Oxygen Controls the Phosphorus Release from Lake Sediments - a Long-Lasting Paradigm in Limnology

The pioneer works of EINSELE, MORTIMER, and OHLE on the linking between phosphorus (P) and iron (Fe) cycles seven decades ago created the theoretical basis for a long-standing paradigm among limnologists i.e., 'oxygen controls the P release from sediments'. While many empirical studies as well as strong correlations between oxygen depletion and P release seem to support this paradigm, various field observations, laboratory experiments, and repeated failures of hypolimnetic oxygenation measures cast doubt on its universal validity. The temporal existence of a thin oxidized sediment surface-layer could affect only fluctuations of the temporary P pool at the sediment surface but not the long-term P retention. On longer time scales P release is the imbalance between P sedimentation and P binding capacity of anoxic sediment layers. The P retention of lake sediments strongly depends on sediment characteristics and land use of the catchment. The presence of redox-insensitive P-binding systems such as Al(OH)(3) and unreducible Fe(III) minerals can enhance the P retention and completely prevent P release even in case of anoxic conditions. Alternative release mechanisms such as a dissolution of calcium-bound P and decomposition of organic P under both, aerobic and anaerobic conditions, are often more important than the redox driven Fe-coupled P cycle. Additionally, bacteria affect P cycling not only by altering the redox conditions but also by releasing P during mineralization of organic matter and by accumulation and release of bacterial P. Since microbial processes consume oxygen and liberate P it is difficult to distinguish whether oxygen depletion is the result or the cause of P release. Nowadays, the old paradigm is discarded and a paradigm shift takes place. Sedimentary P exchange ought to be considered as a complex process which is mainly determined by the amount and species of settled P as well as their subsequent diagenetic transformation in the sediment. The classical paradigm is only valid in special cases since reality is much more complex than suggested by that paradigm.