The Effect of Phosphorus Concentration on the Intrinsic Rate of Increase for Salvinia minima Aaron Jacobs Partners: Andrew Watts Derek Richards Jen Thaete
Introduction: Salvinia minima is an aquatic fern, originally from South America, that has invaded multiple states in the US. They are composed of horizontal shoots that connect multiple plants together and floating leaves. The leaves, also known as fronds, are generally an oval shape and are covered with coarse, white hairs. The hairs main purpose is to act as a water repellent to keep the leaves from sinking when covered with water. Salvinia grows in ponds, lakes and slow moving streams. This invasive species is known to block waterways by covering them with a thick layer, some even 20cm thick, which can block boating and can have a serious impact on the ecosystem of the waterway. The layer of thick vegetation blocks sunlight from deeper plants and organisms and decreases the oxygen concentrations, which limits oxygen for fish and other organisms living in the aquatic habitat. When the masses of plants die, they lower oxygen levels even further. Salvinia are also responsible for clogging hydro power stations as well as irrigation systems 1. Salvinia aren t all bad, they are showing potential as a plant for aquatic phytoremediation. There are few reasons why Salvinia are being more closely examined for phytoremediation. Phytoremediation is the process of using plants as a tool to remove harmful pollutants from the environment 2. Salvinia have a wide habitat range and can survive in temperatures from -3 C to 43 C. They are also capable of outgrowing duckweeds, due to their high reproductive ability. They have a very high growth rate, which is why they are so dangerous in waterways, but this same quality that is viewed as being harmful could potentially be used in a positive way. It also has larger leaves, making it easier to harvest than duckweeds 3. Salvinia also work as a buffer for ph, they have the ability to change a rather acidic environment into a neutral one in an extremely short amount of time. This means that they could be used for 1
both the removal of excess and potentially harmful nutrients and the neutralization of ph in waterways. Salvinia also possess another important quality, they store all the nutrients into their leaf tissues, this would make them a great plant to harvest and be used as a potential fertilizer 4. This lab will examine population growth using exponential models, more specifically geometric growth models since we are dealing with discrete time intervals and not a continuous stream of data, as well as logistic growth models to analyze the data. When a population enters a new habitat, that is rich with nutrients and the population density is low, the population will follow a more exponential-like growth. This is due to density independent factors like the excess nutrients, light intensity and the space available to grow. At some point though, a population will be limited by the amount of individuals, these are density dependent variables. Density dependent variables are limitations brought on by the close proximity of the individuals living in a confined and limited environment. When these types of factors are at work, a population will begin to decrease in growth, reaching that population s carrying capacity, and will follow a logistic model curve more closely 5. The purpose of the first part of this experiment is to examine the natural growth rate of Salvinia and use this as a control group for comparison with the second part of the experiment as well as comparing the differences scene when starting with different initial population sizes. The purpose of the second experiment is testing whether adding nutrients, specifically phosphorus, will alter the population s ability to grow compared to the control. I hypothesize that the Salvinia in the plain water, with no added nutrients, will show a lower growth rate, and a lower carrying capacity, than the Salvinia with added nutrients, due to the lack of extra nutrients. Also, the control population of 24 will grow faster than the control population of 12 plants, but both will come to the same carrying capacity in the long run. 2
Materials and Methods: The experiment involves five separate plastic containers that hold the populations of Salvinia plants. There are two control populations; both only contain water with no extra phosphorus added. One of the controls started out with 12 Salvinia plants and the other control container initially started with 24 plants. The independent variable in experiment one was the amount of plants initially started with and the dependent variable being the resulting growth rate. The other three containers housed the second part of the experiment; all three of these containers initially started with 24 plants. All of these containers were then placed in a greenhouse, which was kept at 21 C during the day and 18 C at night, to increase growth and to keep the temperature consistent. The main difference between experiment two s setup compared to experiment one, was that experiment two containers had 2mL of phosphorus added to each of them. The independent variable of experiment two was the addition of phosphorus, making the dependent variable the resulting growth rates. 2mL of phosphorus were then added to the experiment two containers once a week after that initial addition. All of experiment two was identical; this was done to give a better pool of data that could be averaged together and compared. After the plants had been added to the containers, the amount of leaves, or fronds, were counted. Counting days occurred on the same day once a week, starting the second week, day 14 of the experiment, until the 28 th day of the experiment, which was the last day of counting. Due to evaporation, the plants needed water added three times a week, also due to the accumulation of algae the containers needed to be cleaned once a week. After data collection, there were various data analysis techniques that were utilized, specifically the geometric growth model and the logistic growth model. The geometric growth model was used because the data was collected at discrete time intervals. The logistic growth model was used since the 3
