|
| |
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online May 21, 2007
Journal of Experimental Biology 210, 1874-1884 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02775
Interspecific comparison of hydrodynamic performance and structural properties among intertidal macroalgae
Department of Biological Sciences, University of Rhode Island, Kingston, RI 02881, USA
* Author for correspondence at present address: Hopkins Marine Station, Stanford University, Pacific Grove, CA 93950, USA (e-mail: boller{at}stanford.edu)
Accepted 9 March 2007
 |
Summary |
|---|
 TOP Summary IntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Key words: biomechanics, beam, modulus, ecology, seaweed
|
Introduction |
|---|
|
TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
; Koehl, 1986;
Koehl, 1996;
Holbrook et al., 1991;
Gaylord, 2000;
Denny and Gaylord, 2002). The
drag imposed on intertidal macroalgae can cause individuals to become
dislodged (Dayton, 1973;
Paine, 1979;
Denny et al., 1985;
Carrington, 1990;
Dudgeon and Johnson, 1992) or
become damaged (Blanchette,
1997). For many species, flexibility results in a reduction in
drag via reconfiguration, the process by which a macroalga bends in
response to drag, changing its shape and size relative to the flow
(Vogel, 1984;
Vogel, 1989;
Vogel, 1994;
Koehl, 1986). Thus, traits
that improve hydrodynamic performance (e.g. increased reconfiguration and drag
reduction) may increase survivorship in the wave-swept rocky shore.
Reconfiguration has been characterized using Vogel's E, a measure
of a flexible organism's deviation from the expected relationship between
force and velocity seen in rigid organisms
(Vogel, 1994). Comparisons of
Vogel's Es have been made among morphologies and species of
macroalgae (Carrington, 1990;
Dudgeon and Johnson, 1992;
Gaylord et al., 1994;
Pratt and Johnson, 2002;
Sand-Jensen, 2003;
Harder et al., 2004;
Boller and Carrington, 2006a).
However, this method of characterizing reconfiguration does not address the
mechanisms of reconfiguration: changes in size and shape of the organism
presented to flow. Boller and Carrington describe a mechanistic approach that
separately examines the changes in size and drag coefficient
(CD, the measure of the influence of shape on drag) of
reconfiguring macroalgae (Boller and
Carrington, 2006a). For clarity, we will refer to the effects of
changing frontal area as `size' effects and the effects of changing drag
coefficient as `shape' effects, despite the fact that changing size inherently
requires changing shape. With this approach, direct measurement of frontal
area (the size of the alga interacting with the flow) and calculation of
CD across a range of water velocities allows new measures
of hydrodynamic performance to be defined. These measures characterize the
absolute magnitude of reconfiguration and the water velocity required for full
reconfiguration. Boller and Carrington applied this approach to one species
(Chondrus crispus) (Boller and
Carrington, 2006a); one goal of this study is to apply the
approach to species spanning a broad range of morphologies.
A second goal of this study is to evaluate the influence of material and
structural properties on the hydrodynamic performance of macroalgae.
Reconfiguration is a process in which drag may cause a macroalga to bend into
a smaller and/or reduced CD shape, potentially reducing
its risk of dislodgment. Mechanical adaptations that increase the
reconfiguration of macroalgae should be beneficial and may be reflected in a
correlation between the solid-mechanical characteristics and hydrodynamic
performance. Previous studies have examined this relationship in a theoretical
and experimental approach in two-dimensional flow
(Alben et al., 2002;
Alben et al., 2004), have
characterized algal solid mechanics but assumed hydrodynamic performance based
on environmental exposure (Harder et al.,
2006), or used model macroalgae to vary solid mechanical
properties (Stewart, 2006).
Here, we use an empirical analysis combining the direct quantification of
reconfiguration (Boller and Carrington,
2006a) with beam theory to examine how morphology and tissue
stiffness influence reconfiguration.
In beam theory (Gere and Timoshenko,
1984; Denny,
1988), the deflection of a beam of a given morphology will
increase when the stiffness is decreased. However, beam theory indicates that
not just stiffness, but also the cross-sectional shape and length of the beam
influence bending. Thus, longer beams and less rigid cross-sectional shapes
would also be expected to increase bending. This theory has been previously
applied to large kelps for which overall flexibility was estimated as
`structural flexibility', an index of the flexibility of a beam that is
directly proportional to the length and inversely proportional to stiffness
and cross-sectional size of the beam
(Denny and Gaylord, 2002).
|
In addition to influencing the solid mechanics of macroalgae, morphology
has been demonstrated to influence hydrodynamic characteristics (e.g.
