What effect does phosphorus deposition have on water bodies?

Phosphorus deposition significantly influences aquatic ecosystems, particularly regarding nutrient cycling within water bodies. When phosphorus enters a water body, often through agricultural runoff, wastewater discharge, or atmospheric deposition, it can lead to nutrient enrichment, a process known as eutrophication.

This increased availability of phosphorus supports the rapid growth of algae and aquatic plants, leading to algal blooms. These blooms can result in the depletion of oxygen in surface waters as the algae grow and die, causing a negative impact on fish and other aquatic life. The decomposition of this organic matter consumes oxygen, leading to hypoxic or anoxic conditions, which are detrimental for most aquatic organisms.

As for the notion of saturating deep waters, phosphorus can indeed accumulate in lower layers of stratified water bodies, especially during stratification phases. The increased phytoplankton production can lead to higher organic matter settling down into deeper waters when they die and decompose. Thus, while surface waters may face oxygen depletion, deep waters can experience elevated levels of nutrients, including phosphorus.

This understanding elucidates how phosphorus impacts nutrient dynamics and oxygen availability in water bodies, making it clear why the correct answer focuses on the depletion of surface waters while contributing to the saturation of deeper layers.