Water Flow is More Important for Corals Than Light, Part V

Introduction

In recent years there has been a great amount of
varied and original data produced on lighting and corals for the reef
aquarium. By contrast, there have been much fewer articles on the effects
and importance of water flow in the reef aquarium. Most aquarists are
unaware of the relationship between laminar and turbulent flow and
virtually no one ever discusses water motion for reef aquarium in terms
which actually apply to fluid dynamics. It’s time for the reef aquarium
hobby to catch up in the flow department. Just as aquarium lighting has
received a thorough reevaluation, so too must we start to consider how the
water movement of our aquariums translates into water motion which is
relevant and suitable for coral health.

Describing the ideal flow

For lack of a better value, aquarists describe the amount of water
motion in their reef aquariums in terms of turnover rate. If identical
powerheads are placed at opposite ends of an aquarium but one powerhead is
facing the center of the aquarium and the other powerhead is facing the
aquarium glass, the powerhead which is directed towards the center of the
aquarium will undoubtedly produce faster flow speeds and more circulation
throughout the aquarium. Although the powerheads both have the same
turnover rate their orientation to the main water mass has a great effect
on the flow speeds they will produce. Since flow speed is the critical
measure for determining the rate of gas exchange, turnover does little to
convey how fast a coral will respire and photosynthesize.

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In the natural environment, the reef surface and the corals
which live on it experience mostly random, chaotic flow in the form of
oscillatory surge.
In most cases, aquarists interpret “random,
chaotic flow” to exclude laminar or unidirectional flow. This
interpretation usually translates into a bevy of powerheads and inlets
arranged to resemble what I call a squirt gun firing squad. Although reef
aquariums are an attempt at recreating a natural environment, trying to
reproduce surge with the scale and energy of the natural environment would
take tremendous effort and resources. Whereas the oscillatory surge of the
natural environment entails movement of the entire water mass, the typical
aquarium features small plumes of water movement which lose velocity and
momentum with distance away from the source of water flow (Harker 1998).
Water flow which is exiting a powerhead or other outlet begins as high
speed, unidirectional flow. However, as the flow increases distance from
its source, resistance from other flows and viscous friction cause the
orderly flow to quickly lose momentum. At this point the flow loses
velocity and it increasingly becomes multidirectional, turbulent flow.
Riddle illustrated this phenomenon using a digital electronic flow meter
(Riddle 1996). The flow emanating directly from a Hagen 802 powerhead
displayed velocities upwards of 70cm/s whereas the measured velocity was 0
cm/s only 60 cm away. This does not mean that there was no flow at that
distance but rather it means that either the flow was not properly aligned
with the flow meter or the sum of the multidirectional flows had a net
velocity of 0cm/s. It is true that turbulence leads to an increased rate of
mixing, but fast laminar flow will become turbulent as soon
as it encounters an irregular surface such as that of a coral. The faster
the flow speed, the greater the amount of turbulence produced when the flow
encounters a surface. Although turbulence is the desired end product of
water movement, aquarists should be more focused on producing faster
unidirectional flow.

The Flow Environment

The environment in which fluid movement occurs has a great effect on how
the fluid will behave. The three things that will impact water movement in an
aquarium are the dimensions of the aquarium, the relief of the live rock
reef structure including the corals, and the force and duration of the
water motion.

When considering an aquarium for use as a reef tank, it is important to
remember that the dimensions of the reef tank will have a great influence
on what kind of reef it will be.
The size and shape of the tank will
determine the type of lighting to be used, how maintenance will be
performed, the type of fish and corals it can hold and how water flow will
behave within the glass. As a stickler for water motion I usually consider
the last criteria first. In a small aquarium, the water will be easy to
flow throughout the entire aquarium, mass water movement will occur very
quickly but there will not be much heterogeneity of flow speeds. In a
smaller aquarium, viscous forces cause most of the water movement to move
at a similar speed and it could be hard to provide suitably slow and fast
water flow speeds for the corals which prefer one or the other. In a larger
aquarium, it will take more force to move the entire water mass and it will
take longer for the entire volume to circulate. However, once the entire
water mass of a larger aquarium is moving, it will have more inertia and it
will be less impeded by the reef structure or corals which project into
mainstream flow. It is easier to produce a variety of flow regimes in
larger tanks. Since a larger tank will be governed more by kinetic than
viscous forces it is more likely to feature a narrow band of faster flow at
the surface and a broader band of slower flow at the bottom.

