Plant Population Growth Lab BIOL 220M Pennsylvania State University Jacob Cohen 4/15/2013
Cohen 1 Introduction: In nature, everything is connected; all natural processes and organisms are intrinsically linked. This is especially true for aquatic environments which are generally more fragile than terrestrial environments. Aquatic environments are generally full of plants, animals, and microorganisms and can be some of the most diverse environments on earth. This also means that it is easier to disrupt the balance of these environments. Recently, the most imminent threat to aquatic environments is pollution, and more specifically, pollution from the fertilizers used on large scale farms and agricultural sites. This uncontrolled stream of fertilizers, nitrogen and phosphorus being the two most problematic chemicals, causes major problems in water systems by encouraging algal blooms. These algal blooms consume all the oxygen in the water, killing off almost all other organisms inhabiting the immediate area of the algal bloom. This is a serious issue because algal blooms are becoming much more prevalent across the Chesapeake Bay Watershed (the water system that includes Central Pennsylvania). 1 One way to remove these excess nutrients from the environment is through the use of special types of plants. 2 In the case of the Chesapeake Bay Watershed, two specific plants are being used: Lemna minor and Salvinia minima. To effectively use these plants, scientists must introduce them to polluted areas where they would take up the excess nutrients. The plants would then be collected and turned back into fertilizer. This process is known as phytoremediation. 1 The purpose of the following investigation is to answer a number of questions about how effective the introduction of Salvinia and Lemna would be. It is already known that Lemna minor and Salvinia minima take up extra nutrients in the water and incorporate them into their tissues. However, this investigation aims to answer questions such as how the amount of nutrients affects the growth of the plants and how the plants grow in competition with each other. Once these
Cohen 2 questions are answered, scientists will be able to more accurately and quickly soak up excess nutrients in aquatic environments. For the rest of this report, there will be a focus on answering the second question presented: how the two types of plants grow in competition with each other. It is hypothesized that Salvinia minima will be able to out-compete Lemna minor because Salvinia minima plants are physically larger and will be able to take over the space in the cups faster. This will leave no space for the Lemna minor to absorb the necessary amount of sunlight for the photosynthesis necessary to survive. Materials and Methods: This experiment is one took multiple weeks to complete; however, setting it up took less than an hour. The set up differs slightly, depending upon what experiments are being conducted. However, in all cases, plants were added to 10 oz. plastic containers full of artificial pond water. It was the number and species of plant that differed. In this case, there were two containers, both filled with Salvinia minima. One container had twelve individual plants and the other had twenty-four, these two containers were considered the control and the results of their population growth were compared with the results of other experiments. The second experiment that needs to be set up measured the effects of competition between the two species. This experiment had three containers each filled with twelve individual plants of Salvinia and twelve Lemna plants. When the plants were being removed from the stock culture and placed in the individual cups, it was necessary to be careful that no Lemna was sticking to the Salvinia so that the correct number of plants were placed in the containers. Therefore, the Salvinia plants were all dunked in another dish of water to remove any excess Lemna. For Salvinia, an individual plant generally consists of more than one leaf, but after adding the plants to the containers, it is necessary to count the
Thalli Cohen 3 number of individual leaves, also known as thalli, so that the data can be used later on. When this was complete, the containers were taken to the greenhouse so that the plants could continue to grow in a controlled environment. It was necessary to add new water to the containers every few days to make sure that the plants did not dry out. It was also essential to change containers quite regularly to prevent the build-up of algae which could have out-competed both the Salvinia and Lemna, leaving no data for collection. For the next four weeks, the thalli of the plants in the containers were counted weekly (every Monday) for data collection. Results: Table 1 Number of Salvinia thalli present after each week. Week 1 Week 3 Week 4 Week 5 (Salvinia) 25.66666667 60.3333 171 362 Salvinia Monoculture (12) 27 68 183 393 Salvinia Monoculture (24) 54 123 322 559 600 Thalli vs. Time 400 200 0 0 2 4 6 Time (in weeks) (Salvinia) Salvinia Monculture (12) Salvinia Monculture (24) Figure 1 Comparison of the number of thalli in the containers. The table and graph above illustrate the difference in the growth patterns of the containers. It should be noted that the number of thalli for the Salvinia Monoculture (12)
Natural log of Thalli Cohen 4 container and (Salvinia) containers are almost exactly the same, with the former being slightly greater. Natural log of Thalli vs Time 7 6 5 4 3 2 1 0 y = 0.597x + 3.2853 R² = 0.9706 y = 0.6732x + 2.4867 R² = 0.9715 0 2 4 6 Time (in weeks) Monoculture 12 Monoculture 24 Linear (Monoculture 12) Linear (Monoculture 12) Figure 2 The natural log of the number of thalli plotted against time. This graph helps us find r max, or the maximum rate of increase for a population. This is simply the slope of the two curves plotted. In the case of Monoculture 12, r max = 0.6732 and for Monoculture 24, r max = 0.597. From this data we see that the maximum rate of increase for Monoculture 12 is greater than that of Monoculture 24. Table 2 The growth rates (λ) of every experiment for each week. Week 1 Week 3 Week 4 2.35065 2.83425 2.11696 Salvinia Monoculture 12 2.42308 2.85714 2.12222 Salvinia Monoculture 24 2.54167 2.95082 2.09444
Growth Rate Cohen 5 3 2.5 2 1.5 Growth Rate vs. Thalli Monoculture 12 y = -0.0028x + 2.6733 Monoculture 24 y = -0.0026x + 2.6361 y = -0.0029x + 2.72 1 0.5 0 0 100 200 300 400 Thalli Linear (Monoculture 12) Linear (Monoculture 24) Linear (Salvinia Competition) Figure 3 A comparison of the three experiments and their λs. Table 3 The carrying capacities of the three experiments. Carrying Capacity 598 Salvinia Monoculture 12 593 Salvinia Monoculture 24 629 Using the growth rates of the individual experiments we are able to find equations for the lines of λ vs. Time. These can be used to find the carrying capacity by solving for y=1. In this case, the carrying capacity for all three is similar but the and Salvinia Monoculture 12 are almost identical. Table 4 The degree of competition between Salvinia Monoculture 12 and Salvinia Competition. Week 1 Week 3 Week 4 Degree of Competition 0.16787-0.143078 0.030582 The degree of competition was found by taking λ monoculture λ competition. For every week the degree of competition is extremely close to 0, meaning that the growth curves and λ vs. time curves are very similar.
