The diatom is an autotroph. Explain the meaning of the term autotroph.
Describe the function of the water flea in the food chain shown in Fig. 2.1.
Suggest why the heron population is much smaller than the water flea population.
Fig. 2.2a shows a pond skater moving across the water surface. Fig. 2.2b shows a northern pike. These are not drawn to the same scale. Both animals live in northern countries of the world, where temperatures often fall below $0\,^{\circ}\text{C}$. Describe the importance of water as an environment for the pond skater and the northern pike.
The mud at the bottom of the freshwater ecosystem contains dead organic material and supports the growth of water plants. Outline how nitrogen from the dead organic material becomes available to the growing plants.