Fig. 1.14 shows a cross-section through a river from its source in the mountains to its mouth at the sea. Poor waste management can lead to soil and water p...

(a) Two changes in river characteristics from upper to lower course [2]

Any two from:

• Flow speed decreases from fast in the upper course to slow in the lower course. [1]
• Channel width increases from narrow in the upper course to wide in the lower course. [1]
• Bed material changes from large rocks and boulders in the upper course to fine silt and mud in the lower course. [1]
• Dissolved oxygen concentration decreases from high in the upper course to lower in the lower course. [1]

(b) How organisms change from upper to lower course [3]

1. In the upper course, species adapted to fast-flowing, well-oxygenated, cold water are found, such as stonefly larvae and freshwater shrimp. These organisms have adaptations (flattened bodies, hooks) to cling to rocks in the current. [1]
2. In the middle course, species tolerant of moderate flow and oxygen levels are found, such as mayfly larvae and small fish. Conditions here are less extreme. [1]
3. In the lower course, species tolerant of slow-moving, warmer, lower-oxygen water are found, such as bloodworms and sludge worms (tubifex), along with larger fish. These organisms can tolerate or even thrive in water with lower dissolved oxygen. [1]

(c) How untreated waste from Factory X affects the river downstream [6]

1. The waste may contain organic pollutants (sewage-like material) that are discharged directly into the river. [1]
2. Aerobic decomposing bacteria multiply rapidly to break down this organic waste. Their respiration consumes large amounts of dissolved oxygen, increasing the biological oxygen demand (BOD) of the water. [1]
3. Dissolved oxygen levels drop sharply downstream of the discharge point. Oxygen-sensitive species such as mayfly larvae and trout die because they cannot survive in low-oxygen water. [1]
4. Excess nutrients in the waste (nitrates and phosphates) can cause eutrophication, leading to algal blooms that further reduce light penetration and oxygen levels. [1]
5. Toxic chemicals in industrial waste (such as heavy metals and solvents) can accumulate in the tissues of organisms through bioaccumulation, becoming more concentrated at each trophic level in the food chain. [1]
6. Biodiversity decreases downstream of the discharge point. Only pollution-tolerant species such as bloodworms and tubifex worms survive in the most heavily polluted zone, while sensitive indicator species disappear. [1]