The Effect of Light Intensity on the Rate of Photosynthesis in an Aquatic Plant

The Effect of Light Intensity on the Rate of Photosynthesis in an Aquatic Plant

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The Effect of Light Intensity on the Rate of Photosynthesis in an Aquatic Plant



Introduction

The input variable I will be investigating is light, as light is just
one of the 4 factors required in the green-plant process of
photosynthesis. Photosynthesis is the process by which green-plants
use sunlight, carbon dioxide, water & chlorophyll to produce their own
food source. This process is also affected by the temperature
surrounding the plant (the species of plant we experimented with, pond
weed, photosynthesised best at around 20 degrees centigrade.)

Light, temperature & CO2 are known as limiting factors, and each is as
important as the next in photosynthesis. Light is the factor that is
linked with chlorophyll, a green pigment stored in chloroplasts found
in the palisade cells, in the upper layer of leaves. The light is
absorbed by the chloroplasts to produce the chlorophyll. The chemical
& word equation for photosynthesis is stated below:

Sunlight

Carbon dioxide + water -----------------> Glucose + oxygen

Chlorophyll

S

6CO2 + 6H20 -----------------> C6H12O6 + 6O2

C

Light in photosynthesis

Photosynthesis cannot take place without light, as light is one of the
factors by which the green plant photosynthesises (makes its own food
source) Light provides the plant with energy to make the chlorophyll
which in turn absorbs the light. From this it can be said that the
more light a plant is provided with (the more of a limiting factor is
takes in) then the higher the rate at which the pond weed, or any
plant, will give off a product of photosynthesis i.e. glucose or
oxygen. From this the knowledge can be gained that depriving the
aquatic plant I will be investigating of all light would be a
pointless investigation, as no outcome could be measured. Instead I
will vary the light intensity the plant is provided with. I will do
this by placing the light at different distances from the aquatic
plant. I also thought about varying the wattage of the lamp I will be

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using in the investigation, though I decided against this due to
availability of resources, as many other groups will be carrying out
the investigation too.

Prediction

I predict that by varying the light intensity the pond weed is
provided with, that as the light intensity becomes lower, so will the
outcome I will be measuring (the rate of oxygen bubbles given off by
the plant will decrease), and vice versa. My reason for believing this
is that as mentioned previously, light is one of the limiting factors
required for photosynthesis, and by lowering the amount of light
available to the plant, I will be decreasing the rate at which the
process of photosynthesis takes place in the plant. Similarly, by
providing the plant with a greater intensity of light, I will be
enabling the plants' process of photosynthesis to take place more
intensively, as the factor required (in this case light) is available
at a greater quantity. (The rate of photosynthesis taking place within
the plant will increase)

For example, in terms of when a plant completes the process naturally
(without human interference) it is scientifically known that the
process is greatest at mid-day, when the sun is highest, and from
after this point, when the sun gets lower and less light is available
to the plant, the rate of the process decreases, until nightfall where
no such process takes place.


Variables

The alternative input variables that I could have investigated; I will
keep the same in order to conduct a fair test. They are as follows in
the table below.

[IMAGE]

Explanations

Reason 1

When the green plant is provided with a higher concentration of CO2 in
the environment it is photosynthesising in, the rate of photosynthesis
conducted by the plant will be higher - the availability of one of the
limiting factors was higher. Similarly if the availability of CO2 was
lower, then the rate at which the plant photosynthesised would be
lower too.

Reason 2

When the amount of pond weed is increased, then the surface area at
which the plant can absorb light is also increased. Therefore, the
amount of chlorophyll they produce in total is increased when
comparing 1 plant to 2 plants, or a small plant to a large plant.
Similarly to this, a larger pond weed or more than 1 can take in more
Carbon dioxide

than a smaller sample of the same plant could. Effectively this shows
that if more of these limiting factors can be used by a larger sample
compared to a smaller sample, then the rate or amount at which the
products of photosynthesis are given off will be at a higher level
too.

