Phosphorus sorption-desorption dynamics influence lake response to external phosphorus inputs. Which statement is correct?

Phosphorus sorption-desorption dynamics influence lake response to external phosphorus inputs. Which statement is correct?

• A. Phosphorus sorption permanently removes P from water
• B. Desorption never occurs under natural conditions
• C. Phosphorus sorption onto iron-oxide surfaces is irrelevant
• D. Phosphorus sorption onto iron-oxide surfaces can temporarily remove P, and desorption under reducing conditions or pH shifts releases P back

Answer: (D)

Phosphorus sorption-desorption on iron-oxide surfaces controls how much P stays dissolved in lake water and how much is stored in sediments. Phosphate readily binds to iron-oxide minerals, temporarily removing it from the water column and buffering a rise in dissolved P after external inputs. However, this binding is reversible. Under reducing conditions, ferric iron is reduced and iron oxides can dissolve, releasing the bound phosphate back into the water. Similarly, changes in pH can weaken the sorption bond and trigger desorption, returning P to the dissolved pool. This reversibility means external P inputs can have an immediate buffering effect but can later contribute to sustained or renewed eutrophication as P is released again. Since iron-oxide sorption is a real, dynamic part of lake P cycling, it is not a permanent sink, and desorption can and does occur under natural environmental changes. Therefore, the statement that phosphorus sorption onto iron-oxide surfaces can temporarily remove P, and desorption under reducing conditions or pH shifts releases P back, best captures how these processes influence lake responses to external phosphorus inputs.

Phosphorus sorption-desorption on iron-oxide surfaces controls how much P stays dissolved in lake water and how much is stored in sediments. Phosphate readily binds to iron-oxide minerals, temporarily removing it from the water column and buffering a rise in dissolved P after external inputs. However, this binding is reversible. Under reducing conditions, ferric iron is reduced and iron oxides can dissolve, releasing the bound phosphate back into the water. Similarly, changes in pH can weaken the sorption bond and trigger desorption, returning P to the dissolved pool. This reversibility means external P inputs can have an immediate buffering effect but can later contribute to sustained or renewed eutrophication as P is released again. Since iron-oxide sorption is a real, dynamic part of lake P cycling, it is not a permanent sink, and desorption can and does occur under natural environmental changes.

Therefore, the statement that phosphorus sorption onto iron-oxide surfaces can temporarily remove P, and desorption under reducing conditions or pH shifts releases P back, best captures how these processes influence lake responses to external phosphorus inputs.