To what extent do different conditions affect the growth of duckweed (Lemna) over time?

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To What Extent Do Different Conditions Affect the Growth of Duckweed (Lemna) over Time?
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Background
The growth of aquatic plants depends on some factors prevailing in a pond. These factors include temperature, pH, dissolved oxygen and light intensity (Seymour 2). Sensitive plants such as duckweed respond quickly to change in any of these factors, either by increasing when conditions are favorable or reducing when these factors become limiting. Duckweed represents the smallest ever known angiosperms.
In this study, the aim is to investigate the extent by which different conditions affect the growth of duckweed (Lemna gibba) over time. This study holds that some of these factors are dependent on other, and this necessitates establishing the measure of pH and dissolved oxygen.
Research Question and Hypothesis
Research Question
To what extent do different conditions and temperatures affect the growth of duckweed (Lemna) over time?
Hypothesis
Water temperature in a pond has a significant influence on both the pH and dissolved oxygen conditions in a pond. If the water temperature is essential, then it can be used in controlling the growth of duckweed in pond management.
Methodology
Variables
A variable is a measurable trait that can indicate changes in the status of a condition that is tested (Wright & David 3). In this experiment, three types of variables namely independent, dependent, control and extraneous will be used.

Independent variable in this study is the temperature in of the pond water. This temperature rises and falls based on air conditions or light intensity. Although in different settings, such as in a river or ocean temperature can depend on the depth, light intensity, and length of day, it is not possible to isolate or consider these factors in this study. It is for this reason that temperature remains independent rather than dependent.
Dependent variables in this experiment are the dissolved oxygen (DO) and pH. These variables tend to vary in response to changes in others. Therefore, DO, and pH are the subjects to changes in temperature. Based on some literature reviews, this study holds that the high temperature will results in low level of DO but increased nutrient contents.
Another type of variables is the control factors. These conditions are kept the same for the sake of fairness in the experiment. They include water samples, the depth of the pond, and the location of the ponds. Finally, there are extraneous variables. These are factors that influence the relationship between independent and dependent variables. For example, the number of fish in a bond can affect the level of DO and pH. In these experiments, extraneous variables include the bacteria, worms, and other organisms in the water that could interfere with conditions. Unfortunately, this study does not have the control of these factors and hence will only work with what it can.
Materials and Procedures
The objective of this experiment is to investigate how various water conditions and temperatures affect the growth of ducked in a pond. This study will test water temperature, the nutrients (phosphorus and nitrates) contents, and dissolved oxygen to achieve this objective. The following materials will be used.
Beakers
Water samples from the ponds
Dissolved oxygen sensor
Phosphorus meter
And digital nitrate meter
Methods
The methods used in this experiment involved lab testing and observation. In this experiment, two small homemade ponds previously used for growing duckweed for rabbits were used to obtain the measurements. Attaining these measurements, however, took three weeks. Different conditions of the water in the two ponds were taken, first at the beginning of the experiment, the second week and finally after two weeks. The changes in the growth of the duckweed in the ponds were observed and recorded.

Photo1 Showing Homemade Pond A and Pond B.
The reason for choosing these small ponds was to gain more control of the experiment and also because of the ubiquitous nature of the duckweed and ease of husbandry. Their height and depth of the water could be determined easily, and light intensity was uniform in the two ponds since they were exposed to light at the same time and length (in hours) per day. More importantly, any change in the growth of the duckweed was easily observable in 14 days. Moreover, the experiment was manageable and easy to monitor any changes.
Since this study sought to investigate the effects of water conditions and temperature to the growth of duckweeds, a shield was placed above to protect it from sunlight from 6:00 am to about 11:00 am. In this case, Pond A did not experience the morning light intensity and warming from the sun in the best part of the morning. Pond B remained uncovered every day.
At the beginning of the experiment, the water temperature, DO, and pH was taken and recorded. Then an equal amount of duckweed species was introduced in the two ponds and left for observation. Only Pond A could be covered and uncovered for five hours to differentiate the temperature and light intensity conditions in the two ponds. After seven days, I took another reading and recorded the water parameters. This procedure was finally done on the evening of the last day, and changes in growth of duckweed noted.
The Lab Tests
Lab tests were conducted three times; first at the beginning of the experiment, the second time after seven days and lastly after 21 days. During the lab test, water from each pond was taken, and filtered to remove insoluble impurities. Then 50 mm of water was put in the beakers to test for DO and pH. Then the phosphorus, nitrates and DO meters used to measure each parameter and recorded.

