Water flow is more important for corals

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In the first part of the article series I introduced the
mechanismsby 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. The second
article in the series reviewedsome of the published scientific
research describing how water flow affects coral biology. Some of
topics included the effect of water flow on bleaching, particle
capture, photosynthesis, growth rates, morphology and several
others. In this month’s article I will discuss my own
research demonstrating how flow speed and lighting intensity go
hand in hand. Still to come in the article series is a discussion
of moving beyond “X” times water volume turnover and on to “mass
water movement” techniques.

Introduction

In the spring of 2004 I began a research project to examine
the effect of water flow on the photosynthesis and respiration of
Pocillopora damicornis. Under the supervision of Dr. Brian
Helmuth I designed an experiment to measure the consumption and
production of oxygen of P. damicornis while exposed to
different water flow speeds. I hypothesized that

  • Corals will exhibit a physiological response to varying
    water flow velocities
  • Faster flow would produce higher rates of respiration and
    photosynthesis

Pocillopora damicornis was chosen as it is a common
representative of reef assemblages. The measurements were made
using a recirculating, sealed respirometry chamber. The response
was measured by monitoring the change in dissolved oxygen during
photosynthesis and respiration. Measurements were recorded at
different flow rates and lighting intensities. What
appeared to be a fairly straightforward endeavour eventually
developed into a much longer and more laborious project than I
had expected and the research is still in progress. Although the
current data lacks rigorous statistical analysis and
significance, I invite readers to come to their own conclusions
about the preliminary results from this ongoing research
project.

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Materials and Methods

Test Organism

Pocillopora damicornis was chosen as it is a common
representative of reef assemblages. Although the appearance of
P. damicornis can vary greatly, this species is easily
recognized. The variety of P. damicornis used for this
study is a brown strain which has thin (6-10mm), regularly
bifurcating branches. P. damicornis is found in a wide
range of shallow water habitats so results from the experiment
are relevant for a wide range of ecosystems. Most importantly,
since the project had very little funding, the samples for the
project were produced from long term captive specimens which I
gratuitously obtained from Adam Cesnales, Richard Harker and
Morgan Lidster (thanks guys). To date, photosynthesis curves have
been produced and analyzed for two samples from one genotype and
three samples from another genotype. The current results
presented in this article are from a total of five samples.

Experimental Setup

All measurements were made in an experimental setup consisting
of a sealed recirculating flow chamber placed in a saltwater
bath. The setup included a separate bath for the water pump to
minimize transfer of heat from the water pump to the sealed
chamber. Water temperature was actively controlled to maintain
temperature changes within 1ºC during the 15 minute data
collection experiments and over the course of the twelve hour
photoperiod of data collection. The flow chamber was used to
produce turbulent, mostly linear flow at speeds of 4.5, 7.4 and
10.3 cm/s. To minimize coral stress from transfer between the
captive aquarium and the experimental chamber, the setup was
filled with new seawater from the coral aquarium at the beginning
of each day of data collection. Following transfer to the
experimental chamber, corals were allowed to acclimate in
darkness for 30 minutes at the median experimental flow velocity
of 7.4 cm/s. Data collection began with the measurement of coral
respiration in darkness. Lighting was provided by the use of a
broad spectrum (6500K), 150W double ended metal halide light and
it was attenuated by the use of coarse screening and by varying
the height of the bulb above the coral sample. Light intensity
was increased in increments of 50-80 µmol
photons·m²·s, up to a maximum of 500
µmol photons·m²·s for a total of 6-8
measurements of net photosynthesis including one measurement of
respiration (no photosynthesis). At each lighting intensity,
coral samples were exposed to the three flow speeds in a randomly
selected order. In between measurements, the corals were allowed
to acclimate to changes in flow speed and light intensity and the
chamber was flushed by exchanging water with the seawater bath.
The samples were allowed to acclimate to changes in flow velocity
for five minutes, after respiration the corals were allowed to
acclimate to the initial light intensity for 20 minutes and the
corals were allowed to acclimate for 15 minutes to every
subsequent increase in lighting intensity. After each day of data
collection the coral sample was preserved by freezing at
-80°C and processed for molecular analysis and normalization
at a later date.

