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 p artners and the concentration of soluble reactive P in the interstitial wat er were investigated in the bottom sediments of the oligotrophic Lake Stech lin (north-eastern Ger many). The most important fraction of total P in the top centimeter of the sediment is the redox-sensitive Fe- and Mn-bound P w ith 1.29 g/kg (54 % total P), which decreased to about 0.23 g/kg at a depth of 10-30cm. Its decrease is responsible for a corresponding decrease in to tal P with depth. The release of P due to diagenesis of labile P-compounds 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 st ratification (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-compound s (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 la yer and its redox-dependent P-binding capacity influence the present concen tration of P in the whole water body, and thus contribute to maintaining th e oligotrophic status of Lake Stechlin.