A small pond is full of water plants and fish.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A small pond is full of water plants and fish. Early in the morning, before sunrise, the fish swim up and gulp at the surface. In the late afternoon they swim calmly. Explain the difference.
Need a reminder? The key ideaShow
Gases are exchanged by diffusion across a large, thin, moist surface. Water holds far less dissolved oxygen than air, so fish move a lot of water over their gills. Insects use tracheae, land vertebrates use lungs, and plants use stomata and lenticels. By day photosynthesis uses more carbon dioxide than respiration makes; at night only respiration goes on.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. At night. There is no light, so the plants cannot photosynthesise. The plants and the fish all respire and use oxygen. No oxygen is added.
Step 2. Before sunrise. After a whole night, the dissolved oxygen in the water is at its lowest. The fish come up to the surface, where the water touches the air.
Step 3. In the afternoon. All day the plants have photosynthesised and given out more oxygen than they used. The water is full of dissolved oxygen.
Answer: at night everything respires and nothing adds oxygen, so the water has least oxygen at dawn; by afternoon photosynthesis has filled it up again.
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