Reason 3

The ideal temperature for the species of plant I will be experimenting
with is approximately 20 degrees centigrade; however we found that
this temperature was a little cold for the pond weed, and that at 25
degrees we gained sufficient results. If we had conducted the
experiment with the surrounding temperature at 30 degrees then we may
have found that the rate of photosynthesis would have been very low,
or even that photosynthesis within the pond weed was non existent. The
reason for this is, as mentioned previously each species whether plant
or human has an optimum temperature at which they function best. Above
this temperature the enzymes within the species become denatured, they
are destroyed irreversibly by the high temperatures. In the case of
the pond weed if they enzymes within the plant became denatured then
it would become unable to photosynthesise, as well as not being able
to complete other functions.

Measurements to be taken

The input variable I will change is light intensity the pond weed will
be provided with, as mentioned previously. I will do this by
alternating the distance by which the lamp providing the light is set
from the pond weed. I will use a metre rule to measure the distance.
The smallest distance I will set the lamp at is 0cm from the pond weed
within a glass beaker, and the greatest distance at which I will set
the lamp is 100cm away. I will take 5 measurements in total, ranging
in 25cm intervals. These measurements/distances will be 0cm, 25cm,
50cm, 75cm & 100cm. I will take 1 repeat of each measurement/distance
in order to gather whether the results are moderately similar, to
conduct the fairest test possible. Should the 2 results gained be
totally different, I will then proceed to take a 2nd repeat.

The outcome variable I will be measuring is the amount of oxygen
produced by the aquatic plant known as pond weed. Oxygen is one of 2
products of the green-plant process photosynthesis, the other being
glucose. I chose to measure the amount of oxygen produced over the
amount of glucose, as in an aquatic plant, the oxygen produced is
easily visible with the correct set-up of equipment. (See diagram a
for set-up of equipment & figure a for list of equipment used)

I will measure the amount of oxygen produced by counting the number of
bubbles of oxygen given off by the pond weed within a set time of 3
minutes. I could have measured the amount of oxygen given off by the
pond weed another way though, by measuring the volume of oxygen
present in test tube placed over the funnel covering the pond weed
(See diagram b.) I opted against this procedure though, due to
problems encountered in my preliminary investigation. Alternatively, I
could have chosen to measure the other outcome variable, amount of
glucose present. I could have done this by means of investigating the
amount of starch present in the leaf of the plant (Glucose is
converted promptly into starch by plants as it is insoluble, which
makes it much better for storing within the leaf). This process
though, would obviously be more difficult to measure than the outcome
of oxygen. If I was to investigate starch present; I would have to do
this by means of placing iodine onto the leaves of the plant and
noting the colour of the staining upon the leaves. As I am working
with pond weed, where the leaves are shaped a lot differently to that
of a "normal" household plant, this would be a difficult outcome to
measure accurately. And as both methods (finding starch/ measuring
oxygen given off by the plant) show that photosynthesis is taking
place, I chose to measure the rate of oxygen produced.

Diagram a:

Figure a: List of equipment required, and required measurements

· Large beaker, or alternatively a 2 litre bottle cut in half: Fill
with water at 30 degrees centigrade to 1 litre

· One small lamp: to be connected via plug to an electrical socket

· A metre rule

· A 4 cm piece of pond weed

· Stop clock

· A funnel: large enough to cover pond weed, yet small enough to fit
inside the large beaker & be covered by the water within

Alternatively for the measuring volume of oxygen given off experiment

· A test tube: Large enough to cover the spout of the funnel

Diagram b:

Preliminary Investigation & Results obtained

I also set other guidelines of the kinds above during my preliminary
investigation, which would be the basis of my final experiment. e.g.
range of distances of lamp from pond weed, how I would measure my
outcome variable, the time limit I would set for measuring the outcome
variable etc) My preliminary investigation & results, are as follows
below.

As mentioned previously, my preliminary investigation helped set the
guidelines for my actual investigation, yet initially it was to gain
an idea of the range of distances at which I would set the lamp form
the aquatic plant, to vary light intensity available. Before this
aspect of the investigation, I had no idea of what the outcome
variable results (rate of oxygen bubbles produced) would be, when the
lamp was set at any distance. Similarly, I had no idea at what
distance the plant would cease to photosynthesise, as the lamp was the
only source of intense light available. The lab in which the
experiment took place was not very well supplied with light, if the
lamp was not present, it would not be wrong to say that photosynthesis
would have ceased to take place within the lab at all. From this I
have gained the knowledge that the original light within the lab
(without the aid of the lamp) would not effect the investigation by
very much, if at all. Though I still bared this point in mind..