Koehl and Alberte, 1988;
Dudgeon and Johnson, 1992;
Sand-Jensen, 2003) (see also
Carrington, 1990). For example,
ruffled kelp blades have higher CD than smooth, strap-like
blades (Koehl and Alberte,
1988). With respect to reconfiguration, we hypothesize that more
complex morphologies (where blades, ruffled blades and branched macroalgae
would have increasing complexity) would have a greater capacity for
reconfiguration. That is, a branched macroalga with many places to bend can
potentially spread out at low velocity (have large area and/or
CD) and then reconfigure to a very compact and/or low
CD morphology at high velocity. Simple morphologies with
fewer branch points may have less ability to spread out and thus have less
potential to compact and/or change CD.
In this study, the effects of morphology and structural properties on hydrodynamic performance were explored by quantifying reconfiguration, material properties and morphology for a range of intertidal macroalgae. Hydrodynamic performance was examined for 10 intertidal macroalgal species with a variety of morphologies by direct measurement of reconfiguration over a wide range of water velocities. The morphology and material stiffness of the macroalgae were quantified, and the relationships among reconfiguration, morphology and material stiffness were examined. Further, the application of these hydrodynamic/solid mechanical data to the identification of functional-form groups was explored.
|
Materials and methods |
|---|
|
TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
).
Samples were maintained in the laboratory in 
17°C seawater for up to
three days. In preparation for hydrodynamic testing, individuals were carefully separated from the rock in the laboratory with a razor blade, and the bottom of the holdfast was shaved to a flat surface. Samples of Codium were too large to fit in the experimental apparatus; analyses were conducted on distal branch groups up to 20 cm in height. Algae were attached to the force platform either by the flat bottom of the holdfast or, for Codium, by the proximal end of the cut branch using cyanoacrylate glue.
Hydrodynamic performance
Reconfiguration of each sample was examined using the recirculating
seawater flume as per Boller and Carrington
(Boller and Carrington, 2006a),
with several modifications. Size and drag were quantified at 0.16 m
s–1 steps up to 3 m s–1. Extremely low
velocities (<0.16 m s–1) were not examined and are
excluded from further references to `low' velocities. A high-resolution
digital camera (C770 Ultra Zoom; Olympus Optical Co., Ltd, Tokyo, Japan) was
used to acquire images of the alga projected into the flow. The camera was
interfaced to the computer with Camera Controller software (version 1.7.8;
Pine Tree Computing, LLC., Olivette, MO, USA). Frontal area (±0.01 cm;
AF) was measured from the digital photographs using ImageJ
software (version 1.33; National Institutes of Health, Bethesda, MD, USA).
AF was normalized by Arep, the
representative area at low reconfiguration, i.e. the size of the alga when
realigned such that the stipe is parallel to the flow at U
0.16
m s–1.
Drag (FD) at each velocity was measured using a custom
force platform capable of measuring three-dimensional force
(Boller and Carrington, 2006b).
The platform consisted of a plastic post affixed to a three-axis ceramic force
transducer (Series109 3D TrackStick; CTS Corp., Elkhart, IN, USA) and three
AD623 instrumentation amplifiers (Analog Devices Inc., Norwood, MA, USA)
assembled on a four-layer printed circuit board designed using ExpressPCB
software
(www.ExpressPCB.com).
The amplified analog force was converted to a digital signal (DAS16-AO;
Measurement Computing Corp., Middleboro, MA, USA) and recorded by a PC at a
rate of 100 Hz for 10 s using Softwire software (version 3.1; Measurement
Computing Corp.). The force platform was calibrated for each axis by hanging
weights from a string glued to the surface of the post when the platform was
held on its side. During hydrodynamic tests, the vector average of the three
axes was calculated at each sample interval, and FD at
each specific velocity was calculated as the average magnitude of the 1000
vectors (±0.01 N).
Drag coefficient (CD) was calculated at each velocity
as:
 | (1) |
is the density of the fluid (1025 kg m–3),
U is the velocity of the fluid relative to the organism in m
s–1, and AF is the frontal area at
U.
