Phosphate exchange across the sediment-water interface when shifting from anoxic to oxic conditions an experimental comparison of freshwater and brackish-marine systems

Comparative, experimental studies on sediment cores from freshwater andbrackish-marine conditions reveal major differences in the benthic exchangeof phosphate across the sediment-water interface when shifting from anoxicto oxic conditions. The flux of phosphate to the sediment during this shiftwas found to be mediated mainly by scavenging from newly formed colloidalferric oxohydroxide. The capacity of the iron-rich particles to scavengephosphorus depended on the stoichiometric ratio between dissolved iron andphosphorus built up in the supernatant water during reducing conditions. Thefreshwater system was characterized by high iron to phosphorus ratios in thedissolved phase and thus most of the phosphate was incorporated into thecolloidal iron oxohydroxide during the oxygenation. In contrast, the marinesystems reached lower iron to phosphorus ratios during the anoxic period whichresulted in less efficient phosphate scavenging. Consequently, significantamounts of phosphate remained dissolved in the marine systems after the changeto oxic conditions, possibly increasing the proportion of phosphate recycledto the euphotic zone. Manganese showed a consistent redox-dependent behaviourin all the investigated systems, but interacted neither with phosphate norwith iron.