I decided randomly, that the distances at which I would place the lamp
to complete my preliminary procedure were going to be 0cm, 50cm &
100cm. I could then see that if the process would not give sufficient
results with the lamp at a distance from the pond weed at 50cm, then
process of photosynthesis would definitely cease to take place with
the light source at a distance of 100cm from the pond weed. I also
decided I would take the results of the outcome variable within a 5
minute time limit, and for this aspect of the investigation, I would
not take repeat results. I also decided though, that I would count the
number of bubbles, and that if there were a large amount, or they were
at a large size, or that they were given off too rapidly to count, I
would measure the volume of oxygen in my actual experiment. I set the
equipment up as in diagram b.

The results of my preliminary procedure are as follows below.

[IMAGE]

It was from these results that I decided my smallest & greatest
distances of the light source from the pond weed. Yet it was odd how
the results were so similar, considering the distances were of 50cm
apart. This contradicted my prediction, and it was from this I
realized that there was a factor I had not discovered that was
creating this odd pattern of results. I decided that I would have to
set further limits to my investigation, not to gain such "odd" results
again. These other "rules" I set to be carried out during my actual
experiment, after the problems I encountered during the preliminary
procedure. They are as follows overleaf.

1: Pond weed shall be left to adjust to new light intensity in between
distances for 2 minutes. The reason for this is to gain a more
reliable result of the outcome variable. (This factor was the main
reason for the preliminary results being so similar)

2: The experiment should always take place in the same area of the lab
as previous, the reason for this being that some areas of the lab are
better lit than others.

3: Water in which pond weed is surrounded by should be approx 25
degrees centigrade at all times. Although this is slightly high for
the temperature in which a plant generally photosynthesises best, (20
degrees centigrade) it was the first temperature at which the
experiment worked successfully, and so we decided that it should be
kept constantly at this in order to conduct a fair test.

4: We will measure the outcome variable by counting the number of
oxygen bubbles the plant gives off. The reason for this being that the
oxygen bubbles given off in the preliminary experiment were very small
and not given off frequently enough in order to measure volume. Though
we realize that this process would be more accurate in measuring the
amount of oxygen given off by the pond weed.

5: The time in which I will measure the number of oxygen bubbles given
off by the pond weed will be 3 minutes and not 5. The reason for this
is that it would take a total of 10 minutes to gain the results of
just one measurement if the time limit set for measuring the outcome
variable was 5 minutes. This was a suitable amount of time for the
preliminary procedure though, as we were not taking repeat
measurements.

These new "rules" for the experiment, along with the original factors
I decided to keep constant (the variables I will not change) will make
my results more accurate and reliable.

Safety precautions

· Paper towels must be kept at aid at all times throughout the
experiment, as we are working with water & if it is spilled and not
cleared it is a slipping hazard to others who are unaware of the
spillage.

· Make sure hands are completely dry when placing plug of lamp into
the electrical socket, as an electric shock, however minor may occur.

· As I will be working with glass products (beakers etc) a breakage
must be immediately reported. Likewise if a thermometer, which I will
be taking the temperature of the water with, is broken this must also
be reported immediately, as it contains poisonous mercury.

· Work generally in a safe & mature manner, and be aware of others who
will also be conducting the experiment.

Plan of investigation

3 persons are needed to complete the investigation

· Gather all of the equipment listed in figure a & complete set-up
displayed in diagram a.

· Nip end of weed with nail, it is where the O2 will be seen from.

· Set light source (lamp) at 0cm from pond weed in large beaker

· Leave pond weed to adjust to first light intensity of the light
source for a timed 2 minutes

· Person 1 prepare stop-clock to time outcome variable results for 3
minutes

· Person 2 prepare to count number of oxygen bubbles given off within
this time

· Person 3 prepare to note number of bubbles called out by person 2

· Begin what you have prepared to do after the "adjusting time" of 2
minutes

· Note the results you have gained

· Repeat the test for 0cm once more, if results are a lot different,
repeat a 2nd time

· Leave for 2 minutes to adjust to light source at 25cm away from pond
weed

· Complete the above for light source at a distance of : 25cm, 50cm,
75cm & 100cm

NOTE: It is helpful to begin constructing a rough graph of the results
you are obtaining throughout the experiment in order to see whether
you are gaining accurate results. You should see this if a downward
curve/line is present, if you are recording distance of the lamp from
the plant, on your graph.