Hydrodynamic performance data were pooled within each species, and the
normalized area as a function of velocity (aU) was
described as an exponential decay function:
 | (2) |
is the minimum normalized area of the alga
(a measure of maximum reconfiguration), aR is a
coefficient describing the magnitude of area reduced because of
reconfiguration, and ßa is the reconfiguration coefficient of
area, a term that describes the steepness of the decay function. The drag
coefficient as a function of water velocity (CU) was also
described as an exponential decay process:
 | (3) |
is the minimum CD
(CD at maximum reconfiguration), CR is
a coefficient describing the magnitude of the reduction of
CD because of reconfiguration, and ßC is
the reconfiguration coefficient for CD. Parameters for
both functions were estimated for each species using TableCurve2D software
(version 4.07; Systat Software Inc., Point Richmond, CA, USA). Large sample
normal approximate confidence intervals, an approximation of 99% confidence
intervals, were calculated by multiplying the standard error reported by
TableCurve2D by ±2.5758 [the value of the 99th percentile of a normal
frequency distribution of samples
(Underwood, 1997)].
Significant differences among species and pairwise comparisons were defined by
the lack of overlap of confidence intervals.
Morphology and solid mechanics
The morphology of each species was quantified by measuring five features.
Each sample was first sandwiched between two acrylic sheets and photographed
from above with the digital camera. Planform area (±0.01
cm2) was measured from the photograph with ImageJ. Branch lengths
(±0.01 mm) were measured in ImageJ from the tip of the alga to the
lowest branching point above the stipe (for blade-like species, a branch is
equivalent to a blade). Branch width and thickness were measured either with
digital calipers (±0.01 mm) or with ImageJ using an image of the
cross-section of a branch. Cross-sectional area was then calculated assuming
an ellipse for rounded cross-sections, a rectangle for very flat
cross-sections, or directly measured with ImageJ for complexly shaped
cross-sections. For hollow species (Scytosiphon and
Agardhiella), the thickness of one layer of the tissue was used as
the measure of branch diameter, and the thickness multiplied by the
circumference was used as the measure of cross-sectional area of the
branch.
Standard materials testing was performed as per Carrington et al.
(Carrington et al., 2001) on
samples from each alga using a computer-interfaced tensometer (model 5565;
Instron Corp., Canton, MA, USA) equipped with a noncontacting video
extensometer (model 2663; Instron Corp.). For most species, 3 cm-long samples
were dissected from undamaged and non-reproductive branches in the central
portion of the alga. For blade-like species, rectangular samples (1x3
cm, WxL) were cut from the blade using two single-edge razors affixed to
a 1 cm-wide aluminum block. Stipe and apical tissue was not characterized
mechanically. The ends of each sample were held by a pair of pneumatic grips
lined with fine sandpaper at a pressure of 4 kg cm–2. Two
silver-paint dots, applied approximately 5 mm apart to the center of the
sample, defined the length of the test region. The samples were periodically
wetted with seawater to prevent desiccation. Force (±0.001 N) was
measured with a 50-N load cell at an extension rate of 50 mm
min–1 while the extensometer simultaneously measured
extension (±0.005 mm) as the distance between the two dots. Whereas
material properties may be strain-rate-dependent, algal tissues have been
shown to have relatively constant properties across large ranges of strain
rate (Gaylord et al., 2001),
justifying the use of a single strain rate. Stress (
) was calculated as
the force divided by the initial cross-sectional area of the sample. Strain
(
) was calculated from the extension using the formula
(l–l0)/l0, where
l0 was the initial length of the test region and l was the
length measured by the extensometer. The modulus (E, not to be
confused with Vogel's E), or material stiffness, was calculated as
the slope of the steepest linear portion of the stress–strain curve. An
index of the structural flexibility was calculated for each individual
macroalga as L3/ED4, where L
was the average branch length and D was the average minimum dimension
of the branch cross-sections or the average thickness of the tissue for hollow
species (Denny and Gaylord,
2002).
For statistical analysis, structural flexibility was normalized by log
transformation to remove skew (Underwood,
1997); E was untransformed. One-way analyses of variance
(ANOVAs) were performed on each structural property using Systat software
(version 11.0; Systat Software Inc.). In addition, Tukey's pairwise
comparisons were performed to examine relationships among species.
Correlation of hydrodynamic performance with solid mechanics
The relationships between hydrodynamic performance and solid mechanical
properties were examined among species by calculating four hydrodynamic
performance measures derived from aU and
CD functions for each species
(Boller and Carrington, 2006a).
