1. Introduction

Representatives of the Lemnaceae, the Duckweed family; they are well-adapted for use as bioindicators in water for toxic substances as they multiply rapidly, a significant advantage for this use (L.V.Tsatsenko, 2002). Growing plants and duckweed are not very difficult. Moderate conditions of temperature and light and a liquid medium with the necessary nutrients are essential for healthy growth. Fortunately, duckweeds are adaptable to many different conditions and are easy to cultivate. (mobot.com, 2005). It is also used in environmental remediation (which refers to reducing pollution from a contaminated source. It also can be based on human health and ecological risks [Wikipedia, 2016]) because it takes up excess nitrogen and phosphorous from the water (Apfel, A. 2016). Duckweed is also cheap and simple to grow with little maintenance required. To top it off, they are also fast-growing and fast-changing so changes can be identified within a short period hence reducing the need for a long wait. According to Laurence Girard (2016), duckweed multiply quickly, aggressively invading ponds and bodies of water. This means that the duckweed will increase in number from before to after the experiment. In his book, Wendeou, S. et al.(2013) wrote that duckweed have been used to remove pollutants from different qualities of domestic wastewater and agricultural wastewater. He also mentioned that they grow vigorously. Hence, we have identified duckweed to be an effective bioindicator.

“Water bodies are a major recipient of an extensive array of waste produced by man. These may be discharged directly into watercourses by sewers or pipes from factories or be washed down from agricultural or urban areas, particularly, after heavy rains. Under rather exceptional circumstances, water bodies may become significantly contaminated by the atmospheric deposition of pollutants.” as quoted from Rao, M.K. (2007), author of the book Environmental Pollution and Toxicology. When the above mentioned situations occur, it might affect the ecosystem and even us. If people were to drink the contaminated water, they would be poisoned by the contaminants. If fish living in the water bodies or any other animals were to ingest the water, both directly or indirectly. This would indirectly affect us, provided we eat these animals.

Wendeou, S. et al.(2013) wrote in the chapter of his book named “Influence of Salinity on Duckweed Growth and Duckweed Based Wastewater Treatment System” on how he and the other authors conducted an experiment on the effect of wastewater dissolved salt content on the growth of duckweeds when a duckweed system is employed for municipal wastewater treatment. Although their experiment was slightly different, their experiment also involved pollutants and observed the growth of duckweed. They found that there was a negative effect of salinity on the growth rate of the duckweed. As the conductivity rate increased (amount of salt in the experiment), the growth rate declined. However, as mentioned before, this experiment is different from the one we conducted.

Both the above mentioned information in paragraphs 2 and 3 talk about water pollution. In the second source, the chapter actually stated that “The characteristics that make this plant grow rapidly in polluted waters make it an ideal candidate for large-scale application for nutrient removal and water purification.” The above sentence shows that duckweed are quite hardy which might prove an issue if the duckweed can resist the salt. However, the results from the chapter written by Wendeou, S. et al.(2013) does show that the growth rate of duckweed did decrease as the salinity levels rose.

If our experiment gives us a positive result, it would prove that duckweed are effective bioindicators. This would be useful in the area of water pollution detection because when using duckweed, no machinery is required and as mentioned before, duckweed require minimal resources to grow. Also, if the duckweed show changes in a short period of time, it would also strengthen our claim. This is because an early warning, even by an hour might save many lives or cause less problems to the people directly or indirectly affected. One example where water pollution led to deformities was the Minamata Disaster in Japan. This set of information was stated in the abstract. To add on, the factory passing out the industrial waste into the Minamata Bay moved their pipes and released the contaminants into Shiranui Sea as there were no charges pressed (Tsuda, T., et al.(2009)). Tsuda, T., et al.(2009) stated that the government did not press charges as there was no certainty to the contaminant type in the water. In December 1959, the factory producing the industrial waste installed a purification system in their pipes that discharged the industrial wastewater as written by Tsuda, T., et al.(2009). Many residents believed that the contamination of the water bodies would cease, but the system did not remove the contaminant effectively, and the residents continued to be exposed to the contaminants (Tsuda, T., et al.(2009)). Therefore, we can conclude that many people were affected by this disaster.

In conclusion, if our experiment is a success, health authorities or researchers would be able to determine areas of pollution in water bodies, preventing or minimizing the number of such large-scale disasters in the future.

Fig. 1.1: A diagram showing the conditions necessary for the growth of Duckweed with reference to paragraph 1 of Introduction.

1.1 Research Questions 

The following are our research questions:

  1. Is Duckweed an efficient and dependable bioindicator?
  2. Do government agencies such as the National Environmental Agency (NEA, Singapore) use duckweed to test the health of water bodies?
  3. How can we prove that Duckweed will indeed prove as a good bioindicator? (fast-reacting which means it shows obvious changes to the naked eye)
  4. Where can the Duckweed be used at?

     1.2 Hypothesis

Our hypothesis is that as the salt content rises, the number of leaves the plant has would reduce. We also predict that the leaves will become yellowish and die eventually. Hence, the higher the amount of pollutants present in the water, the lower chance of the duckweed surviving with a lower number of leaves.

The independent variable is the amount of salt in mixed with the water or the salt to water ration. Refer to figure 4.1 for the values. The amount of salt mixed is not affected by any other variable in the experiment. As such the amount of salt added is predetermined. Also, this number will not change throughout the experiment.

The dependent variable is the number of Duckweed. Although the number of Duckweed is predetermined at 8 (of around three leaves per plant), the number of Duckweed left when the experiment is over cannot be predicted. As such, the number of Duckweed left is directly related to the independent variable which is the amount of salt in each set-up/container. As there are 3 set-ups for every value of the independent variable, we will be taking the average of the set-ups.

The controlled variables are as follows: The plant that we will be using, namely Duckweed as well as the containers that the Duckweed are to grow in. Where the Duckweed are going to grow in, namely the conditions of growth which would be the lab. The amount of water in the set-ups, this is directly related to the independent variable but is not part of the independent variable as it will not change no matter the increment or decrement of the values of the independent variable. The number and size of the holes made on the shrink wrap should be as similar as possible. The stirrer being used should also remain the same while ensuring that all the salt has dissolved. The initial number of Duckweed in the set-ups also have to be the same (unchanging), this is similar but not the same as the dependent variable as the dependent variable refers to the number of leaves left after the experiment and not before.

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