Ontario Operator-in-Training (OIT) Practice Exam

Which environmental effect is associated with high nitrogen and phosphorus content in sewage?

1. Reduction of water temperatures

2. Increase in fish populations

3. Oxygen depletion in aquatic environments

4. Improvement of water clarity

The correct answer is: Oxygen depletion in aquatic environments

The association between high nitrogen and phosphorus content in sewage and oxygen depletion in aquatic environments is well documented in environmental science. When these nutrients enter water bodies, they can lead to a process called eutrophication. This process encourages algal blooms, where rapid growth of algae occurs due to the abundance of nutrients. As the algae proliferate, they eventually die and decompose, a process that consumes oxygen in the water. With the increased decomposition, oxygen levels in the water can significantly drop, often leading to hypoxic conditions where aquatic life cannot survive. This depletion of dissolved oxygen can have catastrophic effects on fish and other aquatic organisms, ultimately disrupting the entire ecosystem. In contrast, options such as the reduction of water temperatures, improvement of water clarity, or an increase in fish populations do not directly relate to the consequences of high nutrient loads from sewage. These conditions are typically influenced by other factors and are unlikely to occur as a result of nutrient enrichment. Therefore, it is the oxygen depletion that directly correlates with high nitrogen and phosphorus content in sewage, making it the correct answer.