What is the impact of excessive nutrients in receiving waters?

Excessive nutrients, particularly nitrogen and phosphorus, in receiving waters significantly impact aquatic ecosystems by inducing algal blooms. These blooms occur when nutrient levels are elevated, leading to rapid growth of algae and cyanobacteria. While these organisms might seem beneficial initially, their proliferation can have severe consequences.

Once the algal blooms die off, the decomposition process consumes a large amount of dissolved oxygen in the water, which is critical for the survival of fish and other aquatic organisms. This depletion of oxygen can create hypoxic (low oxygen) or anoxic (no oxygen) conditions, leading to fish kills and a decline in biodiversity. Moreover, some algal blooms produce toxins that can be harmful to aquatic life, pets, and even humans. Therefore, the connection between excessive nutrients and detrimental algal blooms, along with subsequent oxygen depletion, explains why the chosen answer accurately reflects these ecosystem impacts.