You should see an upward curve/line present, if you are recording
light intensity on your graph, the reciprocal of the distance, other
wise stated as 1

D

This is perhaps the best choice of graph to construct, as it clearly
presents the idea that as the light intensity increases, so does the
rate at which oxygen is given off by the aquatic plant. See diagrams c
& d below…

Diagram c Diagram d

Results obtained from experiment : "Investigating if light intensity
effects the rate of photosynthesis in an aquatic plant."

As mentioned previously, the results I have obtained from the
experiment will be of measured in 5 intervals, ranging from 0cm
-100cm, 1 repeat reading taken as well as an original reading & an
average of the 2 results calculated. The results are as tabulated
below.

Test 1

[IMAGE]

Test 2

[IMAGE]

Average result

[IMAGE]

Analysis of results obtained : Curve Graph of results

As mentioned previously, I could construct one of 2 graphs (Diagrams c
& d) though below, I have chosen to illustrate my results in the form
of graph d, which demonstrates the theory behind the investigation
(rate increases with light intensity) more clearly. This is why I
calculated the reciprocal of distance (1 ) In the table of average
results & not just distance as shown in the 2 tables prior to D

the results table.

Graph: Shows the effect of light intensity on the process of
photosynthesis in an aquatic plant.

From my ascending graph , I can see clearly that as the light
intensity the pond weed is provided with is increased, the rate at
which oxygen bubbles are given off by the plant increases also. On
analysing the data, you can also clearly see, that each new light
intensity the pond weed was subjected to, gave results which increased
steadily in comparison to the previous & next result. E.g.

3rd Light intensity = 0.02 Rate = 28.5

4th Light intensity = 0.04 Rate = 35.5

5th Light intensity = 1 Rate = 46.5

As can be seen from the rate results, the 4th result is almost 10
bubbles of oxygen bubbles more than the 3rd result, and almost 10
bubbles less that the 5th result.

This trend/pattern is carried out throughout the whole graph of
results.

The most probable explanation for the results I gained; as mentioned
previously in my introduction, as light is one of the factors required
in order for photosynthesis to take place, as the intensity of the
light is increased, photosynthesis is able to take place at a higher
rate. (see page 1 Light in Photosynthesis)

However, this theory would not continue forever as long as the light
intensity was increased further & further. At a point (which was not
displayed on my graph, so logically this point is higher than the
highest light intensity I provided the pond weed with) the rate of
photosynthesis will stay steady , it will increase no further (see
diagram e). The reason for this being that the other 3 "limiting
factors" (CO2, water & chlorophyll) are required in a higher quantity
too, for photosynthesis to continue increasing in rate.

To summarise: For the rate of photosynthesis to increase beyond a
certain point, it cannot be relied on for just 1 of the 4 limiting
factors to increase, after a certain point ALL must increase for the
rate to increase.

Diagram e

Although I did not predict the information gained from my results
listed above, it is fair to say that my original prediction is indeed
correct. As I did predict the rate would increase with increasing
light intensity, even though I did not conclude that after a certain
point, that increasing rate could no longer purely depend on
increasing light intensity.

Evaluation of Investigation

On analysing the results I gained in the final experiment, I conclude
that my investigation was both considerably accurate & the results
were reliable.

My reasons for believing my results are reliable & accurate are
firstly, they agree with the theory behind the rate of photosynthesis
when light intensity is increased, secondly the result which I gained
compared accurately to my prediction, which was well thought out
before it was made, and finally the results I gained for my final
experiment differed so much to the results of my preliminary procedure
which was very inaccurate. This proves that the new changes I made to
the investigation were vital for reliability & accuracy of results.

Of course it is realized that the results I obtained are not
completely faultless, and that some of this was down to the method I
used & also for this experiment, the conditions in which it took
place.

We found that photosynthesis can be highly influenced by the
surroundings in which it took place. Our experiment took place over
the period of 2 days, and on each day the experiment took place, the
natural light conditions in the lab changed. This did not apply
necessarily to my experiment, as the experiment I did took place in a
fairly shaded area of the lab. On the other hand, other groups that
completed the experiment did it in areas that were better supplied
with natural light. This meant that our investigation was difficult to
start, as the pond weed was finding the conditions we were working
under inadequate for photosynthesis, whereas other groups were finding
the investigation much easier to conduct.

