Final answer:
The most prominent man-made source of phosphorus contamination is the use of phosphorus-containing detergents, which leads to excessive algae growth and eutrophication in lakes.
Step-by-step explanation:
The most prominent man-made source of phosphorus contamination is the use of phosphorus-containing detergents. When these detergents were introduced in the 1950s, the wastewater from normal household activities greatly increased the amount of phosphorus available to algae and other plant life in lakes. This resulted in excessive algae growth, leading to eutrophication and the death of fish due to decreased oxygen levels in the water.