Ecology and Evolution Paper 2014

The effect of increased salt concentrations, specifically sodium chloride and calcium chloride, on the per capita growth rate of Spirodela polyrrhiza

Remy Sass, Bethany Felker, Jennifer Blake

Ecology & Evolution

Thursdays 1:30-4:20, Section 152

Emily Hudson

10 April 2014

Abstract

Population ecology studies the perpetual vacillation of populations in the ecological world. By understanding the concepts of how populations change in response to the environment, better steps can be taken to monitor and predict both population sizes and the effects of the fluctuations. In particular, the response of the small freshwater plant Spirodela polyrhiza to the increase of salt concentrations, specifically increases of CaCl and NaCl, was studied. The knowledge gained in this study can be effectively applied to the study of freshwater ecosystems. It was hypothesized that the higher the concentration of salt, the more negatively the growth rate will be affected. It was also hypothesized that increases in CaCl would more negatively affect the growth rate than increases in NaCl. Various salt concentrations were increased in small populations of duckweed and then were allowed to proliferate for a week. The final population count and growth rates were then analyzed. The results showed that the increase in salt concentration negatively affected the growth rate of S. polyhiza, however, there appeared to be no significant difference between the effects of CaCl and NaCl on the growth rates of the populations. Large concentrations of salt may impede the plants ability to successfully function properly. Therefore, salt concentrations negatively impact the plants growth rate.

Introduction

The world is an incredibly dynamic place. The environment, landscape, and all of the world’s inhabitants are in a constant flux. This concept provides the foundation for the study of populations, or population ecology. Population ecologists observe and analyze changes in population size by investigating how the interactions between the populations and the surrounding environment influence those changes (Molles 2013).

        If a population has unlimited resources and no competition or predation, it has the ability to show exponential growth; nothing will hinder the individuals from continuing to proliferate and from the offspring surviving to reproduce. Realistically, though, resources are not unlimited and competition or predation may be present.  The cap on a population size is due to limiting factors that stunt the continued growth of the population, such as the lack of a particular resource. Carrying capacities, which are the maximum numbers of individuals that an environment and its resources can support, dictate the size of the population when it stops growing exponentially (Molles 2013).

        This experiment studied the effect that the availability of nutrients had on the population growth of the small, freshwater plant Spirodela polyrhiza (S. polyrhiza), more commonly known as duckweed. Duckweed is generally found as dense floating mats in ponds and ditches (Driever et al. 2004), and it prefers conditions with high phosphorous and nitrogen concentrations (Van der Heide et al. 2006). Often, the dense duckweed mats on the water’s surface inhibit the growth of other freshwater plants, such as algae and macrophytes, by restricting the oxygen supply and availability of light. Such inhibitions of growth may greatly reduce the overall biodiversity of the ecosystem. Organisms higher up on the food chain  may also feel the indirect effects of the algae and macrophyte population changes (Van der Heide et al. 2006).

        Understanding the dynamics of population growth for species, such as S. polyrhiza, is vital to the study of populations and their impacts on the environment and vice versa. By understanding how to manipulate the population size of duckweed, the growth and proliferation of other organisms may also be controlled and monitored.

        The purpose of this study was to determine the effects that increasing a highly concentrated and slightly concentrated nutrient in the growth medium, sodium and calcium respectively, would have on the population growth rate of the organism. It was predicted that an increase of 0.1 g of salt, NaCl or CaCl, would negatively affect the growth rate of the duckweed, an increase of 0.05 g of NaCl or CaCl  would have a slight negative effect on the growth rate of the duckweed, and an increase of 0.01 g of NaCl or CaCl would hardly affect the growth rate of the duckweed. These predictions were made based on the concept that plants generally respond to high salt stress by stunting both growth rate and plant size (Maas and Grattan 1999).

Materials/Methods

This experiment took place over a time span of seven days within the Ecology and Evolution lab on the campus of the University of Nebraska-Lincoln. On the first day, 27 plastic 100 mL cups were obtained.  Each cup was filled with 50 mL of distilled water and 50 mL of a Swedish growth medium. The contents of the Swedish growth medium can be found in table 1.

Table 1.  Swedish growth medium concentrations

The 27 cups were then divided into three groups. One group, the control, had nine cups while the other two experimental groups also had nine cups each. NaCl was added in small increasing increments of 0.01 g, 0.05 g, and 0.1 g to one of the experimental groups. Each amount was added to three cups. CaCl was added in the same manner to the other nine cups. No salt was added to the control cups (table 2).

Table 2. Amount of salt added to each sample cup

Group	Control	NaCl	NaCl	NaCl	CaCl	CaCl	CaCl
Cup numbers	Cups 1-9	10-12	13-15	16-18	19-21	22-24	25-27
Amount of salt added	0 g added	0.01 g added	0.05 g added	0.1 g added	0.01 g added	0.05 g added	0.1 g added

After all of the solutions were made, six fronds of duckweed were added to each of the cups and the cups were then placed under a light with a cycle of 18 hours on and 6 hours off for one week.  After the duckweed populations were allowed to grow for one week, the fronds were counted and the amount of water loss was calculated and noted. The results were then statistically analyzed by ANOVA.

