Under stratified conditions with anoxic hypolimnion, what happens to ferric (Fe3+) and manganese oxides, and how does this affect phosphorus release?

• A. They dissolve, releasing bound phosphorus into the pore water and overlying water.
• B. They precipitate and trap phosphorus in sediments.
• C. They remain intact and continue binding phosphorus.
• D. They release phosphorus only if oxygen is present.

Answer: (A)

Under stratified conditions, the deep water becomes anoxic, and iron and manganese oxides in sediments dissolve when redox conditions drop. These oxides normally trap phosphorus by adsorbing it from the water column, especially under oxic conditions. When the hypolimnion is reduced, ferric iron (Fe3+) and manganese oxides are reduced to Fe2+ and Mn2+, causing them to dissolve. The phosphorus that was bound to those oxides is released into the pore water, and diffusion toward the overlying water releases additional phosphorus into the lake water. This internal loading of phosphorus can fuel nutrient-driven processes when mixing reoccurs. If oxygen were present, those oxides would stay solid and continue to bind phosphorus, so release wouldn’t occur; the statement about release only with oxygen is incorrect.