To compare the rate of reconfiguration with increasing water velocity among
species, the critical velocity of area reconfiguration
(Ucrit,a) was defined as the velocity at which
aU came within 5% of the minimum value
(a). The critical velocity of
CD reconfiguration (Ucrit,C) was
defined as the velocity at which CD came within 5% of the
minimum value (C). In addition, two indexes were
calculated to compare reconfiguration at low (0.5 m s–1) and
high (3.0 m s–1) water velocities. `Compaction' was defined
as the proportion of area reconfiguration achieved at a given velocity and was
calculated as 1–aU, where aU was
the solution of Eqn 2 at either
0.5 or 3 m s–1. Drag coefficients were also calculated for
each species at each velocity by solving Eqn
3. The dependence of these four hydrodynamic performance measures
on each of two solid mechanical properties (log-transformed structural
flexibility and modulus) was evaluated using linear regression analysis
(Systat version 11.0).
|
Results |
|---|
|
TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
, aR
and ßa indicated significant differences among species for all
parameters (Table 1).
Ucrit,a ranged from 0.92 to 4.21 m s–1
among species (Table 1). Among
the curves of Fig. 3A, three
general patterns can be identified: two species (Mastocarpus and
Chondrus) exhibited more resistance to reconfiguration with
increasing water velocity (i.e. required higher velocity to reconfigure,
Ucrit,a) and moderate levels of total reconfiguration
(i.e. a). F. distichus, F. vesiculosus, Codium,
Agardhiella and Scytosiphon had relatively less resistance to
reconfiguration, but varying levels of total reconfiguration. Three species
(Laminaria, Petalonia and Grateloupia) displayed the largest
resistance to reconfiguration and high levels of total reconfiguration (low
a).
|
|
|
Drag coefficients were more variable than normalized frontal area within a species (Fig. 2) but were still described well by Eqn 3 (all P<0.01, R2=0.59 to 0.83; Table 2, Fig. 3B). Lack of overlap of confidence intervals indicated significant differences among species. Ucrit,C varied from 0.88 to 1.90 m s–1 among species (Table 2), but less variation in CD reconfiguration was evident, such that groups were not identifiable as in normalized area (Fig. 3B).
|
Morphology and solid mechanics
Planform areas, ranging from 8.03 to 172.62 cm2, were large and
variable compared with representative areas, which ranged from 3.99 to 23.94
cm2 (Table 3).
Average height (range=5.97–23.51 cm), branch length
(range=3.66–18.61 cm) and branch diameters (range=0.06–4.55 mm),
the later two used to calculate structural flexibility, are reported in
Table 3.
|
Significant differences were evident among structural properties (Table 4). E ranged from 0.77 to 25.77 MN m–2 and varied significantly among species (Table 5). Structural flexibility ranged from 1.18x107 to 6.25x1012 m MN–1 and varied significantly (Table 5).
|
|
Correlation of hydrodynamic performance to solid mechanics
Hydrodynamic performance correlated significantly with solid mechanical
properties in some, but not all, instances. No significant relationships were
observed between the critical velocities (Ucrit,a and
Ucrit,C) and solid mechanical properties (all regressions
P>0.05, Fig. 4).
Compaction and drag coefficient correlated significantly with solid mechanical
properties at some velocities (Figs
5,
6). At low velocity (0.5 m
s–1), compaction decreased with modulus (slope=–0.005,
R2=0.39, P=0.05;
Fig. 5B) but had no
relationship with structural flexibility (P>0.05;
Fig. 5A). At high velocity (3 m
s–1), compaction increased with structural flexibility
(slope=0.029, R2=0.73, P<0.01;
Fig. 5C) but was not correlated
with stiffness (Fig. 5D;
P>0.05). For drag coefficient, significant correlations were
evident for both structural flexibility (slope=–0.093,
R2=0.47, P=0.03;
Fig. 6A) and stiffness
(slope=0.026, R2=0.50, P=0.02;
Fig. 6B) at low water velocity
(0.5 m s–1). At high water velocity, no significant
correlations between CD and structural properties were
evident (P>0.05; Fig.