Photo2 Showing nitrate level in Pool B after seven days.
A thermometer was dipped into the pond to measure the temperature of water at the site since it was ineffective to measure the samples in the beaker in which case wrong values would have been obtained. The results at each instance are recorded in the table below.
Temp in OC Dissolved Oxygen mg/l Phosphorus
mg/l Nitrate Observation
Initial test: Pool A
Pool B 22.5
22.5 26.2
26.3 1.7
1.7 6.1
6.1 A handful of duckweed in each pond
Test after 7 days:
Pool A
Pool B 23.2
25.2 23.5
17.8 1.4
1.0 5.2
4.7 Duckweed in Pond B was more than those in Pond A
Test after 14 days:
Pool A
Pool B 24.6
27.4 19.4
12.3 0.9
0.7 4.4
1.2 Duckweed in Pond B almost doubled. Unlike those in pond A
Table 1 showing the results of the lab test
Duckweed population growth
Week pH 4 pH 7 pH 10
0 10 10 10
1 16 14 19
2 22 19 31
3 37 32 54
Discussion and Conclusion
Water temperature is an important factor that determines the duckweed’s growth rate. In the study, the temperature was found to increase even in pond A that did not experience enough exposure to sunlight. Nitrates and phosphorus were also found to decline with time while the levels of dissolved oxygen reduced as well. Therefore, it is correct to conclude that a rise in temperature is associated with a decrease in both DO and nutrients.

The level of pH in the water proved significant, as the change in the population of duckweed was very high. From the graph, it is evident that the growth rate is higher at lower pH levels in the water. pH relates to the level of nutrients in the water. Thus, increase in nutrients increases the growth rate of the duckweeds. Therefore, at pH of 10, the duckweed is at the optimum level of growth thereby growing the fronds.
The cause of this relationship is that duckweed needs high levels of nutrients and dissolved oxygen to grow. These factors decline as these tiny plants increases in number and size. The rise in temperature is caused by both the energy from the sun and the breakdown of nutrients in the water. Due to reduced exposure to sunlight in Pond A, duckweed growth, nutrients and DO reduction, and temperature rise occurred slowly. Unlike trend was observed in Pond B where duckweed doubled after every two days, temperature hiked, and other conditions reduced swiftly during the two-week period.
As nutrients break down, there is a production of energy and released into the water. This explains why eutrophication causes the massive growth of aquatic plants, high water temperatures and increased turbidity of water in a lake or pond. Therefore, the growth of duckweed can be controlled by monitoring temperature of the water and other conditions such as dissolved oxygen and nutrient.
In a closed pond, these conditions necessary for the growth of weed can deteriorate as the population increases leading to toxic conditions. In such situations, some nutrients entering the pond should be reduced, excessive weed removed and some mechanical interventions introduced to increase the amount of DO (Swistock 5). Such mechanical intervention can be the creation of splashing or use of a propeller. With such management techniques, it is easy to reach the maximum sustainable yield of duckweed as fodder crops.
Works Cited
Seymour, Matthew. “Duckweed – How To Grow And Care For Duckweed In The Home Aquarium – Aquarium Tidings.” Aquarium Tidings, 2014, https://aquariumtidings.com/duckweed/.
Swistock, Bryan R. “Duckweed, and Watermeal.” Penn State Extension, 2017, https://extension.psu.edu/duckweed-and-watermeal.
Wright, Linda L., and David A. Lake. “Basics of Research: Variables.” Pt.Armstrong.Edu, 2015, http://www.pt.armstrong.edu/wright/hlpr/text/3.1.variables.htm.