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.

Everything should be made simple as possible, but not simpler!

Albert Einstein (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)