Kyle Smith

1B

Duckweed lab

Question:

Does the surface area of water affect the growth of duckweed?

Background:

Duckweed is an extremely small plant. It has small leaf like structures that have a tiny root on the bottom. These structure float on water in stagnant or slow moving bodies of water, oftentimes ponds. The plant reproduces asexually; it splits into separate parts, similar to cells.  Because of this trait, they tend to grow in numbers extremely fast, or exponentially. This means it would be only a small matter of time before they would be able to cover an entire pond. The rapid growth of this plant would classify it as an r-strategist.

Duckweed gains its nutrients by absorbing them from the water. It absorbs a large amount of nitrates and phosphates and is considered a natural water filter because of this absorption. The organism is able to survive in a multitude of lighting conditions; it is most prolific in an open and extremely sunny area. It does moderately well in a slightly shaded area, although not as much as a sunny area. The area where it does the worst, but is still able to survive, is with incandescent lighting. So, some limiting for duckweed would be things such as nutrient availability, light availability, and space availability.

The limiting factors of duckweed easily lend itself to manipulation in a lab. The three main factors that influence a duckweed population are all relatively easy to control. You can control nutrient availability by using deionized water. Light is easily monitored by placing the experiment in a location where no unwanted light can get in. Space would be handled by placing the duckweed in different sized containers. With all three variables controlled, one could slightly change anything the wanted to see how it would affect the duckweed.

Hypothesis:

If we grow duckweed in containers with different surface areas, then the container with the largest surface area will grow the most duckweed because duckweed grows on the surface and will have more space to reproduce.

Procedure:

1. Obtain 3 different sized containers, with 3 of each size.

2. Set the containers up in rows, labeled 1-3 within each different size.

3. Fill each container with 100ml of water.

4. Measure the surface area of the water in each different size of cup.

5. Add 10 duckweed plants.*

6. Let the duckweed reproduce without changing anything and record the number of duckweed plants every few days in every container.

*There are multiple ways to count duckweed. One is is to count it the plant as 1 if it is connected at all; another way to count it is to count each leaf as 1. For this experiment, however, it is counted as 1 when connected.

Variables:

Data:

Limitations:

        This experiment had some major flaws and weaknesses that were not foreseen. One of the biggest problems had to do with the variable we were testing, surface area. The water that the duckweed was growing in was evaporating over time - something that was not planned for. And, due to the shape of the containers, as the water evaporated, the surface areas of the containers were changed. This was a major flaw in the experiment because it completely changed the independent variable.

Another large problem was that the duckweed was not given enough time to reproduce. The increase of duckweed from the beginning of the experiment to the end of the experiment was very minimal. There was not a significant enough amount of duckweed to fill the surface of the water, thus defeating the purpose of having surface area as the independent variable. Regardless, the water would have evaporated and changed the surface area anyways.

These flaws in the experiment are fairly serious problems related directly to the method of testing and what is being tested; so, realistically, the whole experiment needs to be redesigned. One simple fix to the change in surface area would be to use containers that were perfect cylinders or cubes. This would at least keep the surface area the same, but wouldn’t deal with the evaporation issue.

The evaporation issue could possibly be resolved by adding a certain amount of water every time the duckweed was counted. Although, this method would be very difficult because of the fact that surface area also affects the rate of evaporation. So maybe instead of just adding to what water is left, the duckweed is moved from the evaporated containers to containers of the same size with a new 100ml of water in them.

Overall, surface area is a very complex thing to test with a lot of limitations. It would be very difficult to control all of the necessary elements in order to test it.

Conclusion:

        Unfortunately, any and all data collected in this lab is inconclusive. The data is not sufficient to draw a proper conclusion. The lab had too many flaws to be of any use. The duckweed never reached the edges of the containers, so surface area wouldn’t matter to its growth. Along with that fact, and all of the other problems discussed in the “Limitations” section, there is no conclusion to make from this data. This lab requires a retest with some of the improvements already discussed.