4. Discussion

4.1 Analysis of results

Our experiment has shown that with a higher amount of salt content in the water, the less healthy the duckweed will be with a depleting number of leaves and the yellowing or whitening of the leaves. However, in our first reading, there were some anomalies. The results coming from set-ups E1, E2, E3, F1, F2, F3 were varying with E1 and F1 both being 0 and 4, 4, 1 and 5 in order. When we found the differentiation of these results we had quite a big number, namely 2.16 and 1.81 respectively, while other standard deviation results did not exceed 2. This meant that something had gone wrong during the experiment. We felt that this unusual increase of the number of leaves might have to do with the fact that not all the salt had properly dissolved in the pond water which might have contained some salt. It also might have been related to the changes in the environment (engineering lab) since it was further out when we set-up the experiment. The above stated reasons are also stated in the section labelled “4.5 Areas for improvement”. After repeating the experiment, we found a different set of results that tallied with our hypothesis (as seen from Table 2 and Figure 6.1). Hence, we concluded that the above stated reasons were indeed the cause of the anomalies stated.

4.2 Key findings
The key findings in this experiment is that duckweed is able to survive/sustain itself in water with 0.010 grams/millilitre of salt added (2.00g of salt in 200 ml of water). When the amount of added salt increased to 0.015 grams/millilitre (3.00g of salt in 200 ml of water), the duckweed were unable to survive and started to wither (as seen from how the leaves turned white and yellow) and die. However, there is a stunted growth in the duckweed which applies to every set-up including the set-up with 0.010 grams/millilitre of salt (2.00g of salt in 200 ml of water). We deduced this from the fact that the duckweed in the control set-up multiplied from 8 to 18 duckweed while the set-up with 0.010 grams/millilitre of salt did not or hardly had any change in the number of duckweed. As such, duckweed is an effective bioindicator to a certain extent.

4.3 Explanation of key findings

The key findings mentioned in 4.2 shows that duckweed can indeed be a very useful bioindicator as they are not very tolerant towards pollutants. This means that we can identify polluted water bodies by observing if there is any negative growth of the duckweed in water bodies. Negative growth includes the yellowing and whitening of the leaves and stunted growth. When the plant shows these signs, the person observing the plant can then deduce that the water is contaminated in one way or another.

Our findings are also backed up by an article by yilmaz, D. D. (2007), which mentioned that at high salinity (375 and 500molm−3 NaCl), they observed that there was a decreasing growth rate with increasing water salt level. Especially, at a salinity of 500molm−3 NaCl, the net growth rate became negative (yilmaz, D. D., 2007). Additionally, growth rate of roots was significantly reduced at salinity of 250molm−3 NaCl and significantly reduced at 500molm−3 NaCl compared to a freshwater control (yilmaz, D. D., 2007). Biomass production was inhibited by an increase in salinity (yilmaz, D. D., 2007). The model has suggested that salinity is an important factor in limitation of growth of L. gibba (yilmaz, D. D., 2007).

4.4 Evaluation of Hypothesis

According to the second data sets recorded, our hypothesis was not very accurate with the number and health of the leaves depleting as the salt content rises. We were correct to guess that the health of the duckweed would deteriorate as the amount of salt added increased. However, the duckweed were still able to sustain themselves in water that had 0.010 grams/millilitre of salt added although they did not multiply and grow that was seen in the control set-up and neither were the leaves very healthy. Thus, although our hypothesis is mainly correct, the duckweed with 2.00g of salt in 200 ml of water was able to sustain itself. However, if it was to be compared to the control set-up, one would see a difference in the number and health of the duckweed between the two set-ups.

Other than the fact that the duckweed were less healthy and many of their leaves wilted when salt was added, we also found that they actually produce rapidly. Within the seven days of the experiment day (counting the days we started and ended the experiment, the duckweed in the control set-up grew from 8 to 18 leaves with an increment of 10 leaves in total. This shows that 10/7 of leaves grow per day, henceforth, it shows that duckweed grow rapidly, a trait that would strengthen its usage as a bioindicator (contrast between the growth of the duckweed).

If we look at the article written by yilmaz, D. D. (2007), we will find that the results tally with our own findings.  In our experiment, there was a downward trend as more and more salt was added.  The higher the salt concentration, the lower the number of duckweed leaves.  Likewise, the article by yilmaz, D. D. (2007) mentions that salinity is an important factor in the limitation of growth of duckweed (yilmaz, D. D., 2007).

4.5 Areas for improvement

During the experiment, we experienced countless difficulties with the handling of the shrink wrap. This is not only due to the fact that we were not very proficient in using this item but also the fact that the containers were filled almost to the brim. This made wrapping up the containers a very difficult and tricky job for us. Not only that, because we were careless when poking air holes, the shrink wrap was loose and some parts fell into the water, affecting the water and salt constitution. After the experiment, we realized that we should have shrinked wrapped the container then poked holes using a thin and long compass. This prevents the shrink wrap from curving downwards while ensuring more consistency throughout the experimentation.

Another issue was that not all the salt managed to dissolve in the pond water. We guessed that this was due to the fact that the pond water might also constitute of some salt. Since we were not able to run any tests on the pond water, we were not able to check whether our guess was accurate as there was a lack of time. In the future, we should run tests on the water being used in order to have a better consistency which would in turn ensure more accurate results. We should also do a more in depth research on all the variables in the experiment in order for it to be more reliable. This is to prevent such occurances that would possibly affect the end result of our experiment. Therefore, to prevent such issues in the future, we find it necessary to do a more in depth research in every aspect of the experiment.

One more area of improvement for our experiment is the location of the experiment. For this experiment, all the set-ups were kept in the engineering lab. Since the lab is used and lights are switched off and on, the temperature ranges from time to time with different conditions of the surroundings, there were a lot of variables we did not take in account and into consideration. Hence, we feel that we should have done the experiment in a controlled environment where there will be less outside factors affecting the growth of the duckweed. This ensures the results of the experiment to be more reliable and more accurate than right now. Such an environment can be stimulated by placing the duckweed set-ups in a box or in a closet where it is closed off from everything else. Therefore, no or fewer natural elements will affect the results of our experiment, delivering a more accurate result. The results would also be more accurate as there will be little to no changes in the surroundings.

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