Lemna minor, or duckweed, is a small plant that floats on the surface of stagnant water that is usually rich in nutrients. Phosphorus is an important macronutrient in the growth of aquatic plants. We tested the effect of phosphorus on duckweed population growth using a control medium rich in nutrients and compared it to the population growth in a medium that lacked phosphorus. Each treatment had 5 replicates that began in beakers with 40 duckweed thalli, grown in a laboratory setting for 14 days under 140-W lights at room temperature. A regression showed that there was significant growth in each of the populations (P<0.05), and a t-test resulted in a significant difference between the growth rates of each condition on day 3 (t=1.37, P>0.05) and day 14 (t=5.86, P<0.05). The intrinsic growth rate, r, for the nutrient rich treatment was 0.11 and 0.08 for the nutrient lacking treatment, therefore the medium containing phosphorus had a faster growth rate than that without phosphorus. Phosphorus is a limiting factor in the growth of duckweed, and thus affects the population growth rate.
Lemna minor, more commonly known as duckweed, is a member of the Lemnaceae family (Monette et al. 2006). It is a small aquatic plant found floating on the surface of stagnant, nutrient rich waters. Each plant, known as a thallus, is comprised of a small leaf attached to a single rootlet, which resides below the surface of the water. In order for duckweed to multiply, new thalli develop around the edges of the initial leaf, resulting in clumps. The plant population increases geometrically to rapidly cover the surface of still water in a brief period of time. However, this is only apparent if environmental conditions such as light, nutrients and temperature are not limited (Taylor, 2011).
The nutrient concentration of the water in which duckweed resides greatly affects its growth rate. The fast reproduction rate of Lemna minor reflects the idea that duckweed can absorb large amounts of nutrients such as nitrogen and phosphorus (Monette et al. 2006). Due to their free-floating structure, aquatic plants must receive their nutrients from the water column and the atmosphere, as they cannot achieve nutrients from the sediment below. With the exception of carbon, aquatic plants have a reduced possibility of obtaining nutrients as most of their leaf surface is exposed to the atmosphere, rather than the water. Therefore, when nutrients are limiting the growth rate of duckweed suffers immensely (Scheffer et al. 2003).
Free-floating plants are superior competitors in the competition for light but submerged, rooted aquatic plants compete with free-floating plants for other nutrients. Rooted plants can get their nutrients from sediment as well as the water column, reducing the nutrients available to the free-floating plants (Dickinson et al. 1998). Competition for phosphorus increases with population growth, and the availability of phosphorus in the water column is reduced because of the uptake by submerged macrophytes (Scheffer et al. 2003).
Duckweed requires a medium to high level of phosphorus available, as it is an essential nutrient for aquatic plants. It is involved in the mechanisms of photosynthesis, allowing for the production of organic matter in aquatic environments (Fogg, 2003). Therefore, we hypothesized that the growth of duckweed populations in a medium lacking phosphorus will be lower than that in a medium containing phosphorus. The null hypothesis is that the concentration of phosphorus within the nutrient medium does not affect the growth of duckweed, and there will be no significant difference in the population growth rate of the two control groups.
Materials and Methods
In each of 5 100-ml beakers, 90-ml of growth medium were placed (Table 1). The remaining 5 beakers received a medium similar to the control group, but were lacking potassium phosphate. Table 1. Composition of Lemna Minor culture medium. These ingredients were added to dechlorinated, filtered Antigonish tap water (Taylor, 2011).
Chemical Name Formula Concentration (mg/L) Potassium Nitrate KNO3 350 Calcium Nitrate Ca(NO3)2 ∙ 4H2O 295 Potassium Phosphate KH2PO4 100 Magnesium Sulfate MgSO4 ∙ 7H2O 100 Calcium Carbonate CaCO3 30 Ferric Chloride FeCl3 ∙ 6H2O 0.76 Zinc Sulfate ZnSO4 ∙ 7H2O 0.18 Manganous Chloride MnCl2 ∙ 4H2O 0.18 Boric Acid H3BO3 0.12 ————————————————-
Ammonium Molybdate (NH4)6Mo7O24 ∙ H2O 0.04
A copper penny was placed in each beaker in order to control cyanobacteria, which may compete with the duckweed (Taylor, 2011). Each of the 10 beakers received 40 duckweed thalli. The beakers were then placed into a mesh basket underneath a 430-W high-pressure sodium lamp, which mimics the wavelengths produced by natural sunlight. The temperature of the lamp was 24°C and the room temperature was maintained around 21°C. The beakers were observed every day for 14 days. On each day, we would count the number of thalli in each beaker, top up the beaker with distilled water, and gently stir the medium. The baskets were then placed back under the light, but in a different location than before due to the decline in light intensity with distance. The results were recorded in a table each day to keep track of the population growth progress. The mean number of thalli and standard deviation were calculated each day.
