In the first part of the article series I introduced the
mechanisms by which water flow affects the respiration
and photosynthesis of corals. To recapitulate, I described how
rates of photosynthesis and respiration are driven by diffusion
rates which are dependent on concentration gradients. Since
concentration gradients are greatly affected by water motion, the
respiration and photosynthesis rates of corals is strongly
correlated with the intensity of water movement. This second
article in the series will review some of the published
scientific research describing how water flow affects coral
health and in the next article I will discuss my own research
demonstrating how flow speed and lighting intensity go hand in
The fields of flow related coral research are
inter-disciplinary and they rarely fall into a discrete category.
To simplify the discussion I will break up the discussion of the
effect of water flow based on
arbitrary categories which I have
loosely arranged based on a temporal scale. The short term
effects of water flow which are nearly immediate can be observed
or measured within minutes or hours. These short term effects
include rates of photosynthesis, respiration, calcification,
polyp extension, colony expansion, particle capture and pulsing.
The intermediate effects of water flow are usually observable
within days or weeks and these include growth, calcification,
regeneration, temperature and light stress (such as bleaching),
and colony orientation. On the longest timeline of weeks, months
and years the effects of water flow are mostly discussed in terms
of morphology, reproduction, dispersal, recruitment and ecology.
Any one of these topics could fill volumes but I will focus my
discussion on the topics which are most relevant to aquarium
applications. The topics of focus will include photosynthesis and
respiration, particle capture, bleaching, growth rate and the
interaction between flow and morphology. There are innumerable
publications available for these topics so I will select the
studies which most clearly illustrate the importance of water
flow to coral health from an aquarist’s point of view.
Short Term Effects
Photosynthesis and Respiration
The most fundamental of biological processes are
photosynthesis and respiration. The response of photosynthesis
and respiration to environmental factors such as flow is easily
monitored by measuring oxygen or carbon dioxide consumption or
production. Additionally, the photosynthetic efficiency can be
evaluated by measuring electron transport rate within the
In a beautifully simple experiment, researchers William
Dennison and David Barnes maintained samples of Acropora
formosa under stirred and unstirred lab conditions
(1). The experiment showed that corals maintained
under stirred conditions show significantly higher rates of
photosynthesis, respiration and calcification than corals
maintained in unstirred conditions. Corals maintained under
unstirred conditions photosynthesized and respired about 25% less
than corals in stirred conditions. The calcification rates of
corals in unstirred conditions were also lower than corals in
stirred conditions but this reduction was not statistically
significant. The researchers concluded that the coral’s
metabolic response to water motion would influence rates of coral
growth and the development of reefs overall.
In 2003, Sebens et al. published a paper on the effects of
flow on the physiology of Agaricia tenuifolia
in Belize (2). Interestingly, the study showed that
although the respiration seemed to increase proportionately with
increased water flow, water flow speed had little effect on the
maximum photosynthesis rate of A. tenuifolia. It appears
that this species is able to carry out maximal photosynthesis
even under greatly reduced flow which is in contrast to other
coral species which have increased photosynthesis when they are
exposed to higher water flow speeds.
One of the most thorough and detailed papers I have ever read
about flow effects on coral physiology is “In Situ
measurements of flow effects on primary production and dark
respiration in reef corals” by Patterson et al
(3). The researchers used sealed recirculating
chambers deployed from the underwater research habitat Aquarius
in Key Largo FL. They measured respiration and photosynthesis
rates of Montastrea annularis under flow speeds of
2-16cm/s. Like Denison and Barnes the study also found that there
was an increase of primary production and respiration with
increased flow rates but more importantly, the discussion gives
detailed explanations of the interactions between hydrodynamic
forces and diffusional properties of gases to explain their
results. The discussion goes far beyond the mechanisms I
introduced in my previous article so if you are interested in
learning more about how flow affects gas exchange in corals I
would strongly recommend reading this paper.
When it seems that the majority of coral physiology research
is performed on stony corals, the Dendronepthea research
projects of Dr. Katharina Fabricius are available to help balance
things out. In a classic study of Dendronepthea feeding
behavior (4), Dr. Fabricius demonstrated the flow
dependent particle capture of this beautiful coral group. When
exposed to flow speeds from 0-35cm/s, over 200 samples from 35
colonies of D. hemprichi showed a significantly higher
particle capture at the intermediate flow speed of about 15cm/s.