Another aspect of my method which was faulted by the fact that the
experiment took over 2 days was the fact that different numbers of
people were using the electricity in the lab on different days. This
meant one day, when few people were conducting the investigation out
lamp was supplied with more electricity & was bright, on another
occasion more people were sharing the electricity in the lab & the
lamp providing light to the pond weed was dimmer. Though the lamp was
not necessarily very bright one day & extremely dim the next day of
the experiment, it could have made the difference of 10 or so
centimetres distance to the light intensity supplied to the pond weed.
Secondly, the water in which the pond weed was continually stored may
have contained less carbon dioxide on one aspect of the experiment
than on another aspect. This would indicate that less photosynthesis
would take place, as carbon dioxide is one of the 2 products required
by a green-plant for photosynthesis to take place.

A major aspect that could have affected the reliability of my results
was the fact that I counted bubbles of oxygen. Number of bubbles
indicates no real accurate idea of the amount of oxygen given off by
the plant, but really just the rate at which oxygen is produced. The
bubbles in my experiment were generally all similar in size (I noticed
this, as it was an aspect I had thought about) however, it is
impossible to say that they were all exactly the same size; some
bubbles may have been larger & therefore contained more oxygen. Also
there may have been some bubbles of oxygen that were missed by the
counter, or very small. This particular point is unknown & down to
human error, an aspect which always makes experiments a little
inaccurate.

A way of getting around all the dilemma's caused by counting the
number of bubbles would be to set the experiment up as in diagram b,
and measure volume of oxygen instead. I could not do this in my
experiment as during my preliminary procedure, it was seen that not
enough oxygen would be given off by the plant to measure volume of
oxygen left within the test tube. This procedure (measuring volume of
oxygen) also enables us to prove that the gas given off was indeed
oxygen & therefore prove photosynthesis occurred. You could do this
with the glowing splint test illustrated below in diagram f.

The only down point of only measuring volume is that it is not certain
whether enough oxygen to measure accurately will be produced within a
short time, 3 minutes for example. You would need to complete the
experiment within a longer time, 10 minutes for example. This then
make the investigation very long, if you are to take repeats &
investigate a range of 5 measurements. A way of solving this problem,
and make the investigation even more accurate, is to measure both
volume of oxygen & number of bubbles, with this you can gather amount
& rate effectively & in less time.

Diagram f

On considering my experiment, in hindsight, perhaps I would have
gained an even higher rate of photosynthesis if I had made the
temperature of the water slightly lower. 20 degrees centigrade is the
recommended temperature for photosynthesis, and yet we conducted out
experiment with the water around 25-27 degrees C, as I found that 20
degrees was a little low. If I had altered the water steadily by 1
degree, instead of making it 25 degrees C straight away, I may have
found that the water only needed to be at 22 or 23 degrees, and the
pond weed would have photosynthesised at a higher rate at this
temperature.

On analysing the table of results for my 1st and 2nd test of the final
experiment, it can be seen that the results of the 2nd test are always
slightly lower than the 1st. Not one of the results for the different
light intensities are completely the same. This gives the impression
that my 2nd (repeat) results are more accurate than the 1st. The
reason I believe this is because, as mentioned previously, the pond
weed needed a certain amount of time to adjust to a new light
intensity. We allowed this time to be 2 minutes. Perhaps though, to
make the experiment more accurate & the results more reliable, I
should have allowed 5 minutes instead. I could have seen if this was
right by taking a 3rd result, and noting whether it was very similar
to the 2nd result, but because the 2 results I did take were not
totally unsimilar I did not do this.

I believe that my range of 5 distances (0cm to 100cm) was sufficient,
although as mentioned above, I believe taking a 2nd repeat would have
aided me in gaining a more accurate average result, and then also I
would have discovered if the pond weed needed further time to adjust
to a new light intensity.

These points I have mentioned above, did not seem to affect my results
very badly. I did not have any result that were way off my line of
best fit, and seemed odd to my graph.

Although as can be imagined, they were not all perfectly in a line.
Some of the points on my graph were slightly out of place, and this
could have been down to minor faults in my investigation, but nothing
major and as mentioned before the points on my graph formed a
consistent trend. This proves that my results are reliable and fairly
accurate. My results also appear to agree with the information on the
theories of photosynthesis, found in most biology texts; though I did
not have enough evidence (results) to prove the theory which says that
after a certain point the rate will cease to rise, as explained
previously.

