The natural ageing of a lake is called biomagnification.

Answer

### Step-by-Step Text Solution:

1. **Understanding the Statement**: The question states that "The natural ageing of a lake is called biomagnification." We need to evaluate the accuracy of this statement.

2. **Identifying the Correct Term**: The natural ageing of a lake is actually referred to as **eutrophication**, not biomagnification. Eutrophication is a process that occurs when a body of water becomes overly enriched with nutrients, leading to excessive growth of algae and other aquatic plants.

3. **Explaining Eutrophication**: Eutrophication occurs due to the increased concentration of nutrients, particularly nitrogen and phosphorus, often from agricultural runoff or industrial waste. This nutrient overload promotes the rapid growth of algae (algal blooms), which can cover the water's surface.

4. **Consequences of Eutrophication**: The excessive growth of algae can block sunlight from reaching aquatic plants and deplete oxygen in the water as the algae die and decompose. This can lead to the death of fish and other aquatic organisms due to lack of oxygen.

5. **Understanding Biomagnification**: On the other hand, biomagnification refers to the process where the concentration of toxins, such as pesticides, increases in organisms at higher trophic levels of the food chain. As predators consume prey, the toxins accumulate in their bodies, leading to higher concentrations in top predators.

6. **Conclusion**: Therefore, the statement in the question is **false**. The natural ageing of a lake is called **eutrophication**, not biomagnification.