Mechanisms of phosphorus release from the bottom sediment of the oligotrophic Lake Stechlin: Importance of the permanently oxic sediment surface

The vertical distribution of various P-binding forms, potential P-binding partners and the concentration of soluble reactive P in the interstitial water were investigated in the bottom sediments of the oligotrophic Lake Stechlin (north-eastern Germany). The most important fraction of total P in the top centimeter of the sediment is the redox-sensitive Fe- and Mn-bound P with 1.29 g/kg (54% total P), which decreased to about 0.23 g/kg at a depth of 10-30 cm. Its decrease is responsible for a corresponding decrease in total P with depth. The release of P due to diagenesis of labile P-com-pounds was calculated to be 69 mg P m -2 a -1 . This is concordant with the mean accumulation rate of soluble reactive P in the hypolimnion during summer stratification (70 mg P m -2 a -1 ). The mobilization of reductant-soluble P due to the shift of the top oxic sediment layers into deeper anoxic zones (burial) accounts for 81% of the calculated P-release of labile P-compounds (86 mg P m -2 a -1 , 1970-1996). Although, reductant-soluble P does not add to the permanent burial of P, the thickness of the top oxic sediment layer and its redox-dependent P-binding capacity influence the present concentration of P in the whole water body, and thus contribute to maintaining the oligotrophic status of Lake Stechlin.