Which of the following can indicate declining water quality in a body of water?

Excessive algae growth serves as a significant indicator of declining water quality in a body of water. When nutrient levels, particularly nitrogen and phosphorus, become elevated—often due to runoff from agricultural lands or urban areas—it can lead to algal blooms. These blooms deplete oxygen in the water when they die and decompose, creating hypoxic conditions that can severely harm aquatic life. Additionally, some types of algae can produce toxins that are hazardous to both wildlife and humans. Monitoring for excessive algae growth is thus critical for assessing the overall health of aquatic ecosystems, making it a key indicator of water quality issues.