Explain the phosphorus cycle and why phosphorus is a limiting nutrient in many freshwater systems.

• A. Phosphorus cycles through rocks to soils and waterways; weathering releases phosphate, uptake by organisms, burial in sediments; limited solubility and sedimentation reduce availability, causing P-limitation.
• B. Phosphorus cycles through the atmosphere widely.
• C. Phosphorus is abundant in seawater and never limits.
• D. Phosphorus is highly soluble and easily available.

Answer: (A)

Phosphorus moves through ecosystems in a cycle that involves rocks, soils, water, and living organisms, but not a significant atmospheric phase. Weathering slowly releases phosphate from rocks into soils and into waterways. Organisms take up that phosphate to build molecular compounds like ATP and DNA, and when they die or excrete waste, phosphate returns to the environment. A large portion then binds to minerals and settles into sediments, where it can be buried. This burial removes phosphate from the immediate available pool, and because phosphate tends to be relatively insoluble and readily precipitates with minerals, the amount of dissolved, bioavailable phosphate in freshwater is typically limited. Because freshwater primary production depends on dissolved phosphate, these systems commonly experience phosphorus limitation, which controls algal and overall productivity. Human inputs from fertilizer runoff and wastewater can increase phosphate in waterways, often triggering eutrophication. The other statements aren’t correct because phosphorus doesn’t cycle through the atmosphere in any significant way, and it isn’t universally abundant or always highly soluble and readily available.