6C,D).
|
|
|
|
Discussion |
|---|
|
TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
),
despite the large variation in material and morphological properties among
species. Variation in reconfiguration among species may have ecological
importance at moderate velocities. For example, individuals exhibiting less
reconfiguration will be exposed to more force but may have competitive
advantage for space and light competition and less self-shading. At high
velocities, however, the results suggest that macroalgae act as rigid objects,
with drag increasing with the square of velocity. At these velocities,
variation in reconfiguration will no longer matter. Quantification of drag
coefficients for rigid and flexible intertidal organisms at high velocities is
needed to fully understand the hydrodynamic forces of the wave-swept rocky
intertidal zone.
It should be noted that this measure of the capacity of area
reconfiguration (a) is an underestimate of the
total ability of a species to reconfigure because the extremely low velocity
changes that occur as the macroalga realigns are not included. That is, at
still water, the projected area would be the side of the upright macroalga
(approximately the planform area). After realignment, the projected area is
effectively the top of the macroalga (the representative area).
a characterizes the degree of reconfiguration
starting when the macroalga is the size of the representative area. Further,
the variation in planform areas was much larger than in representative areas,
indicating that extremely low velocity realignment narrowed the effective
range of algal sizes in this study.
The flexibility and reconfiguration of macroalgae has been said to decouple
shape from hydrodynamic force generation
(Denny and Gaylord, 2002).
Here, where the effects of macroalga shape and size are separated, the
mollifying effect of flexibility is also evident. Comparison of low (more
variable) and high (less variable) velocity CD suggests
that drag coefficients converge at high velocities as reconfiguration conforms
algae to a compacted shape (Fig.
3B). Further, all of the C reported
here are similar to those of bluff bodies (i.e. generation, the different
morphologies are hydrodynamically similar, as was found in a previous study
(Carrington, 1990). This
moderating effect of reconfiguration on the hydrodynamics of flexible
macroalgae has probably enabled the evolution of the wide variety of
morphologies seen even in the harsh wave-swept rocky intertidal zone.
In as much as these data are representative of the species in general, comparisons can be made regarding the relative contributions of changes in size and CD among species. Many species have higher Ucrit,a than Ucrit,C, indicating that size continues to change after CD has become constant (Fig. 7). Thus, for these species, reconfiguration at intermediate velocities is dominated by changes in projected area, not changes in CD, suggesting that changes in the overall size of the macroalgae presented to the flow is more important than the shape. However, some species (Codium, F. distichus, F. vesiculosus and Scytosiphon) have similar, low Ucrit,C and Ucrit,a, suggesting that they experience size and shape effects across similar ranges in velocity. Further, these similar Ucrit values are probably because of lower Ucrit,a values, suggesting these species reach their maximum potential reconfiguration across smaller changes in velocity.
|
The dynamic nature of reconfiguration is evident in these results. The
relative magnitudes of parameters among some species change as water velocity
increases, suggesting conclusions drawn from low-velocity studies may not be
appropriate at high velocities (Carrington,
1990; Vogel, 1994;
Bell, 1999;
Denny and Gaylord, 2002;
Boller and Carrington, 2006a).
For area reconfiguration, F. distichus and F. vesiculosus
have lower normalized area than Chondrus and Mastocarpus at
0.75 m s–1 (Fig.
3A), but this relationship is overturned at 3 m
s–1. However, the reconfiguration drag model describes these
velocity-dependent differences where frontal area and CD
approach asymptotes. Only Mastocarpus and Grateloupia were
predicted to continue to reduce in size beyond the range of this study,
suggesting that for the majority of species extrapolation beyond 3 m
s–1 is possible. A high-velocity test of the model is needed
to justify this extrapolation.
Structural properties
Although material stiffness varied among species, all species in the study
have low stiffness compared with other, non-algal biological materials
(Wainwright et al., 1976;
Denny, 1988). The moduli
reported in this study (0.77 to 25.77 MN m–2,
Table 5) are similar to the
range reported elsewhere for macroalgae (1 to 100 MN m–2)
(Dudgeon and Johnson, 1992;
Carrington et al., 2001;
Hale, 2001;
Denny and Gaylord, 2002). Of
note is the exceptionally low modulus of Codium (0.77 MN
m–2) that is probably because of its siphonous growth form
(Graham and Wilcox, 2000).
Codium has multiple siphons that are highly interwoven to form the
thallus. This design probably allows for movement of siphons relative to each
other and thus a lower modulus. Other species in the study (Chondrus,
Mastocarpus and Agardhiella) are pseudoparenchymatous [filaments
that coalesce to form macroscopic thalli
(Graham and Wilcox, 2000)],
and this more integrated construction may yield higher moduli.
