November 14, 2011
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. Introduction
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...