At the lowest flow speeds, particle capture suffers from
unfavorable hydrodynamics and a lack of particle
“traffic” and at the highest flow speed, the polyps
cannot remain open under the force of the water flow. Although
the 1997 Sebens et al. (5) paper also deals with the
effects of water flow on particle capture, it focuses on how the
morphology of Madracis mirabilis interacts with water
flow at different speeds to affect particle capture. Like the
Fabricius paper mentioned above, the experiment determined that
the highest rate of particle capture for M. mirabilis
occurred at flow speeds between 10-15cm/s. However, whereas
Dendronepthea feeding was largely shut down above flow
speeds of 30cm/s, at flow speeds of 40-50cm/s M.
mirabilis still showed about one fourth the overall capture
efficiency demonstrated at the more favorable 10-15cm/s. The
reason for this discrepancy is that whereas
Dendronepthea polyps were largely collapsed under the
force of the water flow, the solid branches of Madracis
protected downstream polyps which remained open to capture food
particles in the gentler turbulent eddies. Additionally, the
Sebens et al paper showed that aggregations of Madracis
are suited to capture particles under specific flow speeds
depending on the density of the branches.
Medium Term Effect
Coral bleaching occurs when reef-building corals expel their
life-giving symbiotic algae which makes them appear stark white.
Although coral bleaching is mostly attributed to unusually high
water temperatures, other factors such as turbidity and water
motion seem to be decisive factors in making bleaching events
more or less severe.
One of the most important coral conservation efforts is the
science of determining where coral are most resistant to
environmental stresses which cause coral bleaching and where they
will be most likely to recover from those stresses. In a review
of the published accounts of coral bleaching events, the paper by
West and Salm (6) describes the relationship between
coral bleaching and several environmental factors including water
flow. Although large-scale weather patterns generate conditions
which lead to mass bleaching, on a local scale the bleaching
response is not uniform. One of the recurring details West and
Salm discovered in their review is that corals which incurred
little to no bleaching or rapid recovery almost always occurred
in areas which were exposed to permanent or seasonal upwelling or
which experienced rapid flow from being in channels or strong
currents. The review concluded that conservation areas which were
conducive to coral resistance and resilience to bleaching stress
would likely be areas which experienced stronger and more
consistent water flow.
Photoinhibition is a precursor to coral bleaching which occurs
when corals receive more light than they can handle. When
photoinhibition occurs, corals begin to stress and their
photosynthesis rate decreases. In corroboration with the
observations of West and Salm, the recent work by Nakamura et al.
(7) has shown that there is a reduction of
photoinhibition by water flow. By exposing samples of
Acropora difitifera to varying water flow speeds and two
different light regimes, Nakamura and his colleagues determined
not only that photoinhibition is reduced at higher flow speeds
but they also showed that at high light intensities and low flow
speeds, photodamage of the coral symbionts is amplified which in
turn makes corals more sensitive to light and temperature
extremes. In a follow up to this experiment, the same group of
researchers looked at the recovery rate of bleached
Stylophora pistillata under similar experimental
conditions (8). Since coral bleaching induces a loss
of zooxanthellae, the researchers exposed the sample corals to
low (3cm/s) and moderate (20cm/s) water flow speeds and measured
zooxanthellae density within the corals over a 7 week period.
While the corals from the moderate flow treatment showed a rapid
increase in zooxanthellae density throughout the study, the low
flow treatment corals barely gained any zooxanthellae density at
all. Basically, the two aforementioned studies by Nakamura et al.
support the position of West and Salm that water flow is
important for making corals resistant to bleaching stressors and
that adequate water motion is also important for helping corals
recover from bleaching stress.
Although strong water flow is beneficial for helping corals
resist bleaching, living in a really stable, comfortable
environment can reduce a coral’s tolerance to occurrences
of thermal stress. In the 2005 study by McLanahan et al
(9), researchers surveyed reef zones around the island
of Mauritius which differed mostly in their abundance of water
flow. The study discovered that coral bleaching was most
prevalent on the reefs which received the most constant and
abundant water flow. Although this finding contradicts the
majority of the literature regarding the relationship between
water flow and coral bleaching, the paper explains that the most
affected corals had reduced acclimation to minor stress events.
Since the most affected corals were acclimated to a very stable
temperature and constant high water flow environment, the corals
did not have to acclimate to minor stress events so they were
less able to tolerate an anomalous high temperature event. The
ecologically dominant corals which suffered the most bleaching
due to this temperature anomaly were Acropora and
The effect of water flow on growth rate is a multi pronged
affair. Growth rate overall is a reflection of how well corals
can photosynthesize, catch food, resist bleaching, avoid disease
and predation. Growth rates are enhanced by flow due to decreased
sedimentation, increased photosynthetic and respiration rates,
greater abundance of food “traffic” and more
efficient food particle capture.
