients to be flushed out of a system before harming seagrasses (Kitheka et al., 1996). In
contrast, long residence times promote the accumulation of nutrients and the growth of phytoplankton
and nuisance algae while suppressing the growth of
seagrasses via low light availability (Rysgaard et al.,
1996; Herbert, 1999).
Coral reefs, seagrass beds, and mangrove forests
often co-occur in tropical coastal systems suggesting an interaction of sorts, determined by tidal flows.
For example, coastal wetlands such as marshes and
mangroves assimilate nutrients leaching from land
and thereby reduce the nutrient level reaching adjacent seagrass systems (Valiela and Cole, 2002) during ebb flows. These authors suggested that landderived N loads from 20 to 1,000 kg N ha−1 year–1
seem to be a critical range for seagrass survival
in shallow waters. When N loads are higher, wetlands are no longer able to remove sufficient N
through denitrification, and N burial, and tidal currents will carry the excess nutrients into the seagrass beds. Excess nutrients can then lead to the
loss of the seagrasses. Therefore, adjoining plant
systems (wetlands and seagrasses) are not isolated
units but are likely to be linked (Valiela and Cole,
2002).
Seagrass beds are among the most productive systems on the planet (Dring, 1994) and experience relatively low grazing losses with most leaf production being shed (Cebrian and Duarte, 2001; Mateo
et al., Chapter 7). This amounts to a considerable
Chapter 8 Fluid Dynamics in Seagrass Ecology
quantity of detrital material which can remain within
the seagrass meadow or can be exported. The fate of
seagrass detritus depends, to a large extent, on the
magnitude of currents, waves, and tides (Ochieng
and Erftemeijer, 1999) and the nature of the leaves:
some leaves float on becoming detached while others sink (Zieman et al., 1979). Floating leaves are
more likely to be exported by tidal currents, but
leaves that sink and form detritus locally may also
be exported onto adjacent beaches or the deep sea
during storm events and/or spring tides (Hemminga
and Nieuwenhuize, 1990; Kirkman and Kendrick,
1997; Ochieng and Erftemeijer, 1999). It appears
that in many instances detritus remains within the
originating ecosystem, being recycled more or less
in situ (Hemminga and Nieuwenhuize, 1991; Paling, 1991). In other cases, large amounts of seagrass detritus are transported into adjacent estuaries contributing to the estuarine carbon cycle (Bach
et al., 1986; Cebrian and Duarte, 2001; Mateo et al.,
Chapter 7). Seagrass fragments have even been
found at great depths in ocean basins (>1,000 m)
where, it is postulated, they comprise an important
food source for a number of invertebrate detritivores (Menzies et al., 1967; Menzies and Rowe,
1969; Wolff, 1976, 1979; Suchanek et al., 1985),
as well as pelagic fishes and crustaceans (Williams
et al., 1987). Litter washed up onto beaches also supports a wide range of invertebrates (Kirkman and
Kendrick, 1997; Ochieng and Erftemeijer, 1999),
and the location where the litter is deposited (high
tide line or storm line) determines where invertebrates will find the highest availability of food. Litter that remains in shallow waters provides protection from erosion (Ochieng and Erftemeijer, 1999)
and a habitat for juvenile fish (Lenanton et al., 1982;
Robertson and Lennaton, 1984), but the reliability
of this habitat depends on the local hydrodynamic
conditions.
Tidal flows do not only link adjacent communities but can also isolate them. For example, in an
estuary in Kenya, organic particles efflux and reflux between mangroves and seagrasses during each
tidal cycle (Hemminga et al., 1994). During the ebb,
POM effluxes from the mangroves reaching the seagrasses; during the flood, particles resuspended in
the seagrass meadow (in part, particles generated
in the mangroves and deposited in the seagrasses)
reach the mangroves (Hemminga et al., 1994). These
particles never make it to the adjacent coral reef
217
due to trapping of the high turbidity plume by the
tide and onshore winds (Fig. 13; Kitheka, 1996;
Kitheka et al., 1996; Miyajima et al., 1998). Turbid waters could be detrimental to the reef-forming
coral polyps (Johannes, 1975). As a result of the
wave attenuation by the coral reef and the tidal isolation of the corals, mangroves, seagrasses and the
coral reef can co-exist just a few kilometers apart
along the Kenyan coast. This process is likely to also
apply to other reef–seagrass–mangrove associations
throughout the world.
