Ammonia in wastewater treatment is considered toxic primarily because it:

Ammonia in wastewater treatment is considered toxic primarily because it stimulates excessive algae growth. When ammonia is present in water, it serves as a nutrient that can lead to algal blooms. These blooms can have several negative ecological impacts, such as depleting oxygen levels in the water as algae die and decompose, which can further harm aquatic life. Additionally, the rapid growth of algae can block sunlight from reaching other aquatic plants and disrupt the natural balance of the ecosystem.

Excessive algae growth can create a cycle of degradation in water quality that is particularly harmful to aquatic environments, leading to hypoxia or anoxic conditions. Thus, while the other potential answers might address issues related to wastewater treatment, the primary concern with ammonia is its role in promoting harmful algal blooms, which can have severe ecological consequences.