Our data was then graphed so we were able to analyze it using statistical methods. A regression was calculated to examine if the slope was significantly different from zero, thus showing that the duckweed grew and there was a linear relationship between the two variables. Two growth curves were produced in order to observe the differences in growth rates between the two groups of thalli, plotting days as the independent variable and number of thalli as the dependent variable.
These means were then ln-transformed to estimate the slopes, which provided the intrinsic growth rates of each treatment to be 0.11 and 0.08 for the control and treatment, respectively. A t-test was used to compare the instantaneous population sizes on three different days of the experiment. On day 3, there was no significant difference between the growth of the control group and the treatment (t=1.37, P>0.05). However, on day 9 (t=4.48, P<0.05) and day 14 (t=5.86, P<0.05) there was a significant difference in growth rates, which therefore must eventually produce a difference in population sizes.
The beakers containing the phosphorus culture medium and those lacking phosphorus both showed a population increase over the span of 14 days (Figure 1). For the medium containing phosphorus, the mean number of thalli grew from 40 ± 0 to 163.6 ± 20.7 (SD) and the medium lacking phosphorus grew from 40 ± 0 to 102.6 ± 10.6.
Figure 1. Growth of Lemna minor, thalli over 14 days under 430-W lights at room temperature. The Control population was grown in a complete growth medium, and the Treatment was in a medium lacking phosphorus. Error bars are standard deviations. N=5 on each day.
Both curves resemble an exponential growth curve, tapering off at the end, but the curve containing phosphorus is a more clearly defined as exponential. This indicates that the growth in the phosphorus nutrient medium was more rapid than the duckweed growth in the phosphorus-lacking medium. In order to achieve a line of best fit, we took the natural logarithm of the means of data. The resulting graph shows the general trend of the population growth of the two populations (Figure 2). The slope of these lines is known as the intrinsic growth rate, r, of the populations. For the population phosphorus rich population we obtained a growth rate of 0.11 and for the phosphorus lacking population the value was 0.08, therefore the nutrient rich population grew at a faster rate than the nutrient lacking population.
Figure 2. Ln-transformed mean number (N=5) of duckweed thalli in phosphorus rich and phosphorus lacking nutrient medium. The intrinsic growth rate, r, is 0.11 for the nutrient rich population and 0.08 for the nutrient lacking population. The mean growth rates were significantly different (t=2.16, P<0.05) between the two populations.
The slopes of the trend lines differed from zero, thus showing that there was growth among both of the populations. A regression was calculated which shows that there is significant growth in each population (P<0.05). In a t-test comparing relative growth rates of the two populations, the calculated t-value was 2.16 and the critical value of t was 1.70 at 28 degrees of freedom. There is a significant difference between our mean growth rates (2.16>1.70, P<0.05). Therefore, we fail to reject our hypothesis that there is a significant difference between the growth rates of Lemna minor grown in a phosphorus rich medium and that grown in a medium lacking phosphorus.
The duckweed grown in the nutrient rich medium had a higher growth rate than the nutrient lacking population, which supports my hypothesis. Former research agrees with these findings, as the productivity of aquatic plants is most likely to be limited by the abundance of phosphate and inorganic nitrogen (Lacoul et al. 2006).
Previous experiments confirm these results in concluding that duckweed is generally indicative of high depth zones that are rich in nutrients (Onaindia et al, 1995). Limiting the abundance of these nutrients will lead to a reduction in plant growth, but introducing concentrations of certain compounds may limit growth as well. The concentration of ammonium ions has a negative effect among aquatic plant growth, and is derived mostly from pollution. Therefore, the presence of aquatic plants can act as indicators of the degree of contamination in a given area (Onaindia et. Al, 1995). These findings can contribute to a future experiment that may test the effect of duckweed concentration on nutrients and compounds within the water column that they reside.
Overall, both of the duckweed populations experienced growth over the span of 14 days. This experiment could lead to further research of the population growth of Lemna minor. We could test the effect of growth in conditions with different concentrations of phosphorus, thus allowing us to observe the ideal growth concentration and if an excess of nutrients may in fact hinder the growth rate. To broaden our research, we could test the effect of phosphorus concentration on terrestrial plants to determine whether it is a limiting factor among their growth patterns as well.
Nutrient concentration has a large impact on the population growth of duckweed. When phosphorus is limiting, the growth of Lemna minor is reduced significantly compared to that grown under ideal conditions. Our results agree with this idea because the population grown in a nutrient rich medium had a higher growth rate than the population grown in a nutrient-lacking medium. Lemna minor has the ability to multiply geometrically, but when placed in a limiting environment, the growth is deferred.
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