Among the species, lower modulus (stiffness) does not always confer greater structural flexibility. Codium has the lowest flexibility because of its extremely large branch diameter, despite its order of magnitude lower modulus and long branches. Other tree-like species have moderately thick and short branches and generally higher stiffness, which also results in low flexibility. High flexibility is seen among blade- and whip-like species, because of their long branch length and small branch diameters.
Do structural properties influence hydrodynamic performance?
Stewart used flexural stiffness
(Stewart, 2006), the product
of the modulus and the second moment of area (a measure of cross-sectional
shape relative to the applied force), to examine the relationship between
hydrodynamic performance and solid mechanics for morphologically identical
model seaweeds. In that study, stiffness significantly influenced drag
generation, but only for the most flexible models that had an order of
magnitude lower modulus. Conversely, Dudgeon and Johnson observed greater
reconfiguration in a stiff species (Mastocarpus) than in a more flexible
species (Chondrus) (Dudgeon and Johnson,
1992). Harder et al. observed a correlation between tissue
stiffness and exposure (Harder et al.,
2006), suggesting that lower stiffness bestows greater
reconfiguration and survival.
In this study, we hypothesized that higher structural flexibility would result in higher rates of reconfiguration and subsequently require lower velocities to fully reconfigure (lower Ucrit,a and Ucrit,C). However, this pattern was not observed (Fig. 4). Structural flexibility was positively correlated with compaction at high velocity and negatively related to CD at low velocity. This pattern may be because of a correlation between structural flexibility and morphology, where blade-like species had higher compaction than more complex species, as discussed below.
More complex morphologies were hypothesized to have greater capacity for
reconfiguration and thus have lower a. Results
suggest the opposite is true: blade-like species had the lowest
a among the species studies. This pattern may be
because of the cross-sectional size and number of an alga's branches when bent
and pushed together. That is, the absolute minimum size a blade-like macroalga
can achieve through reconfiguration may be less than the minimum size of a
dichotomously branching macroalga simply because the blade has less tissue and
can compact tightly because of its simple shape.
The application of beam theory requires some simplifying assumptions that
may be inappropriate for intertidal macroalgae. For example, branches are
assumed to bend as cantilevers. However, the torsion of branches may be
important for the compaction of blades. Thus, variation in torsion modulus
among macroalgal species (Harder et al.,
2006) may influence reconfiguration. Further, tissue stiffness in
compression and tension may differ and influence bending of branches
(Koehl and Wainwright, 1977;
Gaylord and Denny, 1997). A
more complex structural model of macroalgal bending may clarify the
relationship between hydrodynamic performance and structural properties.
Alternatively, an empirical test of whole thallus structural mechanics may
provide the data to correlate solid mechanical and hydrodynamic
performances.
Functional-form groups
The characterization of hydrodynamic performance of the various algal
morphologies found in the rocky intertidal zone may be useful for defining
functional-form groups based on resistance to hydrodynamic disturbance
(Padilla and Allen, 2000).
This functional-form approach to addressing wave disturbance may allow for
better understanding of community and landscape ecology of the rocky shore.
Larger sample sizes incorporating the range of morphological variation within
a species are needed to fully test this application of the data, but some
trends can be pointed out from our results
(Table 6).
|
Blade-like species (Laminaria, Petalonia and Grateloupia)
are more flexible and achieved the highest degree of size reconfiguration (low
a) and generally low C.
However, blades required high velocities to accomplish reconfiguration (high
Ucrit,a). Together, these qualities suggest that
blade-like species have great capacity to reduce the effects of drag, as found
in a previous study (Carrington,
1990). Tree-like species (Chondrus, F. distichus, F.
vesiculosus, Codium and Mastocarpus) are generally stiffer but
variable in the rate of reconfiguration; they are generally less
reconfigurable in size (high a) and have relatively
high drag coefficient (high C), suggesting that
this morphology is least effective at reducing the stress of intertidal flows.
The variation in hydrodynamic performance observed among these species
probably reflects the morphological variation in this loosely termed group.
The performances of whip-like species (Agardhiella and
Scytosiphon) are intermediate to blade- and tree-like species.
Conclusions
The reconfiguration drag model successfully characterizes the hydrodynamic
performance of morphologically distinct intertidal macroalgae. Hydrodynamic,
structural and morphological properties all vary significantly among species.