Number of Thalli Cohen 6 Table 5 Average thalli number of Salvinia and Lemna per week. 1 3 4 5 (Lemna) 12 29.96 51 68.66667 (Salvinia) 25.66667 60.33333 171 362 Thalli vs. Time 400 350 300 250 200 150 100 50 0 0 2 4 6 Time (in weeks) (Lemna) (Salvinia) Figure 4 Comparison of the average number of thalli per week of each species when grown together. There was no data collected for the number of Lemna thalli for week 3, so the number 29.96 was found by using Microsoft Excel to find the exponential equation of best fit: and plugging in x = 3. Using this data, we will be able to find the growth rates of each species when they are grown together. Table 6 The weekly growth rates of each species when grown in competition. 1 3 4 (Lemna) 2.496666667 1.70227 1.346405 (Salvinia) 2.350649351 2.834254 2.116959
Growth Rate Cohen 7 Growth Rate vs. Thalli 3 2.5 2 1.5 1 0.5 0 y = -0.0028x + 2.6733 y = -0.0291x + 2.7513 0 50 100 150 200 Number of Thalli (Lemna) (Salvinia) Linear (Salvinia Competition (Lemna)) Linear (Salvinia Competition (Salvinia)) Figure 5 Plot of the weekly growth rates of each species grown in competition. Table 7 The carrying capacity of each species when grown in competition. Carrying Capacity (Salvinia) 598 (Lemna) 60 The growth rate of Salvinia is significantly larger than that of Lemna. Likewise, the carrying capacity of Salvinia is almost ten times larger than the carrying capacity of Lemna. Discussion: The results suggest that there is no significant influence on the growth of Salvinia when it is competing with Lemna. When the growth curves for and Salvinia Monoculture 12 are compared, there is almost no discernible difference. The same can be said whenever the data from and Salvinia Monoculture 12 are compared. In fact their carrying capacities differ by only five thalli, an almost irrelevant difference. The close correlation between these two sets of data suggests that there is no competition between Salvinia and Lemna because Salvinia grown with Lemna is able to grow as if the Lemna were not present at all. This means that Salvinia completely out-competed Lemna. This finding is supported by the
Cohen 8 results of other experiments done by another group of students who were studying what happens to Lemna when it is in competition with Salvinia. This conclusion suggests that it is unnecessary and pointless to use both Salvinia and Lemna in the same body of water for phytoremediation, as the Salvinia would simply outcompete the Lemna to the point where there was only Salvinia present in the water. This would defeat the purpose of putting both species in the affected water in the first place. There were four other experiments performed to explore the effects of excess nutrients on each species. The results from these experiments show that there is no effect upon the growth of Salvinia or Lemna when nitrogen was added. When phosphorus was added, Salvinia experienced an increase of 30% in r, but Lemna showed no increase in r. These results lead to the conclusion that when there is excess nitrogen in a freshwater system, either Salvinia or Lemna can be used to absorb the nutrient because neither plant will grow faster or have a higher r max value. However, when there is excess phosphorus in the environment, Salvinia should be used to take in the extra nutrients because it experiences a 30% increase in r while Lemna experiences no increase in r. This mean that if Salvinia is used it will be significantly more effective at soaking up the excess phosphorus. Research performed by B. D. Tripathi and Suresh C. Shukla has showed us that using plants as phytoremediators is both cheap and effective. 3 Further research done by K.R. Reddy and W. F. De Busk has showed how effective specific plants are as phytoremediators. Their study included both a species of Salvinia and Lemna, and found that Lemna was able to take up more nitrogen and phosphorus than Salvinia. 4 This research suggests that instead of Salvinia, Lemna should be used to soak up excess nutrients from aquatic environments.
Cohen 9 To test whether or not this research is useful for practical purposes, more experiments could be done in the field. These experiments would use a much larger sample size to see if the results are similar to those found using small-scale experimental techniques. Other experiments could be done to find the amount of nutrients soaked up by the plants before they are unable to take up any more. There are many different directions in which this research can be taken and the more that is learned, the better we will be able to understand the aquatic systems that we depend upon for sustenance. References: 1. Hass C, Burpee D. (2013) A Preliminary Study of the Effects of Excess Nutrients and Interspecies Competition on Population Growth of Lemna minor and Salvinia minima. Penn State Department of Biology Laboratory Manual. 2. Sharma, H. K. et al (2011). Removal of contaminants using plants: A review. Current Trends in Biotechnology and Chemical Research, 1: 11-21. 3. B. D. Tripathi & Suresh C. Shukla. (1991). Biological Treatment of Wastewater by Selected Aquatic Plants. Environmental Pollution, 69: 69-78. 4. Reddy, I.R., and DeBusk, W.F. (1985). Nutrient removal potential of selected aquatic macrophytes. Journal of Environmental Quality, 14: 459-462