As you might expect, the more live rock you have in an aquarium, the
harder it will be to provide adequate water movement to the volume of the
entire tank. Since I prefer to view corals from above, I generally use as
little rock as possible. We have all heard the suggestion that a reef
aquarium requires 1-2 pounds of live rock per gallon but since the density
of live rock varies greatly, this is not a very useful guideline. As you
can see in figure 1, my personal preference is to build a reef structure
which occupies no more than about 25% of the actual volume of the aquarium.
This restriction equates to a live rock arrangement which can be most of
the length of the aquarium but it shouldn’t be much more than half as high
and half a wide as the dimensions of the aquarium. Although new aquarists
might be inclined to call this kind of set-up “empty,” a patient aquarist
knows that this modest amount of rock leaves plenty of room for corals and
growth.


figure1.JPG

Figure 1 This aquarium is a good example of a reef tank
which uses a modest amount of live rock. Although it is a novel shape,
the dimensions of this aquarium made it difficult to produce mass water
movement.

In the ocean, surface currents of water are driven mostly by wind
blowing across the surface of the sea. The amount of water moving in those
currents is proportional to the force of the wind and the duration for
which it blows, which is called the fetch. Since aquarists do not use wind
to move water, for our aquariums we can think of the fetch as the duration
that a mass of water is pushed in a particular direction. Commercially
available “wavemakers” are not designed or constructed on anything more
than the status quo of the coral hobby which is that corals and reef
aquariums need “random, turbulent flow.” Apart from their high price, my
biggest complaint about these pump controlling devices is that their
outlets are switched on and off with such short intervals that they do not
allow for an optimized fetch of water flow. By turning off a pump before it
has had a chance to reach its full water movement potential, a water pump
in this scenario essentially sends out a plume of water movement which
encounters a lot of resistance from the inertia of the water volume. By
increasing the duration that a water pump is turned on, the moving parcel
of water will gain size and momentum so that when the pump is turned off,
the water volume should continue to move through the aquarium for a short
time. The capacity for wavemakers to produce mass water movement can be
ameliorated by increasing the timing interval between pumps and designing
pump circuits which work together to move the entire volume of the
aquarium.

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The final consideration for the flow environment is the placement of
invertebrates in regions of the aquarium which combine suitable lighting
intensities and water flow speeds. Since the upper region of the aquarium
is often the preferred placement for high light corals, it is doubly
advantageous to concentrate the fast water movement in the upper layers of
the aquarium. Be mindful that when water flows around a shape, there is
usually more turbulence and therefore more gas exchange on the downstream
face of the shape. If you are looking at the upstream surface of a coral
for indications of the coral’s behavioral response to water flow, you could
be missing the more significant response on the downstream face of a coral.
The bottom line is that you should do a colony wide inspection of your
corals for indications of the suitability of the flow to which the coral is
exposed.

Mass Water Movement

A moving mass of water will tend to return to a low energy, equilibrium
state by way of several feedback mechanisms. Gravity, pressure, and
friction are forces which ensure that the water at far ends of a vessel
have a shared effect on the movement of the water mass as a whole. As
aquarists trying to replicate flow conditions of the environment where
corals occur naturally, we should be mindful of these forces and we should
employ them to our advantage in order to encourage mass water movement
throughout the entire aquarium.