Measurement of Physiological Response

The coral’s response to water flow was measured by the uptake
and release of oxygen using a fiber optic oxygen (FOXY) sensor
and S2000 spectrophotometer, both of which are manufactured by
Ocean Optics Inc. (OOI). Temperature within the flow chamber was
monitored using a FOXY-TS1 Omega Thermistor. Data from the FOXY
oxygen and temperature probes was plotted and logged using OOI
sensors software, also by Ocean Optics Inc. The oxygen probe was
calibrated at six oxygen concentrations and three temperatures
for a total of 18 calibration points. In addition to the
temperature compensated calibration, at the beginning of each day
of data collection, one or more control experiments were
performed to estimate measurement error from the effect of
temperature on changes in oxygen concentration. Physiological
measurements were normalized to surface area. Since the three
coral genotypes were very similar and grew in the same aquarium,
it is assumed that they contained similar zooxanthellae types and
similar zooxanthellae densities.

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Preliminary Results

Respiration

Respiration showed an increase with an increase in water flow.
The lowest flow speed had the lowest respiration rate, the median
flow speed had an intermediate respiration rate and the highest
flow speed had the highest respiration rate. The plot of the
association between flow speed and respiration rate shows a very
strong linear correlation with a correlation of 98%. Figure 5
illustrates the results of the respiration measurements.

Photosynthesis

Before I delve into the details of the preliminary
photosynthesis curves, I would like to introduce some terminology
which will familiarize readers as to how they should interpret
the current graphs. Figure 6 demonstrates where these critical
points occur on a photosynthesis curve.

  • Respiration: Consumption of oxygen by the organism
    resulting in decrease in O2 concentrations.
  • Compensation: Point at which Photosynthesis (P)
    equals Respiration (R), where there is no net change in O2
    .
  • Maximum Photosynthesis: Maximum photosynthesis can
    be caused by a number of factors including CO2
    limitation and chemical quenching, and is usually described as
    the point at which increasing light intensity no longer results
    in a further increase in the rate of O2
    production.
  • Photoinhibition: The point at which the
    photosynthetic process begins to curtail in an effort to
    prevent photodamage to the photosynthetic apparatus. This is
    the point at which exposing a coral to more light is
    potentially stressful.

Figure 7 shows the results of the average of 5 samples of
P. damicornis from two different genotypes. The tested
corals show an increase in photosynthesis that is closely
associated with an increase in water flow. Although the
compensation point for all three flow speeds is very similar,
from this point all other characteristics of the curve diverge.
The lowest flow speed of 4.2 cm/s exhibited maximum
photosynthesis at a little less than 200 umol photons/m2/s and it
showed photoinhibition at light levels above 270 µmol
photons·m²·s. The median flow speed of 7.4
cm/s exhibited maximum photosynthesis at 300 µmol
photons·m²·s and it showed photoinhibition at
light levels above 350 µmol
photons·m²·s. The highest flow speed of 10
cm/s exhibited maximum photosynthesis at 400 µmol
photons·m²·s and it showed photoinhibition at
light levels above 420 µmol
photons·m²·s. Table 1 summarizes the results
of the respiration and photosynthesis measurements.

Table 1
Water Flow Speed 4.2cm/s 7.4cm/s 10cm/s
Respiration lowest middle highest
Compensation ~same ~same ~same
Maximum Photosynthesis lowest middle highest
Photoinhibition highest middle lowest

Conclusions

Discussion

Leading up to maximum photosynthesis, photochemical reactions
are limited mostly by the input of CO2. Exposure to higher flow
speeds drives a higher rate of diffusion of CO2 into the host.
The greater availability of CO2 leads to higher rates of
photosynthesis much in the way that a supercharger for a car
provides a greater availability of oxygen for fuel to burn,
leading to increased horsepower.

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During peak photosynthesis, oxygen builds up and at extreme
concentrations it leads to the formation of oxygen radicals, a
toxic form of oxygen.