Structural properties are correlated with some aspects of hydrodynamic
performance, but those correlations between performance and structural
properties are velocity dependent. Different mechanisms (size versus
shape change) are responsible for reconfiguration at different velocities.
Functional-form groups based on performance and structural properties are
discernible, suggesting that these physical measurements of the organisms'
interactions with the environment may be useful for understanding the ecology
of these biomechanically complex organisms.
|
|
Acknowledgments |
|---|
|
Footnotes |
|---|
Present address: Department of Biology, University of Washington, Friday
Harbor Laboratories, Friday Harbor, WA 98250, USA 
|
References |
|---|
|
TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Alben, S., Shelley, M. and Zhang, J. (2002). Drag reduction through self-similar bending of a flexible body. Nature 420,479 -481.[CrossRef][Medline]
Alben, S., Shelley, M. and Zhang, J. (2004). How flexibility induces streamlining in a two-dimensional flow. Phys. Fluids 16,1694 -1713.[CrossRef]
Bell, E. C. (1999). Applying flow tank measurements to the surf zone: predicting dislodgment of the Gigartinaceae. Phycol. Res. 47,159 -166.[CrossRef]
Blanchette, C. A. (1997). Size and survival of intertidal plants in response to wave action: a case study with Fucus gardneri. Ecology 78,1563 -1578.[CrossRef]
Boller, M. L. and Carrington, E. (2006a). The
hydrodynamic effects of shape and size change during reconfiguration of a
flexible macroalga. J. Exp. Biol.
209,1894
-1903.
Boller, M. L. and Carrington, E. (2006b). In situ measurements of hydrodynamic forces imposed on Chondrus crispus Stackhouse. J. Exp. Mar. Biol. Ecol. 337,159 -170.[CrossRef]
Carrington, E. (1990). Drag and dislodgment of an intertidal macroalga – consequences of morphological variation in Mastocarpus papillatus Kutzing. J. Exp. Mar. Biol. Ecol. 139,185 -200.[CrossRef]
Carrington, E., Grace, S. P. and Chopin, T. (2001). Life history phases and the biomechanical properties of the red alga Chondrus crispus (Rhodophyta). J. Phycol. 37,699 -704.[CrossRef]
Dayton, P. K. (1973). Dispersion, dispersal, and persistence of annual intertidal alga, Postelsia palmaeformis Ruprecht. Ecology 54,433 -438.[CrossRef]
Denny, M. W. (1988). Biology and Mechanics of the Wave-Swept Environment. Princeton: Princeton University Press.
Denny, M. and Gaylord, B. (2002). The mechanics
of wave-swept algae. J. Exp. Biol.
205,1355
-1362.
Denny, M. W., Daniel, T. L. and Koehl, M. A. R. (1985). Mechanical limits to size in wave-swept organisms. Ecol. Monogr. 55,69 -102.[CrossRef]
Dudgeon, S. R. and Johnson, A. S. (1992). Thick vs thin – thallus morphology and tissue mechanics influence differential drag and dislodgment of two codominant seaweeds. J. Exp. Mar. Biol. Ecol. 165,23 -43.[CrossRef]
Garbary, D. J. and DeWreede, R. E. (1988). Life history phases in natural populations of Gigartinaceae (Rhodophyta): quantification using resorcinol. In Experimental Phycology (ed. C. S. Lobban, D. J. Chapmin and B. P. Kramer), pp.174 -178. Cambridge: Cambridge University Press.
Gaylord, B. (2000). Biological implications of surf-zone flow complexity. Limnol. Oceanogr. 45,174 -188.
Gaylord, B. and Denny, M. W. (1997). Flow and flexibility. I. Effects of size, shape and stiffness in determining wave forces on the stipitate kelps Eisenia arborea and Pterygophora californica. J. Exp. Biol. 200,3141 -3164.[Abstract]
Gaylord, B., Blanchette, C. A. and Denny, M. W. (1994). Mechanical consequences of size in wave-swept algae. Ecol. Monogr. 64,287 -313.[CrossRef]
Gaylord, B., Hale, B. B. and Denny, M. W. (2001). Consequences of transient fluid forces for compliant benthic organisms. J. Exp. Biol. 204,1347 -1360.[Abstract]
Gere, J. M. and Timoshenko, S. P. (1984). Mechanics of Materials. Monterey, CA: Brooks/Cole Engineering Division.
Graham, L. E. and Wilcox, L. W. (2000). Algae. Upper Saddle River, NJ: Prentice Hall.