Results

Our results supported our hypothesis because the addition of 0.1 g, 0.05 g, and 0.01 g of NaCl F(3, 14) = 5.57, p = 0.0099 (Fig. 1), as well as the addition of 0.1 g, 0.05 g, and 0.01 g of CaCl into the growth medium negatively affected the growth rate of the S. polyrhiza, F(3, 14) = 12.55, p = 0.000295 (Fig. 2).

Figure 1. The averages of 0.1 g, 0.05 g, and 0.01 g of NaCl and CaCl added to the 50:50 water and Swedish growth medium was compared to the control group, showing the number of fronds of Spirodela polyrhiza within each cup.

In addition to ANOVA, t-tests were performed in order to determine the significance of each individual experimental group compared to the control group.  The 0.01 g NaCl (p = 0.01975), 0.01 g CaCl (p = 0.01835), 0.05 g CaCl (p = 0.00004989), 0.1 g NaCl (p = 0.0003722 ), and 0.1 CaCl (p = 0.0002885) all displayed significance when compared to the control.  However, the 0.05 NaCl (p = 0.1499) appeared to have very little significance.  

Discussion

Comparable to much of the research that has been done on the effects of salts on S. polyrhiza population growth, our data indicate that the addition of NaCl and CaCl have a significant effect on the growth of the population.  The results indicate that the duckweed responded negatively to the addition of the salts, causing them to die and/or display brown spotting and streaking.

The death of the duckweed plants, as well as the brown spotting and streaking that appeared on their fronds, was likely due to the high salinity of the environment.  According to Maas and Grattan (1999), plants usually respond to high environmental stress (in this case, the environment being the salt in the growth medium), by stunting their per capita growth rate and frond size. Yilmaz (2006) confers that high salinity environments significantly inhibits the growth rate of the duckweed. High salinity environments cause hypertonic cellular environments, and the plant’s cells shrink and may ultimately die. High salinity environments also disrupt the homeostasis of the plant, as well as negatively affect the metabolic rate (Yildiztugay 2014).

After the S. polyrhiza was allowed to grow for one week, there appeared to be a significant difference in the number and condition of the fronds between the control and experimental groups.  Even the smaller concentrations of the salts that were added resulted in a significant decrease in the population growth rate in comparison to the controls group’s rate, which suggests that the environment heavily influenced the capacity to which these plants could grow.

When the t-tests were performed in order to compare each individual concentration to the control, all of the concentrations except for the 0.05 NaCl exhibited significance.  This odd result could be due in part to chance and/or human error in the data collection.

The chlorine within the salt compounds could have affected the results of the experiment. In order to increase the concentrations of the sodium and calcium, the nutrients had to be added as salts. As the sodium and calcium concentrations increased, so did the chlorine concentration.  Therefore, the sodium and calcium alone may not have caused the brown streaking and spotting on the S. polyrhiza fronds; the chlorine may have played a role as well.  

If this experiment were to be replicated, three factors would be taken into consideration. First, it may be possible that a drastic increase in the amount of salt added could potentially wipe out the duckweed population. This experiment showed that even a minute amount of these compounds could start to kill the plant; which is evident in the quantity of brown streaks and spotting on the fronds.  Therefore, if the environment increased in salinity, the stress would be extremely overwhelming to the duckweed, which may result in a sharp decrease in the per capita growth rate and the size of the plant. A similar experiment must be conducted in order to determine a range in maximum and minimum salt concentrations that the duckweed would be able to live in.  

Next, the amount of solution in the cups would also be taken into consideration if this experiment were to be repeated. It is possible that the brown streaks and spots on the S. polyrhiza fronds could have been attributed to the lack of regulation of the evaporation rate. By keeping the amount of liquid stable, evaporation would be eliminated as an outside factor that could alter the results. If the experiment were to be repeated, we would ensure that the solution volume stayed the same by adding water to the cups in order to negate the effects of evaporation.

Lastly, the significance between the increases of calcium and sodium would be analyzed. It would be interesting to further investigate the differing effects that calcium and sodium have on the plants.

This study is relevant to areas outside of the lab as well, especially when discussing the effect of salt on roadside plants.  According to Hudler (date unknown), when salt dissolves in water, the sodium and chloride ions separate from one another and are readily absorbed through the [plant’s] roots.  Therefore, when snow containing salt is plowed off into the roadside landscapes, the salt absorbs a great deal of the water that would otherwise be available to the roots of the plants.  When there is a loss of water and oxygen, the plant’s potential to grow declines.  Overall, it is clear that road salt  has a massive effect on the population growth of plants on the side of the road.  

In conclusion, the addition of the salts had a significant effect on the growth rate and size of the duckweed fronds.  The results collected in our study are in agreement with Maas and Grattan (1999), which suggested that high salinity environments would negatively impact a plant’s growth.  The data acquired from both of these studies will therefore provide insight into what the ideal environment for a duckweed plant is.    

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