Initially, the build up of oxygen in coral tissues inhibits
chemical pathways much like a traffic jam of chemicals. At this
stage, dynamic photoinhibition slows down the process of
photosynthesis to prevent the further build up of oxygen. If a
coral is further exposed to reduced flow or increased lighting
intensity, there will be an increase of oxygen leading to the
formation of oxygen radicals. Oxygen radicals are dangerous
because they destroy important photochemical proteins and cause
irreversible chronic photoinbition. Higher water flow
speeds increase the rate of O2 diffusion out of the
host which prevents light related stress and helps to maintain
maximum photosynthesis. The decreased O2 concentration
maintains a maximum rate of photosynthesis much in the same way
that a water cooling system for high performance computers
removes excess heat and allows the processor to function at a
maximum speed. The relationship between water flow and
photoinhibition is only recently starting to be understood.
Although researchers agree that increased water flow reduces
photoinhibition, the mechanisms of the effects are still
debated.

The data from this research project clearly demonstrates that
water flow speed is critical in driving respiratory and
photochemical reactions. Since the water pump I used for this
project could only provide flow speeds up to 10 cm/s, the effects
of higher flow speeds remains to be seen. It is likely that
maximum photosynthesis at higher flow speeds would eventually
show some form of saturation similar to the photosynthesis curves
for light. At higher flow speeds corals will still only be able
to photosynthesize so much and beyond that, higher flow speeds
will depress coral polyp extension, cause mechanical abrasion to
coral tissue and it will eventually lead to branch breakage.

Application

Having a better knowledge of the environmental factors that
make corals healthier can help aquarists grow corals faster and
it might help us to understand the care requirements of still
difficult species such as Goniopora and
Dendronepthya. More importantly, a better understanding of
the synergy between water flow speed and lighting intensity can
help aquarists make better informed decisions about the equipment
they should employ for their aquariums. Coming back to one of the
first things I discussed in this article series, aquarists often
put much more effort into considerations for lighting than water
flow. As you can see from figure 7, if an aquarists provides very
high light intensities but fails to deliver an equivalent water
flow speed, instead of benefiting it is more likely that their
corals will suffer from photoinhibition stress. The excessive
lighting equipment will not yield the desired increase in growth
rate but instead it will likely cause an increased heat input to
the aquarium, increased algae growth and a considerable waste of
money and energy. If you take nothing else from this article
series just remember that:

The more light a coral receives, the more flow it will
need

Since P. damicornis occurs in a wide range of habitats,
I am going to take some liberty to use the results from this
research and apply it to what one might consider as an average
coral’s basic flow requirements. Figure 8 shows some
suggested minimum water flow speeds for optimum coral growth
relative to the available lighting intensity. Loosely
interpreted, the green line from 0-100 µmol
photons·m²·s is roughly the average amount of
light one could expect from a minimally lit aquarium with normal
output fluorescent or just a few power compact or very high
output fluorescent bulbs. The yellow line from 100-400 µmol
photons·m²·s would represent an aquarium with
moderate intensity lighting provided by mostly power compact or
very high output fluorescent bulbs or a few low intensity metal
halide bulbs. The red line would represent an aquarium that is
really shallow or lit with banks of power compact or very high
output fluorescent bulbs, very high intensity metal halides or a
combination of all of these. As most of you are aware, lighting
intensity within an aquarium can vary greatly depending on the
age of the bulb, type of ballast reflector types and the depth of
the tank. Therefore, since lighting intensity is highest closer
to the light source, another way to interpret the graph would be
to provide higher water flow speeds at the top of the tank,
moderate flow speeds at the middle of the tank and lower flow
speeds at the bottom of the tank. I would like to emphasize that
these suggested water flow speed values are minimums which are
only meant to be a relative guideline as to how aquarists should
match up their aquarium’s lighting equipment with the
delivery of appropriate water flow speeds. In next month’s
article I will discuss why aquarium hobbyists should move away
from gauging the water movement in their aquarium by using “X”
amount of turnover, I will introduce some useful principles of
fluid dynamics and I will also cover various techniques for
producing mass water movement within our coral aquariums.

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Acknowledgements

I would like to acknowledge a handful of persons who were
helpful in helping me to produce this research project. Dr. Brian
Helmuth, members of the Helmuth lab, Art Illingsworth, Richard
Harker, Morgan Lidster, Coral Dynamics, and Alvis Nanney.

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 Jake Adams

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