In order to maximize the output of water flow equipment, aquarists
should design water movement systems so that all the components work
together to minimize resistance and move the entire water mass of the
aquarium. The best way to combine the energy of moving water to produce
maximum water motion for an aquarium is to encourage the formation of a
circular course of water movement called a gyre. Like the wheel, a gyre
takes advantage of feedback mechanisms which preserve momentum by
minimizing resistance. An aquarium gyre somewhat resembles a conveyor belt
of water movement and it is characterized by mostly laminar, unidirectional
flow. By alternating the rotation of the gyre from one side to the other,
it is possible to evenly distribute turbulence on all sides of corals and
therefore increase photosynthesis and respiration. An extreme example of
this technique is exemplified by a special aquarium called a gyre.

Gyre Tanks


figure2.jpg

Figure 2 This horizontal gyre tank produced flow speeds
between 15-22cm/s.

A gyre tank encourages the maximum amount of water motion momentum
because it contains a divider to essentially turn the tank into a circuit.
This specialized aquarium constricts the cross section of the water’s path
so that all of the water is evenly moving in the same direction. A setup
like this mostly dispenses with rock or other ornaments on one face of the
divider so it reduces friction with the usual aquarium reef structure for
at least one side of the flow’s fetch. The divider stretches nearly the
entire length of the aquarium and it can be placed either horizontally or
vertically. In a vertical setup, the divider rests on the middle of the
bottom of the aquarium and it projects out of the water surface. In a
horizontal setup, the divider is flush with the middle of the front and
back panes of the aquarium glass. A vertical gyre tank is good for keeping
tall coral species such as gorgonians, arborescent soft corals and tall
staghorn corals. This type of aquarium can be more aesthetic because it is
easy to hide the pumps and flow outlets behind the divider of the aquarium
and it preserves the viewing area of the tank. However, since the water
mass of a vertical gyre is always in contact with the bottom of the
aquarium and the surface of the water, it has more potential to develop
velocity shear with faster flow at the top and slower flow at the bottom.
In a horizontal gyre tank the powerheads or flow outlets are placed
underneath the divider and they are aligned in a horizontal plane. This
setup preserves the original actual surface area of the aquarium but it
does so at the expense of the height of the aquarium. The larger surface
area and closer proximity to the lighting source makes the horizontal gyre
tank ideal for concentrated efforts of coral culture.

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Figure 2 is an example of a horizontal gyre tank which I built for stony
coral culture. The aquarium is 33 gallons, 4 feet long, 14 inches wide and
12 inches tall. The divider was made out of two pieces of dark plexiglass
which were overlapped in the center. Both pieces of the divider were
unattached and I found that I could vary the speed of the water flow by
adjusting the distance of the gap between the divider and end faces of the
aquarium glass. The water movement was provided by one Seio 820 pump on one
side and two Maxi-jet 1200’s on the other side. A Chauvet light timer was
used to alternate power between the pumps for 5 to 15 minutes to each side.
Since the water flow was so unidirectional in this long aquarium, it was
very simple to measure flow speed. Water velocity was calculated by adding
neutrally buoyant particles to the water and timing how long it took for
them to travel across a distance of the aquarium. Using this technique I
was able to measure water flow speeds between 15-22cm/s throughout the
entire aquarium. These velocities are within the range of ideal flow speeds
for optimum particle capture, respiration and photosynthesis of many
corals. Figure 3 is an image of a vertical gyre tank built and designed my
Michael Janes of Aquatouch. Mr Janes is an octocoral
specialist and he refers to his design as a laminar flow tank. He designed
the aquarium to produce ideal flow conditions while still maintaining
enough vertical space to accommodate tall soft coral species such as
gorgonians. Although this aquarium was designed primarily as a proof of
concept, Mr. Janes continues to work with this type of gyre tank for
studying octocoral species.


figure3.jpg

Figure 3 A vertical gyre tank (a.k.a. laminar flow tank)
designed to accommodate tall coral species. Photo by Michael Janes.

