- Distribute Worksheet: Biological Cycle to Check how much they have learned from the previous exercises.
- Repeat the role of producers (the plants), consumers (the animals) and decomposers in an ecosystem (the bacteria, the fungi, and some insects). Explain to pupils once again that plants and animals produce organic substances, that dead organic substances can decompose to minerals, that plants through the process of photosynthesis, can transform mineral substances into organic substances. Let pupils write down missing data on their worksheet during the discussion.
- Distribute the worksheet Cycle of dissolved oxygen in water to pupils and discuss the oxygen path as a part of the biological cycle: (1) mixing of water surface by wind, waves and currents add atmospheric oxygen to the surface water; (2) oxygen is used for breathing by animals and plants (respiration); (3) oxygen is also used in decomposition of organic matter (poop and dead organisms) on the bottom (respiration and mineralization); (4) oxygen is used for breathing by plants and phytoplankton (respiration); (5) oxygen is produced through the process of photosynthesis by water plants and phytoplankton; Repeat also the basic knowledge about photosynthesis.
- Ask students to think what can happen if there are too many nutrients in water that enters a river as untreated wastewater or as fertilizers washed with rain from fields?
- Distribute the worksheet Can a River Handle Pollution? to Ask them to look at the picture and write a story about what happened and what kind of influence this has on life in a river.
- Consider together with pupils:
- Which processes of cleaning water are the most important in nature? (Biological cleaning with the help of plants and microorganisms).
- What do you imagine under the concept “the biological processes of self-purification of water”? (All living organisms contribute to cleaning water).
- What kind of pollution can nature purify itself? (Organic waste – human and animal secretions).
- What about some other forms of pollution – sewage from households, industry? (Sewage that contains chemicals cannot always be purified by nature alone; it depends upon whether the chemicals are biodegradable or not, or even poisonous).
- What happens when sewage, carrying organic substances, enters the water ecosystem? (Natural organic substances, such as water from toilets and food waste, are decomposed by bacteria into mineral substances that plants can use for their growth. Because of the higher availability of food, there is increased production of plants and also animals. Artificial organic substances, such as different plastics, rubber, or medications, can be biodegradable or not. Such waste can be very dangerous to the environment).
- Why is excessive organic pollution (this means plenty of food for decomposition processes) dangerous to life in water? (bacteria need oxygen for decomposition of organic Plants are producing oxygen only by day. Therefore, when there is plenty of food and organisms grow extensively, they can easily use up the oxygen at night, and as a result, they can die ).
- Do you know that wastewater treatment plants use bacteria to eat organic substances in wastewater to clean it?
- Let pupils work on Worksheet: Biological cycle and waste material, where they should present, using arrows, how substances from wastewater integrate into the biological cycle.
Activity adapted from Vahtar, M., Zdešar, M., Kompare, B., Urbanc-Berčič, O., (2005): Kako se reka očisti? – Priročnik za učitelje 3. Book collection Vodni detektiv. ICRO Domžale, Domžale, Slovenia, p.36.