Although this transplantation experiment by Dr. Ilsa Kuffner
(10) seemed more concerned about the effects of
ultraviolet light on the photoprotective pigments of corals, one
of the factors which was varied was water motion. Dr. Kuffner
transplanted replicate branches of nine different colonies of
Porites compressa. The fragments were distributed
between an ambient (normal) water motion area and a low water
motion area. Corals transplanted to low flow showed decreased
concentrations of photopretective pigments compared to the
samples which were maintained in the normal flow environment.
Additionally, the study determined that the only significant
factor affecting calcification rate was water motion. Corals
transplanted to areas of high water motion showed significantly
higher rates of calcification than those which were part of the
low water motion treatment. The aforementioned study by Dr.
Fabricius (8) also showed than on a longer time scale
the growth rates of D. hemprichi colonies reflect the
efficiency of particle capture with growth rates being highest at
flow speeds between 10-20cm/s. The Sebens et al (2)
paper which studied Agaricia also performed measurements of
Agaricia growth rates under different flow speeds. As expected,
corals which grew in high flow environments also showed higher
growth rates than corals which grew in lower flow environments;
those which grew in sheltered concavities showed greatly reduced
Long Term Effects
On a long time scale of weeks, months and years, the effects
of water flow still include growth and calcification but the
discussion of growth and calcification shifts away from
measurements of length, area, volume and mass and we can begin to
observe and discuss morphology, asexual reproduction, competition
Flow effects on Morphology
Corals are very adaptable animals and different morphs of the
same species were often thought to be new species. Because corals
live in a range of habitats, their skeletons have to grow into
shapes and textures which will best suit the physical environment
which is primarily governed by water flow. An older study by Dr.
David Bottjer describes some interesting characteristics of
Acropora cervicornis morphologies under high and low
water motion environments (11). For this research
project, Dr. Bottjer selected a low flow back-reef area and an
adjacent high flow reef-crest area to investigate the branching
density, angle and orientation of A. cervicornis
colonies which grow in these different flow environments.
Colonies growing in the calmer back reef area exhibited
relatively upright branches with a great deal of spacing between
the branches and no significant orientation with water flow. By
contrast, the branches of colonies occurring in the high flow
reef crest area were markedly different; the branches were much
more densely spaced, they tended to be more horizontally inclined
and the branches were mostly pointing away from the oncoming
flow. The author concluded that the branch characteristics of
high water flow colonies were well suited to reducing the
transfer of momentum from flowing water to branches to reduce
breakage. However, it is unclear whether this morphology is the
result of a genetic response to water motion or the result of
strong flow pruning branches and colonies which grew into ill
suited shapes for the environment or both.
Although any aquarist or scientist will concede to the fact
that water flow has a profound effect on the morphology of corals
and other benthic organisms, very few explanations of the
mechanisms which govern this flow-dependent property have been
proposed. One exception is the 1985 paper by Rinkevich and Loya
(12) on the branching pattern of S.
pistillata. The paper cites previous work by the authors
that showed that S. pistillata grows by dichotomous fission of
the branches, that mature colonies are spherical in shape overall
and that broken branches regrow faster than unbroken branches
such that the spherical shape of the colony is mostly reformed
(13). The authors were primarily interested in the
curious fact that branches of S. pistillata
never fuse even though branches which were broken and grafted to
the same colony would rapidly fuse (14) (Note: go see
for yourself, in your tank, if any of your Stylophora or
Pocillopora branches have fused. Its likely they
haven’t whereas Montipora and Acropora
branches fused quite regularly in mature colonies). The authors
proposed that an isomone (type of hormone) is
responsible for preventing the fusion of branches of S.
pistillata. To test this hypothesis the researchers
attached upward growing branches of S.
pistillata and Millepora alcicornis in pairs
from the same colonies such that they would touch if they grew in
a straight direction. Although the Millepora samples
simply fused, the Stylophora samples grew very close to
each other but then they simultaneously began to grow away from
each other. If there is indeed a hormone which proportionately
governs branch spacing, then water motion would have a profound
impact on coral morphology; In low water flow the substance would
be in higher concentration leading to widely spaced branches and
in high flow the isomone would be less concentrated which would
cause the coral colony to grow more closely spaced branches.
