Mud, or sediment, is an active part of aquatic ecosystems. Sediment varies widely within and among ecosystems in its biotic and abiotic characteristics.
Biotic factors are the living components of a community or larger ecosystem.
Abiotic factors are essentially non-living components that effect the living organisms of a community.
In many ecosystems sediment can release excess phosphorus (a common aquatic pollutant) into the water column causing internal eutrophication.
When studying aquatic ecosystems, people often think about the water and things that live in the water. However, the mud at the bottom of lakes and wetlands – the sediment – is an active part of these ecosystems. A wide diversity of organisms, both macroscopic and microscopic, live in sediments.
Sediments can often be a source of nutrients, especially nitrogen and phosphorus. Nutrients released from sediments are part of an ecosystem’s internal load (as opposed to the external load, which consists of nutrients that come from outside the ecosystems).
Most commonly, sediments release large amounts of phosphorus as phosphate, sometimes causing excessive algal growth, harmful algal blooms (growth by algae that produce toxins), and even fish kills (as dead algae fall to the bottom of an ecosystem, fuel bacterial decomposition, and consume oxygen). These negative effects caused by sediment release of phosphorus are called internal eutrophication.
Life in the Mud
Gather around a picnic table at a local pond or wetland area. Lead the class in a discussion about how the abiotic and biotic components of the pond could interact. Use what the students have said to link the nutrient content of the sediment and water to the activity of living things in the water. For example, nutrients released from the sediment can enhance growth of algae in the water.
Ask students to hypothesize about what they expect to see in the mud. What makes up the sediment? What makes up the pond water? Describe some possible interactions between the sediment and the water column.
- 2 quart jars with lids for each group
- Mud from a pond (enough for about 1/3 of each of the jars)
- Water from a pond–algae WILL be there (enough to fill the other 2/3 of each of the jars)
- Shovel (one for each group)
- Water quality kits for measuring nutrients for each group
- Compound microscope (for each pair of students)
- Microscope slides
- Optional Materials: Dissolved oxygen meter, dissecting microscope, thermometer, conductivity meter, funnel
Experimental Set Up
- Split into small groups and distribute materials evenly. Each group should disperse to a different area around the pond perimeter to begin the experimental set up.
- Each group should utilize the water quality kit to test the pond water and record the data in their journals.
- Each group sets up the jars: one with nothing but pond water in it (control group) and another jar with 1/3 pond sediment and 2/3 water in it (treatment).
- After collecting mud samples, return to the table. Lead students through the process of developing a hypothesis with the guiding questions: What differences do you expect to see in the treatment group and control group in about a week? Why do you think those differences might occur? Possible hypothesis: There will be a greater number of algae in jar with sediment and pond water compared to the jar with only pond water.
- Check the jars after one week. If you do not see obvious responses, check them again after two weeks as it may take some time for visible algal growth to occur.
- Qualitative Observations: appearance of the water and sediment, look for evidence of algae growth—cloudy water and green “slime” on the sediment; any bubbles coming from the sediment, smell, layers in sediment evidenced by color difference or texture changes; macroscopic organisms in either sediment or water; bacterial growth (slime).
- Quantitative Observations: use a microscope to count the algal cells in the water in each jar; if available, test the water in each jar with any available nutrient water testing kit (nitrogen and/or phosphorus), depth of water and sediment over time, water temperature, conductivity and dissolved oxygen, pH.
If you would like to do undertake this outdoor lab activity with you students, I’ve created a free printable student page, Life in the Mud, for your use. If you download it, please leave a little note in the comments.