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 and brackish-marine conditions reveal major differences in the benthic exchange of phosphate across the sediment-water interface when shifting from anoxic to oxic conditions. The flux of phosphate to the sediment during this shift was found to be mediated mainly by scavenging from newly formed colloidal ferric oxohydroxide. The capacity of the iron-rich particles to scavenge phosphorus depended on the stoichiometric ratio between dissolved iron and phosphorus built up in the supernatant water during reducing conditions. The freshwater system was characterized by high iron to phosphorus ratios in the dissolved phase and thus most of the phosphate was incorporated into the colloidal iron oxohydroxide during the oxygenation. In contrast, the marine systems reached lower iron to phosphorus ratios during the anoxic period which resulted in less efficient phosphate scavenging. Consequently, significant amounts of phosphate remained dissolved in the marine systems after the change to oxic conditions, possibly increasing the proportion of phosphate recycled to the euphotic zone. Manganese showed a consistent redox-dependent behaviour in all the investigated systems, but interacted neither with phosphate nor with iron.