What is the main consequence of excess nitrogen or phosphorus in natural systems?

The correct answer highlights the significant environmental consequence of excess nitrogen or phosphorus in natural systems, which is the occurrence of algae blooms and various related changes to the ecosystem. When these nutrients enter bodies of water—often due to agricultural runoff, wastewater discharge, or other human activities—they can lead to nutrient over-enrichment.

This over-enrichment promotes the rapid growth of algae, leading to what is commonly referred to as an algal bloom. Algal blooms can create a variety of problems: they often produce toxins harmful to aquatic life and humans, they can block sunlight from reaching other aquatic plants, and when they decay, they deplete oxygen levels in the water, leading to hypoxic conditions. This series of events can drastically alter the natural balance of aquatic ecosystems, benefiting some species while harming others and potentially leading to significant environmental degradation.

In contrast, increased water clarity typically results from reduced nutrient levels and healthy aquatic vegetation, making it an unlikely consequence of excess nutrients. Reduced aquatic biodiversity often occurs as a result of conditions like hypoxia created by algal blooms, but it is a secondary effect rather than the primary consequence of excess nutrients. Lower temperatures in water bodies are generally not linked to nutrient levels; instead, temperature variations are influenced by climate and environmental conditions