Phosphorus diagenesis in lake sediments: investigations using fractionation techniques

A 60-cm sediment core, representing approximately 50 years of deposition, was collected from the major depositional basin of Onondaga Lake, a calcareous, hypereutrophic system in Syracuse, New York (USA). Sequential chemical extractions were performed to obtain fractional phosphorus (P) profiles, and these suggest that the sediment P available for solubilization and recycling to the water column is contained within the CaCO3-associated, Fe&Al-bound, and extractable biogenic-P fractions. Corrections were applied to account for the presence of refractory P within the CaCO3-associated and Fe&Al-bound fractions. Labile phosphorus comprises ~50% of the particulate P at the time of deposition. Reductions in labile P with depth suggest losses due to diagenesis with subsequent release to the lake water column. Rate constants for diagenesis were calculated from measurements of labile P on dated cores. Two labile components were apparent: one, exhibiting 'fast' decay (k = 4.8 year-1) was located within the upper 1 cm of sediment, and the other, characterized by 'slow' decay (k = 0.1 year-1) was located throughout the sediment to a depth of ~30 cm, below which sediment P was refractory. The ratio of the 'fast' to 'slow' fractions is approximately 1 : 1 in freshly deposited sediment. These findings facititate sediment P modelling and have bearing on the recovery of lakes following reductions in external phosphorus loads.