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

Phosphorus (P) release from sediment particles to the interstitial water ha s been studied extensively, but the contribution of different inorganic P p ools in sediment under differing environmental conditions is not fully unde rstood. This study was undertaken to get more detailed information about th e chemical mobilization mechanisms. Phosphorus mobilization from reserves b ound by Al, Fe, and Ca compounds in response to increased pH and to inorgan ic silicon (Si) enrichments was investigated using a sequential fractionati on analysis and an isotope-labeling technique. The aerobic sediment of Lake Vesijarvi 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 re leased P onto hydrated Al oxides. This finding reveals that P equilibrium b etween 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, resulti ng 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 fa vor the appearance of bloom-forming cyanobacteria.