Gyres in Reef Aquariums

An aquarium does not necessarily need a divider to produce gyres of the
water mass. Although the water movement will not be as complete and uniform
as it is with a gyre tank, it is still advantageous to encourage water
movement to follow a circuitous path. In a reef aquarium with live rock and
coral on the bottom, the water surface of the aquarium provides the least
resistance to moving water. Because of the lack of friction, moving water
which is directed in this region will produce the most momentum of the
water mass. If there is an even transport of the surface water from one
side of the aquarium to the other, the entire water mass should begin to
gain momentum as it is moved at both ends. At one end of the aquarium, the
water will begin to “pile up” and then sink down. At the other end, water
will rise up to replace the volume which is displaced by the water motion.
Although it is easiest to create gyres which follow the top and bottom
surfaces of the aquarium, this is not the only way to create gyres. Figure
4 is a photo of a 180 gallon aquarium with an overflow drain right in the
center of the tank. This aquarium contained a modest amount of live rock
and it was circulated by encouraging mass water movement around the center
overflow. Once again a Chauvet light timer was used to alternate the flow
between two circuits of powerheads. Each circuit contained pumps which were
diagonal to each other and in this fashion the force of both pumps were
working together to move the entire water mass. The center overflow was not
necessarily the most aesthetic design but it was very easy to spin water
around it using only very modest water pumps.


figure4.JPG

Figure 4 Because this 180 gallon was circulated using
mass water movement techniques, only modest equipment was required to
produce adequate water motion.

Not only can mass water movement techniques help aquarists produce
higher water flow speeds in the aquarium but it can also encourage more
water movement through the interstices of live rock and corals with open
growth forms. Normally an aquarist might target one or more plumes of water
movement at corals which require fast water flow speeds. In this scenario,
the turbulent water flow plume encounters a lot of friction on its way to
the desired location of the reef aquarium: it will experience resistance
from the still water around it, it will experience drag from the shape of
the corals it encounters and the turbulent nature of the water flow plume
will do little to preserve the momentum of the water movement. In a
scenario with the employment of mass water movement, the behavior of the
fluid will be much different. The plume of water motion from the same
source will encounter less resistance from the water around it since both
parcels of water are moving in the same direction. The decreased resistance
will straighten out the flow and preserve more momentum. Not only will the
water be moving faster once it reaches a coral, since water is moving away
from the coral on the downstream end, water will be forced through the
normally stagnant water which is present at the interior of corals with
open growth forms. Figure 5 shows an aquarium where dense coral growth
account for a significant portion of the aquarium’s cross section. When
using mass water movement techniques in cases of dense coral growth, water
flow speed can actually accelerate as more water is pushed through spaces
with a smaller area. Aquarists who wish to encourage additional water
movement at the inside of dense coral colonies will see great benefits from
using mass water movement techniques.

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figure 5.jpg

Figure 5 An example of a mature reef aquarium which
exhibits very dense stony coral growth.

Conclusions

The reef aquarium hobby has a long way to go before our understanding of
water flow catches up with what we know about reef aquarium lighting. Like
the “Watts per Gallon” moniker that came before it, the use of “turnover
rate” to describe water movement continues to cripple the progress of more
advanced water movement techniques. By encouraging the formation of one or
more gyres, aquarists are capable of producing more water movement in terms
of overall water flow speed. Since higher flow speeds produce greater
amounts of turbulence, this translates into increased gas exchange and
higher rates of photosynthesis and respiration.

References


  1. Harker, R. 1998. Measuring Turbulent Flow in Reef Tanks. Aquarium
    Frontiers, August Issue.
  2. Janes, M.P. 2005. ANew Era for Sea Fans? International Marine
    Aquarium Conference. Chicago, Illinois.
  3. Riddle, D., 1996. Water motion in the reef aquarium, Part 1. Aquarium
    Frontiers, 3:4, 32-39.
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