Hale, B. B. (2001). Material properties of marine algae and their role in the survival of plants in flow. PhD thesis, Stanford University, USA.
Harder, D. L., Speck, O., Hurd, C. L. and Speck, T. (2004). Reconfiguration as a prerequisite for survival in highly unstable flow-dominated habitats. J. Plant Growth Regul. 23,98 -107.
Harder, D. L., Hurd, C. L. and Speck, T.
(2006). Comparison of mechanical properties of four large,
wave-exposed seaweeds. Am. J. Bot.
93,1426
-1432.
Hoerner, S. F. (1965). Fluid-dynamic Drag. Brick Town, NJ: Published by author.
Holbrook, N. M., Denny, M. W. and Koehl, M. A. R. (1991). Intertidal trees – consequences of aggregation on the mechanical and photosynthetic properties of sea-palms Postelsia palmaeformis Ruprecht. J. Exp. Mar. Biol. Ecol. 146, 39-67.[CrossRef]
Koehl, M. A. R. (1984). How do benthic organisms withstand moving water. Am. Zool. 24, 57-70.
Koehl, M. A. R. (1986). Mechanical design of spicule-reinforced connective tissues. Am. Zool. 26, A38.
Koehl, M. A. R. (1996). When does morphology matter? Annu. Rev. Ecol. Syst. 27,501 -542.[CrossRef]
Koehl, M. A. R. and Alberte, R. S. (1988). Flow, flapping, and photosynthesis of Nereocystis luetkeana – a functional comparison of undulate and flat blade morphologies. Mar. Biol. 99,435 -444.[CrossRef]
Koehl, M. A. R. and Wainwright, S. A. (1977). Mechanical adaptations of a giant kelp. Limnol. Oceanogr. 22,1067 -1071.
Padilla, D. K. and Allen, B. J. (2000). Paradigm lost: reconsidering functional form and group hypotheses in marine ecology. J. Exp. Mar. Biol. Ecol. 250,207 -221.[CrossRef][Medline]
Paine, R. T. (1979). Disaster, catastrophe, and
local persistence of the sea palm Postelsia palmaeformis.Science 205,685
-687.
Pratt, M. C. and Johnson, A. S. (2002). Strength, drag, and dislodgment of two competing intertidal algae from two wave exposures and four seasons. J. Exp. Mar. Biol. Ecol. 272,71 -101.[CrossRef]
Sand-Jensen, K. (2003). Drag and reconfiguration of freshwater macrophytes. Freshw. Biol. 48,271 -283.[CrossRef]
Stewart, H. L. (2006). Hydrodynamic
consequences of flexural stiffness and buoyancy for seaweeds: a study using
physical models. J. Exp. Biol.
209,2170
-2181.
Underwood, A. J. (1997). Experiments in Ecology. Cambridge: Cambridge University Press.
Vogel, S. (1984). Drag and flexibility in sessile organisms. Am. Zool. 24, 37-44.
Vogel, S. (1989). Drag and reconfiguration of
broad leaves in high winds. J. Exp. Bot.
40,941
-948.
Vogel, S. (1994). Life in Moving Fluids. Princeton: Princeton University Press.
Wainwright, S. A., Biggs, W. D., Currey, J. D. and Gosline, J. M. (1976). Mechanical Design in Organisms. London: Edward Arnold.
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati 
Twitter What's this?
This article has been cited by other articles:
|
|
 |
|
  M. A. R. Koehl, W. K. Silk, H. Liang, and L. Mahadevan How kelp produce blade shapes suited to different flow regimes: A new wrinkle Integr. Comp. Biol., December 1, 2008; 48(6): 834 - 851. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
,
), Agardhiella
(
) and Mastocarpus
(
). Form is denoted by the
color of the line, where black=blade-like, red=whip-like and
blue=tree-like.
),
Scytosiphon (
),
Chondrus (
),
Codium (
), F.
distichus (
), F.
vesiculosus (
) and
Mastocarpus (
). Form
is denoted by the color of the line, where black=blade-like, red=whip-like and
blue=tree-like.
), Agardhiella
(
), Scytosiphon
(
), Codium
(
), Chondrus
(
), F. distichus
(
), F. vesiculosus
(
) and Mastocarpus
(
). Form is denoted by the
color of the line, where black=blade-like, red=whip-like and
blue=tree-like.
