What is the main environmental issue caused by eutrophication?

Eutrophication is primarily characterized by an excessive richness of nutrients, particularly nitrogen and phosphorus, in bodies of water. This nutrient overload often results from agricultural runoff, wastewater discharge, and industrial processes. When nutrient levels increase significantly, it stimulates a rapid growth of algae, known as algal blooms. These blooms can lead to a range of ecological problems, including decreased oxygen levels in the water, which can create dead zones where aquatic life cannot survive. The excessive algae can also block sunlight, preventing photosynthesis in aquatic plants, further disrupting the ecosystem. Thus, the correct answer highlights the central issue of nutrient over-enrichment that drives the eutrophication process, ultimately impacting water quality and aquatic habitats.