I found it quite challenging to complete this investigation as fairly
& reliably as possible, though my results indicate this was reasonably
well attained. I found it hard to conduct the experiment fairly mainly
because it was hard to keep to the rules & limitations we set, as it
was difficult to try and make the plant photosynthesise under our
conditions, it is a living thing & cannot be forced. However, we did
finally overcome this by adjusting the surroundings in which the
experiment was taking place slightly without breaking the rules we set
to conduct a fair test. We found difficultly also in controlling the
other variables which cannot be adjusted easily, CO2 for example. A
variable such as temperature could easily be adjusted to the
conditions we decided to set our test under, but to adjust the levels
of carbon dioxide; a natural gas in the surrounding air (or water in
the case of our experiment) was extremely difficult. However, after
our experiment had taken place we found that by changing the water
after each test for the rate at which oxygen was given off, we would
be providing the pond weed with the same levels of carbon dioxide each
time. We did not do this in our experiment, though we did change the
water several times, though not as consistently as this.

Plan for a new & improved experiment (changes to be made)

New equipment required: Pond weed (7-10cm)

5 different wattage bulbs for lamp (40 W, 60 W, 80 W, 100 W, 120 W)

Test tube

* Ensure pond weed (approx 7-10cm) has previously been kept in a
well lit area, is very healthy & photosynthesising well ( Nip end
of weed with nail)

* Set up experiment in a dark room immediately (as in diagram b) -
Ensure room available for at least 3 hours & you are not sharing
electricity

* Place pond weed in fresh water (approx 23 degrees C)

* Place 40 W bulb 15 cm away from pond weed & leave to adjust for a
timed 5 minutes

* Begin investigation: Leave pond weed to photosynthesise for 5
minutes, counting number of bubbles given off, and noting general
size in mm, after 5 minutes calculate amount of oxygen left in top
of test tube. ( You also wish to conduct the glowing splint test
to test the gas given off was oxygen diagram f)

* Change water to ensure levels of CO2 are the same for the next
test.

* Take a repeat & if result is over 5 bubbles more than original
result, take a 2nd repeat

* Repeat test for 60 W, 80 W, 100 W & 120 W bulbs. Refilling water
after each test.

With this you should gain even more accurate & reliable results,
although it is a very strict experiment, and the actions of the plant
cannot be fully controlled.

Further Experiment

For further work, I could introduce the adjusting of a different
limiting factor into the experiment, CO2 for example. By adjusting the
levels of carbon dioxide available to the plant I could still measure
whether this affects the rate of photosynthesis in pondweed.

I would conduct this experiment by exposing the water in which the
pond weed was surrounded by to a range of levels of lime water, which
would decrease the levels of CO2 when the amount of lime water is
increased.

Plan for the investigation

Note: Conduct experiment in a well lit area, make sure light levels
are never adjusted.

* Ensure pond weed (approx 7-10cm) has previously been kept in a
well lit area, is very healthy & photosynthesising well ( Nip end
of weed with nail)

* Set up pond weed, as in Diagram b, in a large beaker of
approximately 1 litre of fresh water at 23 degrees centigrade,
covered by a funnel & test tube.

* Time for 5 minutes to allow the pond weed to adjust to the natural
levels of the surroundings it is in

* Begin investigation: Time with a stop clock for 5 minutes,
counting the number of bubbles given off by the pondweed, then if
there is a sufficient amount of gas within the test tube covering
the funnel, calculate the amount of oxygen given off by the pond
weed.

* Change water to ensure initial CO2 levels are the same for the
next test

* Take a repeat result. If the repeat is over a sensible amount
above the original result, take a 2nd repeat to gain a fair
average.

* Repeat test for 4 more different levels of limewater to which the
pondweed will be exposed. These being

Note: Change the water after each test to ensure same starting level
of Carbon dioxide in water. Even though we are measuring whether the
rate of photosynthesis is affected by different levels of exposure to
CO2, It would only be a fair test if the different range of levels can
clearly be measured, which they cannot be clearly measured if the test
is conducted with the same water, even though the levels of CO2 would
be different.
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