What describes the introduction of excess nutrients to an ecosystem?

The introduction of excess nutrients to an ecosystem is best described as pollution, specifically nutrient pollution. This occurs when substances such as fertilizers, sewage, and industrial waste lead to an overload of nutrients, particularly nitrogen and phosphorus, in water bodies. This can result in algal blooms, which deplete oxygen in the water and harm aquatic life, causing significant disruptions to the ecosystem's balance. Nutrient pollution affects water quality, disrupts food webs, and can lead to dead zones where life cannot be sustained.

Other options like overfishing, habitat loss, and overhunting relate to the depletion of species or destruction of habitats but do not specifically address the issue of nutrient excess that causes eutrophication and related environmental problems. Thus, pollution accurately captures the phenomenon of nutrient overload in ecosystems.