Although it is quite possible that there are one or more hormones
which control branch patterns in Stylophora, other
Pocilloporids and other corals, I believe that polyp extension is
a more plausible spacing control for this group. Since polyp
extension can be largely explained by water flow as well, the
physiological-repellent effect might be a factor of polyps
touching each other when branches grow closely to one another,
thereby stimulating the branches to grow away from each
Here is one for the zoanthid lovers. Although little
consideration is usually paid to the flow requirements of
“Zoa’s” as a group, a study by Dr. M. Koehl
(15) produced a thorough investigation of the
relationship between water flow and the morphology of zoanthid
colonies. The species used were Palythoa caribaeorum and
P. variabilis. P. carribaeorum colonies are
characterized by having solid mad of interconnected polyps
whereas the polyps of P. variabilis are about twice as tall and
wide and they are only loosely connected. These two carribean
species of zoanthids are usually found in environments with
different amounts of water motion. It appears that the smaller
polyps and more rigid colonies of P.
carribaeorum are best suited to resisting drag forces
which occur in higher water flow. As water flow becomes reduced,
corals like zoanthids which grow close to the substrate start to
experience difficulties with catching food, gas exchange and
sedimentation. P. variabilis has developed
several adaptations to overcome these difficulties. First of all,
the taller polyps of P. variabilis stick out
from the substrate into more mainstream water flow so that it
experiences relatively similar flow speeds as P.
carribaeorum even though it generally occurs in calmer
environments. The longer tentacles and larger oral disc of P.
variabilis allows it to catch more food particles even
in reduced water flow. Also, the flexibility of the loosely
connected polyps is useful for dealing with the increased
sedimentation which occurs at lower flow speeds. Furthermore,
P. variabilis lives up to its name because it
has polyps which can look quite different depending on whether
they are solitary or colonial. Since solitary polyps have no
neighbours with which it can share the force of water motion,
these tend to be shorter and thickened at their bases whereas
colonial polyps are much taller and narrower at the base. On the
contrary, polyps which occur at the leading edge of colonies tend
to resemble solitary polyps since these also experience more
force from water motion than polyps at the interior of zoanthid
Morphology effects on water flow
Just as water flow affects morphology, the morphology of
corals is specially designed to alter water flow patterns to suit
particular niches. In reality, water flow and morphology effects
go hand in hand but it is clear that water flow affects
morphology first. For example, the microhabitat which occurs at
the interior of zoanthid colonies might foster increased growth
rates and this phenomenon might help to explain why under certain
conditions, small clusters of zoanthid polyps tend to multiply
much faster than individual or loosely aggregated zoanthid
colonies. But before zoanthids can create a suitable microhabitat
for themselves, the solitary founder polyp must first acclimate
to the water flow to which it is exposed.
As you might expect, a higher density aggregation of corals
with many more upstream branches to disrupt water flow is more
efficient at reducing high water flow than a lower density
aggregation of the same size. Accordingly, Sebens et al (9)
determined that denser aggregations of M.
mirabilis were better suited to capturing food particles
at high flow speeds because of their increased ability to
effectively reduce water flow. However, the lower density
aggregations with more spacing between branches were better
suited to capture food particles under lower flow conditions
because they allowed more water to pass through the colony.
Although most of us think of morphology as the three dimensional
shape of our corals (is. Massive, branching, plating), it also
includes the 3d elements which project from the two dimensional
surface to alter the texture or roughness of our corals. In 2001,
Gardella and Edmunds (16) examined how surface
roughness of two massive carribean corals affects the small scale
properties of water flow and how this relates to diffusion and
gas exchange. The experiment used differently sized skeletons of
Dichocoenia stokesii and
Stephanocoenia michilini and measured
downstream turbulence under several flow rates. They compared the
downstream turbulence to the surface roughness of their sample
skeletons and they found a significant correlation in the smaller
colonies they used. They found that the surface roughness between
these corals showed little effect on turbulence although the
taller Dichocoenia colonies had greater downstream
turbulence, especially at higher flow rates. Therefore, the
taller Dichocoenia colonies had a greater effect on turbulence
which increased the potential for high diffusion and gas exchange
I would like to reiterate that the effect of water flow on
coral health is a continuum across the arbitrary topics which I
have outlined. For example, the long term effect of high
photosynthesis and calcification rates on a coral will yield a
high growth rate and the long term effect of high growth rate
will yield more sexual and asexual reproduction and it will also
have an effect on morphology. Likewise, long term changes to
coral morphology are usually to improve the characteristics of
water motion through a coral colony to increase gas exchange and
particle capture. In other words, corals depend on strong water
flow for their immediate health and they grow into forms which
reflect the flow conditions of their environment. To apply this
to aquariums, you should not only consider how you can provide
adequate water flow for your corals now but you should also be
mindful of ways to keep up the flow characteristics of your tank
as the corals mature and begin to alter and obstruct flow in your
tank. In next month’s article I will discuss some research of my
own wherein I examined the effect of water flow at different
lighting intensities on the photosynthesis and respiration of
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