populations are limited by their environment, they will reach a limit of growth, which is known as the carrying capacity. The exponential growth model was also used to find r max values, under the assumption of continuous growth. Lastly, the formula to find the geometric rate of increase was used so that the carrying capacity could be calculated for the various populations 5. Geometric Rate of Increase: λ =!!!!!! Exponential Growth Model:!"!" = r!"#n Logistic Growth Model:!" = r!"!"#n!!!! Results: Figure 1. 450 Population Growth of Salvinia Control Populations Number of Fronds 400 350 300 250 200 150 100 50 Count(12) Count(24) 0 0 5 10 15 20 25 30 Time (Days) Figure 1. Scatter plot showing the growth rates of Salvinia control populations. This graph depicts the control population, the blue line representing the container starting with 12 Salvinia plants and the red line representing the container that started with 24 Salvinia plants. 4
Figure 2. Ln(N) vs. Time for Control Populations ln(n) 7 6 5 4 3 2 1 y = 0.0763x + 3.9391 y = 0.0923x + 3.2195 Count(12) Count(24) Linear (Count(12)) Linear (Count(24)) 0 0 5 10 15 20 25 30 Time (Days) Figure 2. Scatter plot showing the natural log versus time for the control populations. This graph was made to use the slopes of the lines in order to find r max. Figure 3. Lambda Values 4 3.5 3 2.5 2 1.5 1 0.5 0 Lambda Values vs. N t for Control Populations y = - 0.0882x + 3.4867 y = - 0.0062x + 3.0739 Count(12) Count(24) Linear (Count(12)) Linear (Count(24)) 0 50 100 150 200 250 300 Number of Fronds Figure 3. Scatter plot of lambda versus population size for the control populations. This graph s best-fit line equations were used to find carrying capacities of the populations. 5
Figure 4. Population Growth of Salvinia for Experimental Phosphorus Populations Number of Fronds 700 600 500 400 300 200 100 0 0 5 10 15 20 25 30 Time (Days) Count(Phosphorus) Count(24) Figure 4. Scatter plot showing the growth rates of Salvinia for the control and experimental populations. This graph shows a red line representing the control container that started with 24 Salvinia plants and a pink line representing the average of the three containers for the experimental phosphorus plant populations. Figure 5. Lambda Values 3.5 3 2.5 2 1.5 1 0.5 0 Lambda Values vs. N t for Experimental Phosphorus Populaitons 0 100 200 300 400 500 Number of Fronds y = - 0.0043x + 3.2999 y = - 0.0062x + 3.0739 Count(Phosphorus) Count(24 control Linear (Count(Phosphorus)) Linear (Count(24 control) Figure 5. Scatter plot of lambda versus population size for the control and experimental populations. This graph s best-fit line equations were used to find carrying capacities of the populations. 6
Table 1. Carrying Capacities Control Group (12) Control Group (24) Experimental Group (phosphorus) 235 335 535 Table 1. Chart representing carrying capacities found. The carrying capacities in this table are color coded with the data they came from, these values were found using the best-fit lines from the lambda vs. population size graphs. Carrying Capacity Calculations Control Group (12) Control Group (24) Experimental Group (Phosphorus) y = - 0.0882x + 3.4867 y = - 0.0062x + 3.0739 y = - 0.0043x + 3.2999 Set y=1 and solve for x Set y=1 and solve for x Set y=1 and solve for x 1 = - 0.0882x + 3.4867 1 = - 0.0062x + 3.0739 1 = - 0.0043x + 3.2999 x = 235 x = 335 x = 535 Table 2. Carrying Capacities Using Surface Area Number of Layers 1 2 3 Carrying Capacity 207 414 621 Table 2. Chart showing the carrying capacities found using surface areas. The carrying capacities in this table are based on the layering behavior of Salvinia. Sample Calculation for Surface Area Carrying Capacities K = Surface Area of Container Surface Surface Area of Salvinia Frond K = 5809mm! 28mm! K = 207 7
Discussion: The results showed that Salvinia minima could be a good candidate for phytoremediation. The first component of the results focused on the population growth and how populations of Salvinia react to different initial population sizes as well as the introduction of phosphorus. Figure 1 qualitatively seemed to show that the populations starting at different initial amounts of plants grow at the same rate. This figure also showed that starting with more plants consistently shows a higher amount of plants as time continues. But when this data was more closely looked at it told a different story. Figure 2 showed that the slopes, or r max value, were not the same. This means that the two control populations actually grew at different rates. The results show that the control population that started at 12 grew at a faster rate than the starting at 24 group. This means that my initial hypothesis was wrong, which looking back makes sense. The population that started with 12 plants would have less initial competition and more resources available to each plant, this would allow for a more exponential growth. For more information about the population dynamics of these two groups I found the carrying capacities for the two populations using the best-fit lines from figure 3 and the surface area of the water in the container. Table 2 shows the carrying capacities found for the container using the surface area of the container and seeing how many Salvinia fronds could fit into the space. The main problem with using this method is that it doesn t account for the behavior of the Salvinia; Salvinia has round leaves, which don t fit perfectly together so there are gaps between leaves. Another behavior of Salvinia is that it forms layers under the surface of the water, I tried to account for this by multiplying the carrying capacity for one layer by 2 and 3, this is shown in table 2. A better method for finding the carrying capacity is using the best-fit line from figure 3. The carrying capacity for the control group with 12 initial plants, as shown in table 1, was 235. This 8
