Phosphorus mobilization from various sediment pools in response to increased pH and silicate concentration.

Phosphorus (P) release from sediment particles to the interstitial water has been studied extensively, but the contribution of different inorganic P pools in sediment under differing environmental conditions is not fully understood. This study was undertaken to get more detailed information about the chemical mobilization mechanisms. Phosphorus mobilization from reserves bound by Al, Fe, and Ca compounds in response to increased pH and to inorganic silicon (Si) enrichments was investigated using a sequential fractionation analysis and an isotope-labeling technique. The aerobic sediment of Lake Vesijärvi had a high P retention capacity, and Fe-bound P was the largest inorganic P pool as well as the main source of released P. High Si addition (47 mg Si L-1 sediment) released more P to the interstitial water than did the elevation of pH from 6.6 to 9.5, since Si lowered the resorption of released P onto hydrated Al oxides. This finding reveals that P equilibrium between Fe-bound and Al-bound P in sediments regulates P net mobilization to the interstitial water under aerobic conditions. Furthermore, elevated pH combined with high Si enrichment had a positive synergistic effect, resulting in the most substantial P mobilization. This synergism may cause a self-fueled increase in the internal loading of P. It accentuates the effect of diatom sedimentation on P fluxes in eutrophic lakes with high pH and may favor the appearance of bloom-forming cyanobacteria.