2. Linking Seagrasses and Adjacent
Ecosystems via Wave Attenuation
Several plant and animal communities (mangroves,
marshes, corals, and oyster reefs) adjacent to seagrass meadows tend to attenuate waves, protecting
shorelines from erosion (Knutson et al., 1982; Knutson, 1988; Massel et al., 1999; Möller et al., 1999).
The decrease in waves by mangroves and marshes
leads to sediment deposition (Othman, 1994) and,
consequently, reduced water turbidity. A mangrove
forest as narrow as 50–150 m can completely attenuate wave heights of up to 1 m (Othman, 1994), while
a marsh can attenuate more than 80% of the incoming wave energy (Möller et al., 1999). Theoretically,
seagrasses could benefit from this wave reduction
especially during ebb flows as the water mass that
resides in the mangroves or marshes will have lower
turbidity. In turn, sub-tidal seagrass meadows adjacent to mangroves and marshes can minimize the
impact of waves on marshes and mangroves via wave
attenuation (van Katwijk, 2000).
Seagrasses also require relatively sheltered conditions in order to become established and thrive
(Fonseca and Bell, 1998; Robbins and Bell, 2000;
Koch, 2001), conditions that may have been previously found in the shelter of the once extensive
oyster reefs in Chesapeake Bay (USA) and of sand
bars in Tampa Bay (USA) (Lewis, 2002). Whereas
some wave attenuation is expected to be beneficial
for seagrass establishment, excessive wave attenuation may also be detrimental to seagrasses. When
wave energy is extremely low, sediments tend to be
relatively fine and to have elevated organic content
(Wanless, 1981; Almasi et al., 1987; Fonseca, 1996;
Fonseca and Bell, 1998), conditions which are not
always favorable to seagrasses (Koch, 2001).
218
E. W. Koch, J. D. Ackerman, J. Verduin and M. van Keulen
Fig. 13. Hydrodynamic link (or lack thereof) between a coral reef, seagrass bed and mangrove forest in Gazi Bay (Kenya) via the flux
of oceanic water, seagrass-affected water and mangrove-affected water during a neap high tide (a), a spring low tide (b), and a neap low
tide (c). E and Q f refer to evaporation and freshwater input, respectively. Note that when the tide is rising (flood) during a neap high tide
(a), the oceanic water (OW) moves over the coral reef into the seagrass habitat pushing the seagrass-affected water (SGW—open circles)
toward the mangrove habitat and the mangrove-affected water (MW—dots) is restricted to the mangrove forest. In contrast, during a
spring low tide (b), the oceanic water is restricted to the area offshore from the coral reef, the seagrass-affected water extends to the
coral reef and the mangrove affected water covers the seagrass and mangrove habitats (but does not reach the coral reef). If only a neap
low tide (c) occurs, the oceanic water penetrates farther into the lagoon than during the spring low tide and the seagrass-affected water
remains in the seagrass habitat, not reaching the coral reef. Under these conditions, the mangrove-affected water extends somewhat into
the seagrass habitat but not to the same extent as during the spring low tide. Source: Kitheka (1997). [Reproduced with permission from
Academic Press/Elsevier].
VIII. Summary and Outlook
Fluid dynamics is an essential component of seagrass ecology as it affects every aspect of the plants
and their habitats, from the smallest to the largest
scales. Over the last decades, we have begun to understand how seagrass beds attenuate waves and currents. Now we begin a new phase of fine tuning previous findings and revising classical concepts. This
is leading to exciting new developments such as (i)
how currents interact with seagrass canopies as if
the plants were part of a mixing layer instead of
only a perturbation in the benthic boundary layer;
(ii) the relative role of deposition and resuspension in seagrass beds, which may be quite dynamic;
and (iii) how the aforementioned processes are affected by unsteady conditions. It is also becoming
clear that not all seagrasses have the same biomechanical properties, and major differences exist in
the extent to which different seagrasses influence,
Chapter 8 Fluid Dynamics in Seagrass Ecology
and are influenced by, water flow. Vegetated flows
are amongst the most complicated and least understood types of water flow which makes this subject
a great gateway for exciting new developments and
advancements in science.
Acknowledgments
Dr. Heidi Nepf, Dr. Mark Fonseca, and an anonymous reviewer provided valuable comments on an
earlier draft of the manuscript. Evamaria Koch would
like to thank the Cooperative Institute for Coastal
and Estuarine Environmental Technology (CICEET)
for financial support to collect the data presented in
Fig. 7. Joe Ackerman would like to acknowledge the
support of funds from NSERC and the Canada Research Chair program. Tracey Saxby is thanked for
making the figures for this chapter.
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