was compared to the carrying capacity of the control group starting at 24 plants, which was 335. My initial hypothesis was that the carrying capacities would be the same, which should be seen since the amount of space in the long run is equal, being that both groups are in the same size containers. There are a few reasons that could explain this discrepancy. The trials only lasted for 28 days, this is a relatively short amount of time and the populations may not have been allowed to grow for a long enough time. Another explanation is that there was a build up of algae in the containers, this algae could have been acting as competition for the plants causing them to grow at a decelerated pace. Lastly poor counting could have been a reason for the inconsistency, since a thick layer of Salvinia develops, it can become difficult to count the fonds and some may have been over or under counted. The second part of this experiment looked more into an aspect that could show if Salvinia minima would make a good plant for phytoremediation. The second part of the results showed how adding nutrient, specifically phosphorus, would alter the population dynamics of the Salvinia populations when compared to a control group with no added nutrients. Figure 4 showed a how the population growth of the two populations compared to one another. The control group that started with 24 plants showed a much slower rate of growth compared to the experimental population. Initially the two populations showed the same rate of growth, which could have been to a result of a time lag, but after that the added phosphorus populations grew at a much more accelerated rate. This agrees with my hypothesis that the nutrient rich environment would cause an increase in growth rate. This is a good sign that the Salvinia could be a good candidate as a plant to use in the process of phytoremediation. The reason this is a good sign is because the ability to hold nutrients and grow at an accelerated rate shows that Salvinia would be able to take potentially harmful nutrients out of waterways at an accelerated rate and could then 9
be harvested as a natural fertilizer due to it s ability to hold nutrients. The results also looked more into the long-term affect of adding phosphorus, by taking the carrying capacities of the two populations into account and comparing them. The best-fit lines from figure 5 were used to determine the carrying capacities as seen in table 1. The carrying capacity for the control group was 335 and the carrying capacity of the experimental group was 535. These results make sense since available space and resources limit carrying capacity, by increasing phosphorus the experimental population was able to grow to a higher carrying capacity than the control group, which agrees with my hypothesis. The sources of error for this part of the experiment are the same as for the first part, with the addition of poor measuring of nutrient. It is possible that not enough or to much nutrient was added to the experimental containers, which could have lead to skewed results. The purpose of this experiment was to determine if the Salvinia minima would make a good plant for phytoremediation in waterways that contain elevated levels of possibly harmful nutrients. The results showed that Salvinia minima could work in this situation, but further research will need to be conducted to support these findings. In future experiments, more plants should be tested since a possible source of error was the very small sample sizes. Phosphorus should be further analyzed and tested, but since Salvinia minima showed such positive results other nutrients should also be examined. It may turn out that Salvinia minima can absorb a large amount of various harmful nutrients, making it a great candidate for phytoremediation. 10
Reference: 1. Salvinia minima (aquatic plant, fern). (2012, October 4). Retrieved 4 10, 2013, from Global Invasive Species Database: http://www.issg.org/database/species/ecology.asp?si=570&fr=1&sts=&lan g=en 2. Chaney, R. L. (200, June). Phytoremediation: Using Plants To Clean Up Soils. Agricultural Research magazine. 3. Eugenia J. Olguin, G. S.- G.- P. (2007). Assessment of the Phytoremediaiton Potential of Salvinia minima Baker Compared to Spirodela polyyhiza in High- strength Organic Wastewater. Water, Air, & Soil Pollution, 181, 135-147. 4. Safaa H. Al- Hamdani, C. B. (2008). Physiological Responses of Salvinia Minima to Different Phosphorus and Nitrogen Concentrations. American Fern Journal, 98, 71-82. 5. Hass, C.A., D. Burpee, R. Meisel, and A. Ward. 2013. A Preliminary Study of the Effects of Excess Nutrients and Interspecies Competition on Population Growth of Lemna minor and Salvinia minima In A Laboratory Manual for Biology 220W: Populations and Communities. (Burpee, D. and C. Hass, eds.) Department of Biology, The Pennsylvania State University, University Park, PA. Adapated from Beiswenger, J. M. 1993. Experiments To Teach Ecology. A Project of the Education Committee of the Ecological Society of America. Ecological Society of America, Tempe, AZ. pp. 83-105. 11