Organisms and Interactions with the Environment: Student Sheet

(Download the MS Word version of this document.)

Background

Organisms interact with the environment in which they live. They can change the environment. This can lead to the process of succession. Succession is a replacement of one community by another in a progression to a climax community. For example, when a volcano erupts, it may wipe out the climax community. The first organisms to colonize the area may be mosses and lichens. These plants may produce acids as a waste produce to break down rocks in the formation of soil. Once soil formation is begun, ferns may come in and colonize the area. Its rhizoids help to form more soil, and as they die and decompose, the soil may become more rich and suitable for different types of organisms. It is easy to see that organisms can change the environment they interact with.

Productivity is the rate at which organisms create new biomass. Photosynthetic autotrophs create new biomass as a result of photosynthesis. Productivity is the amount of organic matter produced from solar energy in a given area during a given period of time.

Gross primary productivity is the total amount of organic matter produced by autotrophs including the organic matter the autotrophs use for cellular respiration.

Net primary productivity is the amount of organic matter produced in a community in a given time that is available for heterotrophs. It is equal to the gross productivity minus the amount of energy expended by the metabolic activities of the photosynthetic organisms.

In general, the biomass of all the organisms (heterotrophs and autotrophs) increases as result of its net primary productivity.

Purpose

The purpose of this lab is to take the measurement of several factors of pond water. After 48 hours, the factors will be measured again to determine if the organisms have changed their environment. The measurements to be taken are temperature, pH, and dissolved oxygen.

In the pond water, there could be autotrophs such as algae. These primary consumers obtain their energy by photosynthesis.

6 CO₂ + 6 H₂O ---> C₆H₁₂O₆ + 6O₂

This process would increase the amount of dissolved oxygen. The pH of the water would decrease because the carbon dioxide comes from the carbonic acid found in the water.

H₂CO₃ ---> H₂O + CO₂

In the pond water, there are heterotrophs such as protozoans, small worms, or hydra. These organisms decrease the amount of dissolved oxygen as they obtain their energy from cellular respiration.

C₆H₁₂O₆ + 6O₂ ---> 6 CO₂ + 6 H₂O

This process would decrease the pH of the water because as carbon dioxide is produced, and it is converted to carbonic acid.

H₂O + CO₂ ---> H₂CO₃

Materials

LaMotte Dissolved Oxygen Testing Kit
3 60ml water sampling jars
Thermometer or temperature probe
Universal testing paper or pH probe
Aluminum foil
Light

Procedure

Obtain 60 ml of pond water in a water sampling bottle. Measure its pH and temperature and record.
Following the instruction on the LaMott kit, titrate the water and determine the amount of dissolved oxygen in the water sample.
Obtain two water sampling bottles and fill both with pond water.
Cover one with aluminum foil. Put both bottles under a light for 48 hours.
After 48 hours, measure the pH and temperature of both bottles and record.
Using the LaMott kit, determine the amount of dissolve oxygen in both water samples. Determine if the pH, dissolved oxygen has changed during this time.

Data Table

	Temperature	pH	Dissolve Oxygen
Water after 48 hrs. in light
Original water sample
Change

Water after 48 hours in the dark
Original water sample
Change

Gross productivity=Light bottle O2 ppm -Dark bottle O2 ppm
Net productivity=Light bottle O2 ppm -Initial bottle O2 ppm
Rate of respiration=Initial bottle O2 ppm -Dark bottle O2 ppm

Questions

1. What process was stopped once the water was covered with aluminum foil?

2. What processes were occurring in the light bottle?

3. What evidence is there, that organisms can change their environment?

4. What was the difference between the measurements and results from the light bottle and the dark bottle?

5. What is the net productivity for the pond water and what is the gross productivity? Explain why these two values are different.

6. Explain what does it mean if the value for gross productivity is positive versus negative.

7. Explain what does it mean if the value for net productivity is positive versus negative.

8. Write a conclusion for this experiment.

[printer-friendly]