Zone of Recovery

The growth of algae reappears in the Zone of Recovery in a polluted river. This zone is characterized by a gradual improvement in water quality and ecosystem health after a period of pollution or degradation. Here's an explanation of why the correct answer is option 'B':

1. Pollution Zones in a River
Rivers are divided into different pollution zones based on the level of pollution and the impact on the ecosystem. These zones help in understanding and managing the effects of pollution on water quality and aquatic life. The four main pollution zones are:
- Zone of Clean Water: This zone represents a healthy and unpolluted river. It has good water quality and supports a diverse range of aquatic life.
- Zone of Recovery: This zone follows the pollution source and is characterized by a gradual improvement in water quality and ecosystem health.
- Zone of Active Decomposition: This zone is located downstream from the pollution source and experiences high levels of pollution. It is characterized by low dissolved oxygen levels and the decomposition of organic matter.
- Zone of Degradation: This zone is the most polluted and occurs immediately downstream from the pollution source. The water quality is severely degraded, and the ecosystem is significantly impacted.

2. Growth of Algae
Algae are microscopic aquatic plants that play a vital role in the ecosystem. They are primary producers that convert sunlight and nutrients into organic matter through photosynthesis. While algae are essential for a healthy ecosystem, their overgrowth can lead to water quality issues.

3. Algae Growth in the Zone of Recovery
In the zone of recovery, the water quality gradually improves as the pollution is diluted and flushed downstream. The decrease in pollution levels leads to an increase in dissolved oxygen, which is crucial for the growth of aquatic plants such as algae. Algae are quick to colonize and reproduce, taking advantage of the improved conditions in the zone of recovery.

4. Significance of Algae Growth
The growth of algae in the zone of recovery indicates the resilience and recovery of the ecosystem. Algae serve as indicators of water quality, and their presence suggests that the conditions are becoming suitable for other aquatic organisms to thrive. As the ecosystem continues to recover, more diverse species of plants and animals will colonize the area.

In conclusion, the growth of algae reappears in the zone of recovery in a polluted river. This zone represents a stage of gradual improvement in water quality and ecosystem health after a period of pollution or degradation.